Endocrine control of energy metabolism in the cow: correlations of hormones and metabolites in high and low yielding cows for stages of lactation

Effect of residual methane emission on physiological characteristics and carcass performance in Japanese Black cattle

This study investigated the physiological characteristics and carcass performance associated with residual methane emissions (RME), and the effects of bull differences on CH4-related traits in Japanese Black cattle. Enteric methane (CH4) emissions from 156 Japanese Black cattle (111 heifers and 45 steers) were measured during early fattening using the sniffer method. Various physiological parameters were investigated to clarify the physiological traits between the high, middle, and low RME groups. CH4-related traits were examined to determine whether bull differences affected progeny CH4 emissions. Ruminal butyrate and NH3 concentrations were significantly higher in the high-RME group than in the low-RME group, whereas the propionate content was significantly higher in the low-RME group. Blood urea nitrogen, β-hydroxybutyric acid, and insulin concentrations were significantly higher, and blood amino acids were lower in the high-RME group than in the other groups. No significant differences were observed in the carcass traits and beef fat composition between RME groups. CH4-related traits were significantly different among bull herds. Our results show that CH4-related traits are heritable, wherein bull differences affect progeny CH4 production capability, and that the above-mentioned rumen fermentations and blood metabolites could be used to evaluate enteric methanogenesis in Japanese Black cattle.

Effects of different vitamin A supplies on performance and the risk of ketosis in transition cows

This experiment investigated the effects of feeding low and high supplies of vitamin A (VA) during the transition period on plasma metabolites, prevalence of ketosis, and early milk production. In a randomized complete block design, 42 prefresh Holstein cows and 21 heifers were blocked by parity and calving date and assigned to 1 of 3 dietary treatments (n = 21 per treatment unless noted): CON, a transition diet with supplemental VA (75,000 IU/d) to meet the requirement; LVA, a transition diet with no supplemental VA; or HVA, a transition diet receiving supplemental VA (187,500 IU/d) 2.5 times greater than the requirement. Experimental periods were prepartum (-14 d prepartum), postpartum (1 to 30 d in milk), and carryover period (31 to 58 d in milk; common lactating diet with adequate VA was fed). Differences in dry matter intake in the pre- and postpartum periods and milk yield were not detected among treatment. Milk fat, protein, and lactose yields were similar among treatments and not affected by VA. Somatic cell count increased linearly with increasing VA. Body weight and body condition score decreased postpartum, but no VA effect was observed. Plasma retinol concentrations (n = 10 per treatment) decreased at d 2 postpartum and increased as lactation progressed, but the concentrations were unaffected by treatment. Plasma β-carotene (n = 10 per treatment) had a treatment by time interaction and its concentration decreased after parturition and remained low for 2 wk. Plasma fatty acids and β-hydroxybutyrate did not differ among treatments. Milk retinol concentration and yield (n = 10 per treatment) increased as VA supply increased. Segmented neutrophils (%) decreased, and lymphocytes (%) increased in blood with increasing VA supply. In conclusion, providing different supplies of VA did not affect production, mobilization of body fat, and risk of ketosis; however, excessive VA supply may have negatively affected the immune response, in part contributing to increased milk somatic cell counts during early lactation.

Invited review: The influence of immune activation on transition cow health and performance-A critical evaluation of traditional dogmas

The progression from gestation into lactation represents the transition period, and it is accompanied by marked physiological, metabolic, and inflammatory adjustments. The entire lactation and a cow's opportunity to have an additional lactation are heavily dependent on how successfully she adapts during the periparturient period. Additionally, a disproportionate amount of health care and culling occurs early following parturition. Thus, lactation maladaptation has been a heavily researched area of dairy science for more than 50 yr. It was traditionally thought that excessive adipose tissue mobilization in large part dictated transition period success. Further, the magnitude of hypocalcemia has also been assumed to partly control whether a cow effectively navigates the first few months of lactation. The canon became that adipose tissue released nonesterified fatty acids (NEFA) and the resulting hepatic-derived ketones coupled with hypocalcemia lead to immune suppression, which is responsible for transition disorders (e.g., mastitis, metritis, retained placenta, poor fertility). In other words, the dogma evolved that these metabolites and hypocalcemia were causal to transition cow problems and that large efforts should be enlisted to prevent increased NEFA, hyperketonemia, and subclinical hypocalcemia. However, despite intensive academic and industry focus, the periparturient period remains a large hurdle to animal welfare, farm profitability, and dairy sustainability. Thus, it stands to reason that there are alternative explanations to periparturient failures. Recently, it has become firmly established that immune activation and the ipso facto inflammatory response are a normal component of transition cow biology. The origin of immune activation likely stems from the mammary gland, tissue trauma during parturition, and the gastrointestinal tract. If inflammation becomes pathological, it reduces feed intake and causes hypocalcemia. Our tenet is that immune system utilization of glucose and its induction of hypophagia are responsible for the extensive increase in NEFA and ketones, and this explains why they (and the severity of hypocalcemia) are correlated with poor health, production, and reproduction outcomes. In this review, we argue that changes in circulating NEFA, ketones, and calcium are simply reflective of either (1) normal homeorhetic adjustments that healthy, high-producing cows use to prioritize milk synthesis or (2) the consequence of immune activation and its sequelae.

Mulberry Leaf Flavonoids Improve Milk Production, Antioxidant, and Metabolic Status of Water Buffaloes

This study was aimed to evaluate the effect of mulberry leaf flavonoids (MLF) on oxidative stress, metabolic hormones, and milk production in Murrah buffaloes. Forty multiparous Murrah buffaloes (4 ± 1 lactations) with similar body weight (average 600 ± 50 Kg) and stage of lactation (90 ± 20 d) were randomly selected for this trial. Four treatment groups (10 buffaloes per group) with different doses of MLF included; control (0 g/d), MLF15 (15 g/d), MLF30 (30 g/d), and MLF45 (45 g/d). Buffaloes were fed with total mix ration consisting of grass (Pennisetum purpureum schum), brewery's grain and concentrate mixture for 5 weeks. Meteorological data including ambient temperature and relative humidity were recorded using the online dust monitoring system to calculate temperature-humidity index (THI). After 1 week of the adaptation, milk yield was recorded daily while physiological parameters (respiratory rate, rectal, and body surface temperature), and milk composition were measured weekly. At the end of the trial, blood samples were collected to analyze serum metabolic hormones including estradiol (E2), growth hormone (GH), prolactin (PRL), Tri-iodothyronine (T3), and Thyroxine (T4). Moreover, serum heat shock proteins (HSP), antioxidants enzymes including malondialdehyde (MDA), total antioxidant capacity (T-AOC), superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) and blood biochemical indices were also analyzed. Results revealed a decrease (P = 0.012) in serum MDA level while increasing (P < 0.01) the HSP and serum GHS-Px contents in supplemented buffaloes. Treatment showed a linear and quadratic decrease (p = 0.001) in the serum T-AOC while reducing CAT contents linearly (p = 0.012) as compared to the control. However, no effect of treatment on serum SOD content was observed. Treatment resulted a linear increase (p = 0.001) in serum GH and PRL hormones while increasing serum E2 levels linearly (P < 0.001) and quadratically (P = 0.025). Treatment increased (p = 0.038) the daily milk yield as compared to the control. However, increase (P < 0.05) in serum T3 and T4 contents, fat corrected milk (4%) and milk protein (%) was observed only in MLF45. Moreover, we observed no change in serum biochemical indices except insulin which linearly increased (p = 0.002) in MLF45. Our findings indicated that MLF at 45 g per day is an appropriate level to enhance milk performance and alleviate heat stress in buffaloes.

Consequences of dietary energy source and energy level on energy balance, lactogenic hormones, and lactation curve characteristics of cows after a short or omitted dry period

Omitting the dry period (DP) generally reduces milk production in the subsequent lactation. The aim of this study was to evaluate the effect of dietary energy source-glucogenic (G) or lipogenic (L)-and energy level-standard (std) or low-on milk production; energy balance (EB); lactogenic hormones insulin, insulin-like growth factor 1 (IGF-1), and growth hormone (GH); and lactation curve characteristics between wk 1 and 44 postpartum in cows after a 0-d or 30-d DP. Cows (n = 110) were assigned randomly to 3 transition treatments: a 30-d DP with a standard energy level required for expected milk yield [30-d DP(std)], a 0-d DP with the same energy level as cows with a 30-d DP [0-d DP(std)], and a 0-d DP with a low energy level [0-d DP(low)]. In wk 1 to 7, cows were fed the same basal ration but the level of concentrate increased to 6.7 kg/d for cows fed the low energy level and to 8.5 kg/d for cows fed the standard energy level in wk 4. From wk 8 postpartum onward, cows received a G ration (mainly consisting of corn silage and grass silage) or an L ration (mainly consisting of grass silage and sugar beet pulp) with the same energy level contrast (low or std) as in early lactation. Cows fed the G ration had greater milk, lactose, and protein yields, lower milk fat percentage, greater dry matter and energy intakes, and greater plasma IGF-1 concentration compared with cows fed the L ration. Dietary energy source did not affect EB or lactation curve characteristics. In cows with a 0-d DP, the reduced energy level decreased energy intake, EB, and weekly body weight gain, but did not affect milk production or lactation curve characteristics. A 30-d DP resulted in a greater total predicted lactation yield, initial milk yield after calving, peak milk yield, energy intake, energy output in milk, days to conception [only when compared with 0-d DP(low)], plasma GH concentration [only when compared with 0-d DP(std)], and decreased weekly body weight gain compared with a 0-d DP. A 30-d DP decreased both the increasing and the declining slope parameters of the lactation curve and the relative rate of decline in milk yield (indicating greater lactation persistency) compared with a 0-d DP, and decreased plasma insulin and IGF-1 concentration, and EB. In conclusion, feeding a G ration after wk 7 in milk improved energy intake and milk production, but did not affect EB compared with an L ration. For cows without a DP, a reduced dietary energy level did not affect milk production and lactation curve characteristics, but did decrease EB and weekly body weight gain. A 30-d DP increased milk yield and lactation persistency, but decreased milk fat and protein content, EB, and plasma insulin and IGF-1, compared with a 0-d DP.

Thyroid hormones, blood plasma metabolites and haematological parameters in relationship to milk yield in dairy cows

To study their relationship to milk yield, the concentrations, in jugular venous blood, of thyroxine iodine (T4I), thyroxine (T4), 3,5,3'-tri-iodothyronine (T3), glucose, non-esterified fatty acids (NEFA), triglycerides, phospholipids, cholesterol, total protein, albumin, urea, haemoglobin and packed cell volume (PCV) have been measured in 36 cows (Simmental, Swiss Brown, Holstein and Simmental × Holstein) of different ages during a full lactation, pregnancy, dry period, parturition and 150 days of the ensuing lactation. Thyroid hormones and triglycerides were negatively, and total protein, globulin, cholesterol and phospholipids were positively, correlated with uncorrected or corrected milk yield during several periods of lactation, whereas glucose, NEFA, albumin, urea, haemoglobin and packed cell volume were not correlated with milk yield. The 10 animals with the highest milk yield (18·9 to 23·5 kg/day) exhibited significantly lower values of T4I, T4, T3 and glucose, significantly higher levels of total protein and globulin and tended to have higher levels of NEFA than the 10 cows with the lowest milk yield (10·9 to 14·3 kg/day) throughout or during certain periods of lactation, whereas concentrations of triglycerides, phospholipids, cholesterol, albumin, haemoglobin and PCV did not differ. Changes in T4I, T4, T3, glucose and total protein during lactation were also influenced by age, presumably associated with an increase in milk production with age. T3 was consistently lowest and cholesterol and phospholipids, during later stages of lactation, were highest in Holsteins, which had the highest milk yields of all breeds. Changes of blood parameters were mainly caused by shifts in energy and protein metabolism in association with level of milk production

Relationships between secretory patterns of growth hormone, prolactin and body reserves and milk yield in dairy ewes under different photoperiod and feeding conditions

Dairy ewes (Prealpes breed) were subjected to differential photoperiod treatments: either long (15·5 light (L): 8·5 dark (D) or short (8·5 L: 15·5 D). Treatments started 15 days before lambing and continued until the 5th week of lactation. After lambing the 22 ewes of each photoperiodic treatment were divided in two groups and fed on either a high or low plane (19 v. 10·5 MJ/day and 370 v. 237 g crude protein per day). Milk yield was not affected by photoperiod but significantly modified by feeding levels (1154 v. 789 ml/day). Protein content of the milk was reduced under long days (—4·9 g/1) compared with short days, while no effect was observed for fat content. Body chemical composition changes, estimated by the deuterium oxide (heavy water) dilution technique between weeks 1 and 4, were affected by both treatments, with a significant interaction between photoperiod and diet for body protein changes: under short days lipid and protein changes were closely related while under long days they were independent.

Blood samples were taken on day 23 of lactation from six ewes per group, every 30 min from 07.30 to 15.30 h for growth hormone (GH) and prolactin (PRL) measurements. The timing of GH and PRL pulses were different, but there was evidence that secretory patterns of each hormone were synchronized across ewes within the four treatments. Long photoperiod increased plasma PRL baseline rather than pulse amplitude, with no effect of diet. Undernutrition mainly increased GH pulse amplitude rather than baseline concentration, with no significant effects of photoperiod.

High spontaneous concentrations of GH were associated with body lipid mobilization (R2 = 0·51) and were also involved in the control of protein changes (R2 = 0·71). These results indicate that GH (and PRL) are involved in nutrient partitioning in the lactating ewe.

Thyroid hormones and daily gain in cattle. Relationships between plasma total thyroxine, free thyroxine, triiodothyronine and average daily gain during the first year of life in Belgian blue and Friesian cattle

Correlation between circulating thyroid hormones and live-weight gain during the first year of life were observed in four trials on cattle of the two main Belgian breeds: Belgian Blue and Friesian. Thyroid hormones assayed and considered as potential predictors of growth rate were: thyroxine (T4), triiodothyronine (T3) and free thyroxine (FT4). Triiodothyronine uptake (T,U), thyroid stimulating hormone (TSH) and free thyroxine index (FTI) were also estimated.

No significant correlation was found between live-weight gain and T4 or T3 in 13 Belgian Blue heifers (trial 1). In 49 bulls submitted to a performance test, T4 and FTI increased from 3 to 12 months whilst T3U decreased (trial 2). Significant correlations between live-weight gain and T4 and between live-weight gain and FTI were obtained from samples taken between 66 and 95 days. Live-weight gain was correlated with T3U when animals were between 96 and 125 days, and 126 to 155 days of age. It is possible that the interaction between total T4 and T3U is important in controlling growth. In 13 young calves, beef merit, expressed as live-weight gain in 1 year, was highly correlated with T4 concentration at 8 to 10 days of age (trial 3). T3 and FT4 did not seem to be directly associated with the growth processes. In trial 4, live-weight gains of 42 young animals housed on a commercial farm were negatively correlated with T4 and with FTI. The conflicting results encountered may perhaps be explained by the lack of information on the use of anabolic preparations in field conditions. T4 concentration and FTI may be considered as potential parameters for the estimation of beef merit but the results are insufficient to draw definite conclusions.

Effects of dietary protein and energy intakes on growth hormone, insulin, glucose tolerance and fatty acid synthesis in young wether sheep

A group of 16 Romney wether sheep fitted with rumen and abomasal cannulae and aged 11 to 12 months was allocated to either high or low crude protein (CP) pelleted diets (220 (HP) and 120 (LP) g CP per kg) to give four daily intakes of dry matter (356, 711, 1067 and 1422 g) within each diet, giving two sheep per treatment. Diets were given hourly for 40 days during which time growth rates, plasma concentrations of insulin (I) and growth hormone (GH) were determined, a glucose tolerance test was performed and relative rates of fatty acid (FA) synthesis were determined.

Live-weight changes ranged from –46 to 215 g/day and wool growth ranged from 86 to 210 mg per 100 cm2daily.

Plasma GH concentrations were significantly higher (P< 0·001) in sheep given LP diets (21·2 (s.e. 4·2) μg/1) than in those given HP diets (10·1 (s.e. 1·8) μg/1) and negatively correlated with both energy (E) (r= –0·62;P< 0·01) and nitrogen (N) (r= –0·63;P< 0·01) intakes which proportionately accounted for 0·47 of the variance in plasma GH concentration (P< 0·01). Plasma I concentrations were higher (P< 0·05) in sheep given HP diets (2·77 (s.e. 0·34) μg/1) than in sheep given LP diets (1·86 (s.e. 0·23) μg/1). Intakes of E and N proportionately accounted for 0·38 of variance in I concentration which was primarily determined by N intake.

After intravenous infusion of glucose (150 mg/min) for 30 min, significant differences in the rate at which plasma glucose and I concentrations declined to pre-infusion levels were evident with different daily intakes of the pelleted diets.

Synthesis of FA in adipose tissue was on average 1·7 times more rapid in sheep given HP than in sheep given LP diets (P< 0·05) and increased approximately three-fold with increasing intake. Partial correlations showed differences in E and N intakes were able to account for 0·73 (P< 0·01) of the variance in FA synthesis rates, with N intake having a greater influence than E intake (partial correlations:r= 0·53;P< 0·05 andr= 0·35;P> 0·05 respectively).

Plasma metabolite and hormone concentrations in Friesian calves of low or high genetic merit: effects of sex and age

This study investigated the potential use of blood metabolite and hormone concentrations as genetic markers for milk fat production and their possible interactions with sex and age. Two groups of calves, one from the Massey University high breeding index (HBI) line of dairy cattle (seven males, eight females) and the other from the low breeding index (LBI) line (four males, 11 females), were studied at 3·5 months and 7 months of age. The average breeding indices (BI) of the calves based on ancestry BI were 138 (s.d. 4·4) and 111 (s.d. 2·3) respectively. Serial blood sampling regimens were conducted in relation to feeding (chaffed lucerne hay at 1·3 times maintenance energy requirement), during an intravenous urea load (120 mg/kg live weight) and during fasting (63 h) and refeeding. Urea spaces and fractional decay constants at each age were estimated by a single compartment distribution model based on plasma urea concentrations following the intravenous urea load.

Plasma concentrations of urea, creatinine and non-esterified fatty acids (NEFA) were not significantly different between the lines in any of the periods examined and at either age. In contrast, plasma concentrations of glucose and insulin were greater in the HBI calves than in the LBI calves although these differences were restricted mainly to the period immediately after feeding. Urea space at 7 months of age was also greater in the HBI animals than in the LBI animals. Plasma concentrations of all hormones and metabolites except insulin were significantly influenced by sex and/or age.

The study does not confirm previous findings that genetic merit for dairying is expressed in terms of plasma levels of urea and NEFA, particularly those during a fast. However, the results of the present study are consistent with previous observations of differences in glucose and insulin metabolism between the tivo Massey University genetic merit lines. These traits may therefore have potential as genetic markers for milk fat production.

Clinical evaluation of growth hormone secretion in cattle using insulin tolerance test

Growth hormone secretion was evaluated in cattle. Clinically healthy bovine growth hormone (bGH) concentrations were 10.7 +/- 1.6 ng/ml in Holstein and 7.8 +/- 3.9 ng/ml in Japanese black cattle. The bGH concentration alternated at three-hour intervals, and tended to be higher at midnight and lower in the morning and before feeding. Insulin tolerance test (ITT) at an insulin dosage of 0.25 U/kg showed a significant increase of bGH concentration to 331 +/- 153% at 60 to 90 min after injection. In ITT applied to five under-growth calves of Japanese black cattle, the basal bGH concentrations were lower and peak values after insulin injection were shown to be significantly low. The ITT is useful for the clinical examination of bGH secretion in cattle.

5'-deiodinase activity and circulating thyronines in lactating cows

To investigate the correlation between lactation and thyroid hormone metabolism, the authors studied concentrations of total and free thyroxine (T4 and fT4), triiodothyronine (T3 and fT3), and reverse triiodothyronine (rT3) in plasma and milk, as well as liver and mammary gland 5'-deiodinase (5'D) activity in dry, early, middle, and late lactating dairy cows. Cows in early lactation show lower plasma levels of T4 and rT3 than dry, middle, and late lactating animals, whereas T3 shows the lowest plasma levels in the dry period; free T4 and T3 show a similar pattern. In early lactation there is a clear decrease in liver 5'D associated with a notable increase in mammary 5'D. Concentrations of T4 and T3 in milk drop significantly in the first few days after delivery, whereas rT3 increases up to the fourth month. The findings suggest a relationship between the hypothyroid status of lactating cows and the rearrangement of organ-specific 5'-deiodinase activity related to the maintenance of the udder's function.

Breed differences in growth hormone and insulin secretion between lactating Japanese Black cows (beef type) and Holstein cows (dairy type)

This study was performed to clarify the levels of growth hormone (GH) and insulin (INS) secretions and the glucose response to INS during lactation in a representative beef breed in Japan, Japanese Black cows, and to compare them with their counterparts in a dairy breed, Holstein cows. Six Japanese Black and seven Holstein primiparous cows received a single intravenous injection of GH-releasing factor (GRF; 0.25 microg/kg), glucose (112.5 mg/kg), or INS (0.2 U/kg) from late pregnancy (2 weeks antepartum) to mid-lactation (6 months postpartum). Japanese Black cows had one-tenth of the total milk yield of Holstein cows during lactation, and significantly lower GRF-induced GH and higher glucose-induced INS secretions than Holstein cows at all stages. In Japanese Black cows, even with lactation, these secretions remained essentially unchanged, whilst Holstein cows showed higher GH and lower INS secretions after the onset of lactation as compared with cows in late pregnancy. Both breeds had similar glucose response to INS at the respective stages. These results suggest that, during lactation, Japanese Black cows may minimize the catabolic effects of GH and sustain the anabolic effects of INS, in contrast with Holstein cows, but have similar ability to inhibit INS-mediated glucose utilization in peripheral tissues to Holstein cows.

Propionibacteria fed to dairy cows: effects on energy balance, plasma metabolites and hormones, and reproduction

To determine the effect of feeding Propionibacteria on energy balance, milk yield, and composition, metabolites and hormones of early-lactating dairy cows, multiparous Holstein cows were individually fed a total mixed ration from -2 to 12 wk postpartum with no addition (control, n = 10) or with an additional 17 g of Propionibacteria culture daily (Treated, n = 9). Daily feed intake and milk production were recorded. Plasma cholesterol, nonesterified fatty acids (NEFA), leptin, insulin, glucose, insulin-like growth factor-I (IGF-I), IGF-binding proteins (IGFBP), and progesterone concentrations were measured up to twice weekly. Cows fed supplemental Propionibacteria had improved energy balance at wk 1 of lactation and had lower DMI per kg of body weight than control cows on wk 3 to 7, 10, and 12. Cows fed Propionibacteria had a greater percentage of milk protein and solids-not-fat and plasma NEFA concentrations than did control cows only at wk 1 of lactation. Treatment did not affect milk production or percentage of milk fat and lactose. Leptin levels were greater in treated than control cows throughout the study. Plasma glucose, insulin, cholesterol, IGFBP-3, and IGF-I concentrations were not affected by feeding Propionibacteria, but those variables increased with week postpartum. Plasma IGFBP-2 and IGFBP-5 levels decreased with week postpartum. Measures of reproductive and ovarian function did not differ between Propionibacteria-treated and control cows. Feeding Propionibacteria culture to transition and early lactating dairy cows may hold potential for improved milk protein production and metabolic efficiency during early lactation, without affecting reproductive function.

Factors affecting body tissue mobilization in early lactation dairy cows. 2. Effect of dietary fat on mobilization of body fat and protein

Twenty-two multiparous Holstein cows were fed either a control diet or a control diet plus 3% added fat (dry matter basis) to determine the effect of added dietary fat on body tissue mobilization and milk production. Body composition measurements were taken using the D2O dilution technique at -2, 5, and 12 wk postpartum. Cows fed added fat produced 2.7 kg/d more milk than did those fed the control diet alone, but milk production, milk composition, and dry matter intake were not affected by diet. The maximum amount of body tissue loss occurred between -2 and 5 wk postpartum when cows fed both diets mobilized 46 kg of body fat and 12 kg of body protein. Between 5 and 12 wk postpartum, only small changes in both body protein and body fat were observed. Even though cows fed added fat showed a tendency toward reduced body fat mobilization (66 kg for cows fed the control diet vs. 37 kg for cows fed the control diet plus added fat) and increased body protein mobilization (4.8 kg for cows fed the control diet vs. 19.5 kg for cows fed the control diet plus added fat), the differences were not significant. Apparent differences in fat mobilization between diets might have been due to initial body fat stores (159 kg for cows fed the control diet vs. 126 kg for cows fed the control diet plus added fat). Across diets, one unit of change in body condition score corresponded to about 55 kg of empty body fat. Supplemental dietary fat did not reduce body tissue mobilization in early lactation.

Factors affecting body tissue mobilization in early lactation dairy cows. 1. Effect of dietary protein on mobilization of body fat and protein

Twenty Holstein cows were fed diets that were formulated with 16 or 19% crude protein (CP) that contained, respectively, 6 and 9% ruminally undegradable protein (RUP) (dry matter basis) to study the effect of increased RUP on tissue mobilization and production parameters. Cows were enrolled in the study from -14 to 114 d postpartum. Body composition measurements using the D2O dilution technique were made at -2, 5, and 12 wk postpartum. Maximum loss of body tissue occurred between wk 2 prepartum and wk 5 postpartum during which time cows fed both treatments mobilized a mean of 54 kg of body fat and 21 kg of body protein. Cows continued to mobilize 18 kg of body fat through wk 12 postpartum, but the amount of body protein was unchanged. One unit of change in body condition score corresponded to about 40 kg of empty body fat. Partitioning of empty body energy between empty body fat and protein indicated that, for each unit of change in body energy, 93% was lost or gained as body fat, and body protein accounted for only 7%. Increasing RUP in the diet had no effect on the postpartum amounts of empty body protein, empty body fat, or empty body energy. Milk production was 39.8 kg for cows fed the 16% CP diet and 42.4 kg for cows fed the 19% CP diet. There was an interaction of treatment by week postpartum. Both dry matter intake and milk production were lower during the first 6 wk postpartum but were greater thereafter for cows fed the 19% CP diet than for cows fed the 16% CP diet. Milk CP percentage was higher (3.08% vs. 2.89%), and milk CP yield tended to be greater (1.29 vs. 1.15 kg/d), for cows fed the 19% CP diet.

Thyroxine, triiodothyronine and reverse-triiodothyronine concentrations in blood plasma in relation to lactational stage, milk yield, energy and dietary protein intake in Estonian dairy cows

Average levels of thyroxine (T4), triiodothyronine (T3) and reverse-triiodothyronine (rT3) in blood plasma of 159 Estonian Red and Estonian Black and White cows were 55.2 nmol/L, 1.78 nmol/L and 0.25 nmol/L respectively. Animals were grouped according to stage of lactation. The T4 level was significantly lower during the early stage of lactation (45.1 nmol/L), compared with later stages, but increased as the stage of lactation progressed (late stage of lactation - 56.7, dry cows 64.3 nmol/L). The T3 level was significantly higher at the late stage of lactation (1.93 nmol/L) compared with the early stage of lactation (1.71 nmol/L) and level in dry cows (1.71 nmol/L). rT3 showed a trend similar to that found for T4 (lowest plasma concentration in early lactation, 0.19; highest in the dry period, 0.33; late lactation, 0.24 nmol/L). Levels of all thyroid hormones were negatively related to the daily milk yield (T4-r = -.51, rT3-r = -.47, calculated thyroid index as rT3 x T3 x T4-r = -.52, for all p < .0001; T3-r = -.32, p < .01). Plasma thyroid hormone concentrations were affected by energy and dietary protein intake. Differences were found in thyroid hormone levels between the 2 breeds and between summer and winter holding periods for Estonian Red cows, which could all be explained by differences in the feeding level and daily milk yield. It is suggested that maintaining low levels of thyroid hormones in early lactation may be one of the dairy cow's mechanisms of reducing metabolic demand.

Effect of selection for milk yield and dietary energy on yield traits, bovine somatotropin, and plasma urea nitrogen in dairy cows

Effects of genetic merit on energy intake, milk yield, fat and protein percentages, BW, BW change, plasma concentration of bST, and plasma concentration of urea N were determined for 139 heifers. Heifers, daughters of bulls of high genetic merit (average +408 kg of PTA for milk) or of average genetic merit (average -153 kg of PTA for milk), were allotted to either a high or low energy diet. Heifers of high genetic merit yielded 8.1% more milk and had 7.7% higher bST concentration than did heifers of average genetic merit, which were 3% heavier than heifers of high genetic merit. There was no significant effect of genetic merit group on energy intake, plasma concentration of urea N, or percentages of fat and protein. Heifers fed the high energy diet consumed 35.1% more energy, yielded 11.8% more milk with a lower fat percentage, and weighed 3% more than did heifers fed the low energy diet. The high energy diet depressed bST concentration by 13.3% and plasma concentration of urea N by 14.2% compared with concentrations for heifers fed the low energy diet. Correlations among bST, BW, and energy intake were negative and significant. Correlations of bST concentration with milk yield, fat percentage, and protein percentage were not significant. Body weight, BW change, milk yield, and energy intake were negatively correlated with plasma concentration of urea N.

Bovine ketosis and somatotrophin: risk factors for ketosis and effects of ketosis on health and production

Aspects of the metabolism and health of 63 cows which had been treated with different amounts of bovine somatotrophin (BST) daily in the preceding lactation and 25 control cows were studied. The aims of the study were first, to identify cows with ketotic conditions, either by measurements of blood metabolite concentrations or by clinical observations, secondly, to evaluate the risk of such conditions in cows treated with BST in the preceding lactation, and thirdly, to examine the metabolic and production consequences of the ketotic conditions in an environment in which the cows' health, body condition and nutrition were closely observed. The cows were categorised objectively by the use of cluster analysis into non-ketotic cows and ketonaemic cows, on the basis of their plasma metabolite concentrations. Twelve of the control cows and none of the cows previously treated with BST were classified as ketonaemic. Similarly, nine of the control cows but only two of the cows previously treated with BST had clinical ketosis. Some, but not all, of the decrease in the risk of clinical ketosis was attributable to the lower body condition score of the cows previously treated with BST. The clinically ketotic cows had a greater risk of other illness in the first 10 days post partum than their herdmates, but the ketonaemic cows had a significantly lower risk of other disease in the first 10 days post partum. The ketonaemic control cows were significantly heavier than the non-ketotic control cows, but they maintained a higher dry matter intake than the latter cows, probably a key factor in reducing the risk of clinical ketosis.

Insulin receptor binding and kinase activity in liver and skeletal muscles of lactating goats

Lactation in goats has been shown to modify in vivo insulin action. [Debras, E., J. Grizard, E. Aina, S. Tesseraud, C. Champredon, and M. Arnal. Am. J. Physiol. 256 (Endocrinol. Metab. 19): E295-E302, 1989]. To further elucidate the mechanism of insulin action, we studied insulin binding and insulin receptor tyrosine kinase activity in solubilized and partially purified receptor preparations from liver and skeletal muscles (longissimus dorsi, tensor fascia lata, diaphragm, and masseter) from lactating and nonlactating goats. Lactation did not alter insulin receptors in the various skeletal muscles and had a minor influence on liver receptors (where only a 20% increase in receptor number was visible, P less than 0.05). Insulin-stimulated autophosphorylation and the kinase activity against polyglutamyltyrosine (4:1) were not significantly modified in skeletal muscle receptor preparations from lactating goats when compared with nonlactating animals. They tended to decrease in liver preparations, but not significantly. Thus the changes in insulin action in vivo during lactation in goats were not related to modifications in insulin kinase activity but were probably localized at a postreceptor level.

Regulation of fatty acid metabolism and gluconeogenesis by growth hormone and insulin in sheep hepatocyte cultures. Effects of lactation and pregnancy

Primary monolayer hepatocyte cultures derived from non-mated, pregnant and lactating sheep were used to investigate the interactions between the effects of growth hormone and insulin on (i) the partitioning of fatty acid metabolism between oxidation and esterification, and (ii) the rate of gluconeogenesis. In hepatocytes from lactating sheep the rates of gluconeogenesis, ketogenesis and very-low-density lipoprotein secretion were approx. 2-fold higher than in cells from non-mated or pregnant animals. There was no apparent difference in the rates of fatty acid uptake between the three groups of sheep cells. Growth hormone stimulated gluconeogenesis only in hepatocytes from non-mated sheep. It has no effect on the flux of fatty acid towards ketone body formation. Growth hormone inhibited intracellular accumulation of acylglycerol from exogenous fatty acid. Insulin alone had no such effect, but it blunted the effect of growth hormone when the two hormones were present together. The data suggest that major differences may exist between ruminants and non-ruminants in the response of liver metabolism both to lactation per se and to the effects of growth hormone and insulin.

Response to recombinant bovine somatotropin in dairy cows with different genetic merit for milk production

Thirty-nine multiparous cows obtained from two genetic lines were utilized to determine the effect of genetic merit on lactation response to long-term administration of recombinant bST. Cow index ranged from -70 to 456 (mean = 183) and -494 to -88 (mean = -288) kg milk for high and low genetic groups, respectively. Cows were blocked by calving date and randomly assigned to treatment within genetic group. Treatments were 0, 10.3, 20.6, and 30.9 mg somatotropin injected daily from wk 14 through 44 postpartum. Cows were fed one of two total mixed rations. Diet 1 (NE1 = 1.65 Mcal/kg, CP = 18%, and ADF = 22%) was fed from start of lactation to at least 4 wk after initiation of treatment. Cows were switched to diet 2 (NE1 = 1.56 Mcal/kg, CP = 16%, and ADF = 27%) when milk output fell below 25 kg/d. Forty-four week lactation yields were 9800 and 9447 kg milk; 364 and 354 kg fat; and 322 and 309 kg protein for high and low genetic groups, respectively. Milk, milk fat, or protein yield due to somatotropin did not differ between genetic groups. Increasing dosage of bST increased milk, 4% FCM, fat, and protein yields in a linear fashion. Percentages of fat and protein of milk were similar for all treatment groups. Body weight changes were not significantly different among treatments, but condition score changes decreased linearly with increasing dose of bST. Long-term treatment with recombinant bST had no apparent effect on incidence of health problems or reproduction.

Treatment of dairy cows with recombinant bovine somatotropin: genetic and phenotypic aspects

Thirty-four multiparous and primiparous Holstein cows were utilized to examine the association between the response to long-term administration of recombinant bovine somatotropin and the cow's phenotypic and genetic production potential. Cows representing a range of phenotypic and genetic production potentials were assigned to one of four treatment groups: 0, 12.5, 25.0, or 50.0 mg recombinant bovine somatotropin daily. They were injected daily for 266 d beginning on d 24 to 35 postpartum. Pretreatment milk and fat yields were used to predict daily yields over the lactation and allowed treatment groups to serve as their own controls. Actual minus predicted yield estimated the response to treatment for milk, fat, and FCM for each cow. Milk composition (fat, protein, and lactose percentage) was not significantly affected by treatment. Response in yield for milk, fat, and FCM was significant during the treatment period (266 d). Milk yield increased by 18.5, 19.9, and 21.4%; fat yield increased by 13.4, 20.3, and 18.1%; and FCM increased by 16.3%, 19.7%, and 21.1% after receipt of 12.5, 25.0 and 50.0 mg recombinant bovine somatotropin, respectively. Differences in response were not significant. The dramatic effect recombinant bovine somatotropin has on production requires that alternative approaches be adopted in the future for accurate genetic evaluation of sires and dams if somatotropin is discriminantly used in the national herd.

Effect of bovine growth hormone on incorporation of [14C]acetate into lipids by co-cultures of bovine mammary, liver, and adipose tissue explants

Incorporation of [14C]acetate into lipids was measured in 24 hr co-cultures of mammary, liver and adipose tissue from Holstein cows at 53, 210 and 318 d of lactation in the presence or absence of bovine growth hormone. Little (less than 1%) of the labeled lipids appeared in the media relative to that incorporated into the tissue. In mammary tissue, incorporation of [14C]acetate was highest into triglycerides (16,298 cpm/mg mammary tissue), followed by phospholipids (1,887 cpm), free fatty acids (1,252 cpm), diglycerides (708 cpm), free cholesterol (360 cpm) and monoglycerides (93 cpm). Bovine growth hormone did not increase incorporation of [14C]acetate when mammary or adipose tissue were incubated separately. However, in the presence of liver and adipose tissue, bovine growth hormone significantly increased the incorporation of [14C]acetate into triglycerides, diglycerides, free fatty acids and free cholesterol by mammary tissue. These results suggest that bovine growth hormone acts on mammary tissue indirectly through liver and adipose tissue to increase lipid synthesis. This mechanism may play a role in the action of bovine growth hormone in vivo to increase milk and milk fat production.

Teat stimulation-induced release of prolactin and basal concentrations of prolactin and growth hormone in pregnant dairy and beef heifers

The physiological bases for differences in milk production between breeds of cattle selected for beef or milk production are largely unknown. This study was conducted to determine concentrations of prolactin (PRL) and growth hormone (GH) in serum before and after teat stimulation in primiparous Hereford and Holstein heifers. Blood was collected from 6 beef and 9 dairy heifers at 115, 175, 230 and 250 d of gestation. Sampling times were -15, -10, -5, 0, 2, 4, 6, 8, 10, 12, 15, 20, 25 and 30 min relative to test stimulation. Mean areas under PRL response curves for beef and dairy heifers at 115, 175, 230 and 250 d of gestation were 427, 447, 556, 273 and 243, 189, 167, 343 ng/ml/30 min, respectively. Calculations of area (but not basal levels) excluded instances when no PRL response to test stimulation was obtained (22%). Neither stage of gestation nor breed affected PRL response. Basal PRL did not differ between breeds and was 1.8, 2.6, 2.4 and 9.2 ng/ml at 115, 175, 230 and 250 d of gestation. GH did not differ between breeds and was 6.6, 6.2, 5.5 and 7.4 ng/ml at 115, 175, 230 and 250 d. No difference between breeds was apparent with regard to PRL or GH secretion during first gestation.

Insulin sensitivity and responsiveness during lactation and dry period in goats

To investigate the role of insulin in partitioning nutrients between the mammary gland and other tissues during lactation in ruminants, euglycemic-hyperinsulinemic clamps were performed in goats during early lactation (15-26 days postpartum), midlactation (78-91 days postpartum), and dry period (169-194 days postpartum). Insulin was infused at 0.4, 0.7, 1.9, 4.4, and 10 micrograms/min. Basal plasma glucose was constant during all periods despite the fact that basal glucose utilization was approximately 3 times higher during lactation than dry period. Basal plasma insulin was similar during early lactation and dry period but increased during midlactation. Insulin infusion resulted in a dose-dependent stimulation of glucose utilization. The insulin-stimulated glucose utilization above basal was greatly impaired during early lactation when compared with dry period, but this only occurred at very high plasma insulin. Insulin infusion also resulted in a decrease in glucose production; the maximal insulin effect is achieved at the lowest insulin infusion rate. The ability of insulin to decrease glucose production was significantly improved during early lactation when compared with dry period. This phenomenon may provide a mechanism to save gluconeogenic substrates during early lactation. In contrast, midlactation did not result in any significant change in insulin action with both glucose utilization and glucose production.

Effects of feed intake and sodium bicarbonate on milk production and concentrations of hormones and metabolites in plasma of cows

Eight Holstein cows were used to investigate the effects of DM intake and sodium bicarbonate on lactational performance and concentrations of hormones and metabolites in plasma. Cows were fed a diet with or without 1.0% sodium bicarbonate (dry matter basis) in a switchback design. Four cows were fed ad libitum and four cows were fed approximately 80% of their recommended nutrient requirements by restriction of DM intake throughout the three 21-d periods. Supplementing the diet with sodium bicarbonate increased DM intake of cows fed ad libitum. There was a feed intake by sodium bicarbonate interaction for production of 4% FCM. This interaction may be explained by the difference in DM intake of cows fed ad libitum or restricted amounts of feed and supplemented with sodium bicarbonate. Cows fed restricted amounts of feed had lower milk, milk fat, milk protein, milk SNF, and milk energy yields. Restriction of feed intake increased plasma concentrations of somatotropin and nonesterified fatty acids but decreased concentrations of insulin, triidothyronine, thyroxine, glucagon, and prolactin. In contrast, feeding supplemental sodium bicarbonate did not affect concentrations of hormones or metabolites in plasma at either feed intake.

Physiological, productive, and economic benefits of shade, spray, and fan system versus shade for Holstein cows during summer heat

During the 1984 summer, effects of spray and fan in freestall areas and feeding areas on milk yield, plasma growth hormone and prolactin, freestall utilization, and economic significance for Holstein cows were studied. Weekly milk and rectal temperatures at morning and afternoon milkings were higher for cows in shade than in shade with spray and fan cooling. Compared with rectal temperature, milk temperature measured in the clawpiece provided a more reliable indicator of heat stress than in the milk meter. Diurnal pattern of rectal temperature from 4-h measurements showed an average of 12 h in which maximal rectal temperatures of cows in shade were greater than 39 degrees C, but cows in the shade plus spray plus fan group had rectal temperatures below 39 degrees C all day. Plasma growth hormone was higher and prolactin lower for cows in shade plus spray and fan than cows in shade only. Cows cooled with spray and fan under shade produced 2 kg/cow per d more than cows in shade alone. Economic analysis showed a net income of 22 cents/cow per day for spray plus fan under shade. Results suggest that milk temperature is a practical technique to assess heat stress in dairy cattle, and the use of spray plus fan is a profitable means to maximize cow comfort and lessen stress-induced decline in milk production.

Growth hormone response to growth hormone-releasing factor in selected and control Holstein heifer calves

Holstein heifer calves from selected and control lines were treated with growth hormone-releasing factor to determine whether the magnitude of the pituitary growth hormone response is associated with genetic potential for milk yield. Cows in the selected line were bred to highest Predicted Difference milk sires available each year, while control line cows were bred to bulls that were average for 1964 milk production. Eight calves from each line of similar age (206 to 210 days) and weight (205 kg average) were injected intravenously with .1 microgram/kg growth hormone-releasing factor. Dams' first lactation 305-day mature equivalent records were 10,192 +/- 495 kg for selected calves and 6,908 +/- 376 kg for control calves. Before treatment, GH tended to be higher in selected than control calves (mean +/- SE: selected = 7.5 +/- 1.8, control = 4.6 +/- .4 ng/ml). After injection, peak GH were reached within 20 min and were similar in selected and control calves, averaging 66 +/- 16 and 73 +/- 13 ng/ml, respectively. Results indicate that the magnitude of the growth hormone response to growth hormone-releasing factor is not related to future performance of dairy calves.

Glucagon, insulin, growth hormone, and some blood metabolites during energy restriction ketonemia of lactating cows

Six Holstein cows were sampled hourly for 24 h for plasma concentrations of hormones and metabolites. Cows were sampled at about 2 wk prepartum, at 3 wk postpartum, during a ketonemia induced by feed restriction to 54% of ad libitum intake, and after a recovery period. They were fed long alfalfa hay postpartum. The onset of lactation caused concentrations of growth hormone, glucagon, acetoacetate, beta-hydroxybutyrate, and total amino acids of plasma to increase and those of glucose and insulin to decrease. Feed restriction exacerbated changes at 3 wk postpartum except for total amino acids and glucagon, which both decreased to prepartal concentrations. Resumption of ad libitum feeding caused most hormones and metabolites to return to prepartum concentrations. Diurnal variations in response to feeding twice daily were most evident for growth hormone, free fatty acids, and total amino acids. The 3-wk postpartum and ketonemic periods gave the greatest responses to feeding. Molar ratios of insulin to glucagon and insulin to growth hormone tended to decrease at 3 wk postpartum and decreased further in ketonemia, demonstrating hormonal adaptations to decreased energy intake during lactation. Lactation ketosis results from more than severe energy deficit.

A review of ovarian follicular cysts in cows, with comparisons to the condition in women, rats and rabbits

Some morphological and endocrinological aspects of cystic follicles in cows, women, rats, and rabbits were reviewed. The etiology of follicular cysts remains a mystery, in part due to the difficulty of studying them during formation, particularly in women and cows. Investigations of the morphological and endocrinological nature of cystic follicles during their formation should enhance our understanding of the factors that cause them. Rats and rabbits may provide useful and convenient experimental models for the study of factors which contribute toward cyst formation.

Effect of exogenous growth hormone in early and late lactation on lactational performance of dairy cows

Bovine growth hormone (51.5 IU/day) and placebo injections were administered for 10 days to four Holstein cows in early lactation (wk 12) and again in late lactation (wk 35). Milk productions in the last 5 days of each period were compared. In early lactation, growth hormone increased milk yield by 15%, fat yield by 17%, protein yield by 14%, and lactose yield by 21%. In late lactation the respective increases were 31, 42, 18, and 35%. For responses of early and late lactation to growth hormone on a quantitative basis, increases for milk yield (4.3 versus 3.9 kg/day) and milk energy secretion (3.3 versus 3.4 Mcal/day) were similar. Concurrent with these increased milk yields, ad libitum intakes of a complete mixed diet declined during the period of growth hormone treatment by 3% in early lactation and 16% in late lactation. During the 6 h immediately following injections of growth hormone, blood plasma concentrations of growth hormone were elevated about 400% in early lactation and 700% in late lactation. Concentrations in plasma of free fatty acids were also higher during growth hormone treatment in late lactation but not in early lactation. Treatments did not affect plasma concentrations of glucose, insulin, glucagon, prolactin, tri-iodothyronine, thyroxine, or cortisol in either early or late lactation. Daily administration of growth hormone in early or late lactation resulted in similar and substantial increases of milk yield and efficiency of milk production.

Iodine metabolism and thyroid hormone relationships in growing sheep fed on kale (Brassica oleracea) and ryegrass (Lolium perenne)-clover (Trifolium repens) fresh-forage diets

1. Kale (Brassica oleracea) and ryegrass (Lolium perenne)–clover (Trifolium repens) pasture, grown under similar soil conditions, were grazed in the vegetative state by growing lambs of 23·6 kg initial live weight for 24 weeks. The kale and pasture contained respectively 20 and 270 μg iodine/kg dry matter (DM). The kale also contained 8 μmol total glucosinolates/g DM and 11·5 g S-methyl-L-cysteine sulphoxide (SMCO)/kg DM, both of which were nondetectable in the pasture diet.

2. Intramuscular injections of 1 (475 mg) were given during weeks 1 and 12 to half the forty-eight lambs grazing each forage. Wool growth, live-weight gain and cytochrome oxidase (EC 1.9.3.1) activity of biopsied hind-limb muscle were measured at 6-week intervals. Jugular blood samples were removed every 6 weeks for the determination of haematological factors and serum thyroid hormone concentrations. All animals were slaughtered at the end of the experiment and thyroid weight, thyroid I content, and the weight and cytochrome oxidase activity of heart muscle determined.

3. Serum concentrations of thyroxine (T4) increased from 20 to 48 nmol/l during the 24 weeks that control lambs grazed ryegrass-clover pasture. I supplementation increased the concentration and total amount of I in the thyroid gland and increased serum T4 concentration, but did not affect any other values measured in the lambs grazing the pasture herbage. Serum concentrations of triiodothyronine (T3) were stable at 2 nmol/l for both groups.

4. Control lambs grazing kale for 24 weeks showed marked thyroid enlargement and depletion of thyroid I. By week 6, serum T4 and T3 concentrations had declined to 2–5 nmol/l and 1 nmol/l respectively and were stable at these values for the remainder of the experiment. I supplementation eliminated the thyroid depletion of this element, caused serum T4 concentration to rise and stabilize at 90 nmol/l by week 18, and T3 concentration to stabilize at 2 nmol/l by week 6. From week 6 onwards, wool growth was increased 13% by I supplementation, whereas empty body growth was unaffected.

5. Lambs grazing kale developed haemolytic anaemia, due to rumen fermentation of SMCO. I supplementation enabled the lambs to resist the anaemia better by increasing erythrocyte reduced gluthathione (GSH) content. Relative to pasture-fed animals, lambs grazing kale and supplemented with I showed increased heart muscle weight and cytochrome oxidase activity. This represented a compensatory mechanism for the reduced blood oxygen-carrying capacity caused by the anaemia. I-deficient (i.e. control) lambs grazing kale showed reduced cytochrome oxidase activity in both heart and hind-limb muscle.

6. The findings are in accord with T3 having a greater biological potency than T4 for regulating rates of body and wool growth. Increases in heart weight, heart cytochrome oxidase content and erythrocyte GSH content of kale-fed lambs were, however, associated with elevation in serum T4 and not T3 concentration.

7. I requirements of growing sheep and cattle consuming the pasture diets are discussed. Because of its better relationship to production traits, it is considered that requirements should be based on the ability to maintain T3 rather than T4 concentrations. On this basis, requirements could be met by diets containing 180–270 μg I/kg DM.

Effect of environmental heat and dietary fiber on thyroid physiology of lactating cows

Six lactating dairy cows were assigned randomly to a diet either of low fiber (30% corn silage) or of high fiber (70% corn silage). Both groups were subjected to 10 days thermoneutral (17.6 degrees C) and 10 days of environmental heat (31.2 degrees C) to determine effects of dietary fiber content and environmental heat on concentrations of thyroxine and triiodothyronine in plasma, milk, urine, and feces. During heat there were significant reductions in concentrations of thyroxine and triiodothyronine in plasma and in excretion of thyroxine and triiodothyronine in milk. Excretion of thyroxine and triiodothyronine increased in urine during heat stress, whereas concentration and excretion of thyroxine and triiodothyronine in feces decreased. Dietary fiber content had little effect upon thyroxine and triiodothyronine concentrations in plasma or excretion in milk, feces, and urine. Excretion of triiodothyronine in milk was negatively correlated with rectal temperature and positively with content in plasma. The reduction in thyroxine and triiodothyronine of plasma and the decline in excretion of thyroxine and triiodothyronine suggested that environmental heat reduced synthesis of both hormones.

Metabolic limits to milk production, especially roles of growth hormone and insulin

Potential of the bovine mammary gland to synthesize milk probably is determined fully at parturition. Realization of that potential depends on the gland receiving adequate amounts of nutrients in the correct proportions. Provided the cow is fed adequately in relation to her potential yield, supply to the body of those nutrients that are absorbed directly from the gut is not likely to limit milk synthesis. Glucose and long-chain fatty acids, however, are not absorbed from the gut in adequate amounts, and metabolic factors (primarily hormonal regulation) determining the supply of these may be important limits to milk synthesis. Even when amounts and proportions of metabolites entering circulation are adequate for maintenance and maximum milk synthesis, the cow's potential cannot be realized fully unless nutrients are channeled selectively towards the mammary gland. This occurs naturally in the high-yielding cow in early lactation and probably can be maintained artificially into midlactation by use of growth hormone. Consequences of this on milk output, health, and reproductive performance are considered.