Relationship of dietary crude protein to composition of uterine secretions and blood in high-producing postpartum dairy cows

Effects of varying levels of undegradable intake protein on endocrine and metabolic function of young post-partum beef cows

Twelve Hereford x Angus heifers, 2.5 year, 492 +/- 17 kg (BCS = 5 +/- 0.5), were randomly assigned to one of three supplements, stratified by calving date and calf sex. Treatments consisted of a daily equivalent of: (1) low undegradable intake protein (UIP) (L: 908 g per cow per day; 36% CP, 108 g UIP), (2) middle UIP (M: 908 g per cow per day; 36% CP, 165 g UIP), and (3) high UIP (H: 908 g M + 243 g feather meal per cow per day; 46% CP, 335 g UIP). Cows were fed sudan grass hay (7.3% CP, as fed) daily at 2% BW (as fed). Supplement was individually fed twice weekly from Week 2 to Week 11 post-partum. Cow body weight (BW), backfat (BF) and rumpfat (RF) thicknesses decreased in all cows, (P < 0.05) yet did not differ among treatments (P > 0.10). There were no differences among treatments in calf BW (P > 0.10). Serum insulin, blood urea nitrogen (BUN), milk components and yield did not differ among treatments (P > 0.10). Area under the curve (AUC) for serum LH was greater (P = 0.07) in H versus L and M (809 versus 599 and 607 +/- 69 U, respectively). No differences were observed in FSH AUC or mean serum concentrations (P > 0.10). Uterine pH did not differ among treatments or between supplement versus non-supplement days (P > 0.10). Serum progesterone remained below I ng/ml for all cows indicating absence of return to estrus. Under the conditions of this study, BW, BF, RF, serum insulin, BUN, milk components, and yield, uterine pH and serum FSH were not affected by level of UIP. However, supplement containing high levels of UIP enhanced GnRH-induced LH release.

Milk urea testing as a tool to monitor reproductive performance in Ontario dairy herds

Dairy herd improvement test-day data, including milk urea concentrations measured using infrared test method, were collected from 60 commercial Ontario Holstein dairy herds for a 13-mo period between December 1, 1995, and December 31, 1996. The objective of the study was to describe, at the cow and the group level, the relationship between DHI milk urea concentrations and reproductive performance in commercial dairy herds. When interpreted at the cow level, there was no association between milk urea and the risk for pregnancy from an insemination occurring within the 45-d period preceding test day. However, a negative curvilinear relationship existed between milk urea and the risk for pregnancy from a first, second, or third insemination event occurring within the 45-d period following test day, with the odds for pregnancy being highest when the milk urea on the test day preceding the insemination was either below 4.5 mmol/L or greater than 6.49 mmol/L, compared with a concentration between 4.5 and 6.49 mmol/L. When interpreted at the group level, there was no association between group mean milk urea for cows between 50 and 180 DIM, and the group conception rate for cows receiving a first, second, or third insemination event in the 45-d period either preceding or following test day. Thus, while DHI milk urea measurements may be useful as a management tool to improve the efficiency of production or reduce nitrogen excretion, through helping to optimize the efficiency of protein utilization, they may have limited utility as a monitoring or diagnostic tool for reproductive performance. The results of this study suggest that good fertility may be achieved across a broad range of milk urea concentrations.

Effect of nutrition on endocrine parameters, ovarian physiology, and oocyte and embryo development

Reproductive efficiency in high yielding dairy cows has decreased over the past 50 years, despite significant gains in genetic selection for increased milk output. One possible reason for this decline has been a change in the nutritional intake to meet the increased energy and protein demands for higher milk production. Excess energy intake in sheep will lead to significant reductions in progesterone concentrations; the effects in cattle are not so clear. Nutrition, unless radically changed, will have little effect on gonadotropin concentrations in ruminants, and this is in contrast to the situation for pigs and for primates, where very short-term nutritional changes manifest themselves in altered gonadotropin secretion. Cattle with reduced energy intake have smaller dominant follicles and more three-wave cycles, compared with animals on higher feed intakes. One of the main areas where nutrition influences reproductive efficiency is at the level of embryo production. Several studies indicate that excess energy intake reduces the response to superovulation and also decrease the yield of embryos and alters expression of some gene constructs within the developing embryo. The mechanism of this effect is not clear but indications are that the quality of the oocytes may be compromised. Indeed recent data indicate that nutritional changes around the time of mating may have detrimental effects on the establishment of pregnancy in heifers. Thus, nutritional balancing is critical for high-yielding dairy cows, in particular. The challenge remains to modify nutritional and management strategies in such cows to maintain the levels of production made possible by genetic selection and still maintain an acceptable level of fertility.

Supplemental Dietary Protein for Grazing Dairy Cows: Reproduction, Condition Loss, Plasma Metabolites, and Insulin

An experiment was conducted over a 2-yr period to investigate the influence of grain crude protein (CP) and rumen undegradable protein (RUP) concentration on reproduction and energy status of dairy cows grazing annual ryegrass (Lolium multiflorum) and oats (Avena sativa). Holstein cows (n = 122) were blocked by calving group [partum (0 d postpartum) vs. postpartum (41 +/- 19 d postpartum at study initiation)] and assigned to grain supplements containing high CP [22.8% of dry matter (DM)], moderate CP (16.6%), or moderate CP (16.2%)] supplemented with RUP from blood meal and corn gluten meal. Postpartum condition loss was greater and first-service pregnancy rate was lower for partum-group cows receiving high CP grain supplements compared with control cows receiving moderate CP supplements. The RUP supplements reduced grain consumption, increased days to first estrus, and reduced first-service pregnancy rate of partum-group cows. The reproduction of postpartum group cows was unaffected by protein supplements. Plasma urea nitrogen was higher for cows fed high CP diets, but plasma ammonia nitrogen, glycated hemoglobin, nonesterified fatty acids, beta-hydoxybutyrate, glucose, and insulin concentrations were similar to cows fed moderate CP. Excess postpartum condition loss, coupled with inconsistent protein supplement effects on days to first service and first-service pregnancy rate, suggest that energy deprivation may have contributed to the low fertility experienced by grazing cows in this study.

Effect of diet quantity and urea supplementation on oocyte and embryo quality in sheep

The objective was to investigate the effects of dietary energy and urea supplementation on oocyte and embryo quality in sheep using in vivo and in vitro experimental models. Sixty-three ewes were fed grass meal at 0.5 or 2.0 times maintenance energy requirements (MER). The diet was supplemented with feed grade urea (U) for half of the ewes on each energy treatment. Ewes were stimulated with 1000 IU eCG and either slaughtered on the day of pessary withdrawal, for in vitro embryo production, or mated and slaughtered on Day 5 for embryo recovery. Urea decreased cleavage rate (48.3 vs 39.7%) and consequently blastocyst rate (41.6 vs 36.8%) but the differences were not significant. Oocytes from animals on 2.0 MER had a lower cleavage rate (54.9 vs 36.0%) and blastocyst yield (49.3 vs 31.4%) than those on 0.5 MER. However, there was an interaction between urea and energy for cleavage (P = 0.04) and blastocyst yield (P = 0.03) indicating a variable response to urea in the presence of high energy. This was manifested by a decrease in cleavage rate in the presence of urea and high energy (22%, 8 of 36), and a reduction in blastocyst development (19%, 7 of 36). When blastocyst development rate was expressed as a proportion of cleaved oocytes there was no difference between groups; in addition, there was no difference between groups in terms of blastocyst hatching rate (overall mean 66.1%) or blastocyst cell number on Day 8 (overall mean +/- SEM, 138.4 +/- 9.0, n=61). The effect of urea on cleavage rate in vivo was more severe. Urea supplementation reduced (P<0.001) the cleavage rate (93 vs 62%). Despite this, the yield of blastocysts was unaffected. Oocytes from ewes on 0.5 MER exhibited a lower (P<0.05) cleavage rate than those on 2.0 MER (66 vs 87%). This effect was also apparent at the blastocyst stage (40.0 vs 50.9%), although the difference was no longer significant. There were no differences in hatching rate (overall mean 70.7%) or blastocyst cell numbers (overall mean +/- SEM, 166.3 +/- 15.6, n=40). Collectively, these results suggest that both high dietary energy and urea content influence subsequent embryo development in vitro, and the deleterious effects of urea are likely influenced by concomitant energy intake.

Effects of ammonia during different stages of culture on development of in vitro produced bovine embryos

The effects of various concentrations of ammonia in the media during in vitro fertilization (IVF), culture (IVC), and throughout maturation (IVM), IVF, and IVC were evaluated using a randomized complete block design. Ammonia was added to the media at various concentrations during IVF (experiment 1), during IVC (experiment 2), and throughout IVM, IVF, and IVC (experiment 3). In the first experiment, there was a significant (P<0.05) increase in embryos developed to blastocyst, and to expanding and hatching blastocyst, in IVF media containing moderate concentrations of ammonia compared with that in the IVF control media. In the second experiment, ammonia in the IVC media increased (P<0.05) the proportion of degenerate ova and decreased (P<0.05) the proportion of ova that developed to blastocysts. In experiment 3, cleavage rates tended (P=0.06) to be greater for control groups than for treatment groups. The proportion of ova developing to morula was greater (P<0.05) in media containing moderate concentrations of ammonia than that in the control groups. These results indicate that the effect of ammonia on development of preimplantation bovine embryos depends on the concentration of ammonia and the stage of development when exposure to ammonia occurs.

Plasma concentration of urea, ammonia, glutamine around calving, and the relation of hepatic triglyceride, to plasma ammonia removal and blood acid-base balance

Two experiments were conducted to test the following two hypotheses: 1) fatty liver could hamper hepatic conversion of ammonia to urea and increase circulating ammonia or Gln% [Gln% = Gln x 100/(Gln + Glu)] in cows around parturition; 2) decreased ureagenesis might cause alkalosis and in turn reduce blood Ca. In the first experiment, 14 Holstein cows were monitored from 27 d prepartum to 35 d postpartum. There was a rise in circulating ammonia and Gln% at calving, suggesting an increase in ammonia passing to and through the liver. Stepwise regression analysis revealed the following relationship for plasma samples at 22 h and liver triglyceride at 2 d postpartum: ammonia (microM) = 32.1+/-0.89 triglyceride (% DM), Gln% = 71.2 + 0.23 triglyceride (% DM) + 1.31 urea (mM). The positive correlation between liver triglyceride and plasma ammonia and Gln% suggests that hepatic triglyceride accumulation might inhibit ureagenesis, thereby increasing ammonia concentration at the perivenous hepatocytes where Gln synthesis occurs and increasing ammonia concentration in blood leaving the liver. In the second experiment, 28 rats were used to determine whether hepatic triglyceride accumulation, induced by choline deficiency, affects urinary ammonia N and blood pH homeostasis. There was a trend for a positive correlation between urinary ammonia N and liver triglyceride. No correlation between liver triglyceride and blood pH, bicarbonate, pCO2 or plasma Ca was found. In conclusion, hepatic triglyceride accumulation may inhibit ureagenesis and result in increased circulating ammonia, Gln% and urinary ammonia N in vivo. Hepatic triglyceride accumulation did not affect blood pH homeostasis.

The effects of the early uterine environment on the subsequent development of embryo and fetus

Synchrony between the embryo and the uterine endometrium is essential for the establishment of pregnancy and birth in people and livestock. When asynchronous conditions occur a variety of complication result that include failure of the embryo to implant, early embryonic mortality, retarded development and growth, and accelerated development and growth. These complications all appear to be induced within the first week of embryo development and not withstanding the immediate endpoint of large or small size at birth, may alter the course of development throughout the life of the animal. Progesterone appears to play a causative role in establishing the abnormal growth of the fetus by decelerating or accelerating embryonic development. This may act through increasing the transport of blood born growth factors into the uterine lumen or by stimulating the release of growth factors from the endometrium directly. It can not be ruled out that progesterone mediated abundance of, or absence of, appropriate nutrition may bring about the same lifelong outcome. In vitro culture situations that include serum and/or co-culture can also bring about these abnormalities of growth. It is hypothesized that exposure to growth factors "out of phase" may result in an irreversible induction of abnormal development. The described abnormalities that occur in sheep and cattle have not yet been described for children resulting from IVF.

Metabolic and yield responses of multiparous Holstein cows to prepartum rumen-undegradable protein

The effects of concentration of rumen-undegradable protein (RUP) in prepartum diets on performance and metabolic measures were determined. Pregnant, nonlactating cows were assigned to one of three dietary treatments that differed in concentrations of crude protein and RUP. The crude protein and RUP percentages (dry matter basis) in the diets were 11.7 and 3.1%, 15.6 and 6.8%, and 20.6 and 10.6%, which were identified as low, medium, and high protein diets, respectively. Dietary treatments were initiated 6 wk prior to expected calving date and were fed until parturition. All cows were fed similar postpartum diets through wk 10 of lactation. Prepartum intakes of dry matter and rumen-degradable protein were similar among treatments. Yields of milk, protein, and fat were not affected by prepartum RUP. Concentrations of plasma urea N in cows fed the medium and high protein diets were elevated during the prepartum period even though intakes of rumen-degradable protein were similar for cows on all treatments. Cows fed the medium and high protein diets had greater prepartum concentrations of Val, Ile, and Leu but not other essential amino acids in plasma. Dietary treatments did not affect prepartum or postpartum contents of triglycerides in liver or plasma nonesterified fatty acid concentrations. Thus, supplemental RUP fed to cows for 6 wk prepartum affected protein metabolism but not measures of energy metabolism.

Bulk milk urea concentrations and their relationship with cow fertility in grazing dairy herds in southern Chile

Milk urea determination is being used as a broad indicator of protein/energy imbalance in dairy herds. The main purpose of this study was to compare blood and bulk milk urea values in grazing herds, to evaluate their seasonal variation under South Chilean conditions, and to examine their potential relationships with herd fertility. The association between herd blood urea concentration (mean of seven lactating cows) and bulk milk urea concentration (tank containing milk from the previous 24 h) was determined in 21 diary herds. Reference values, seasonal and herd variance, and the frequency of herds with values outside a range of 2.5 to 7.3 mmol/l were determined in bulk milk samples obtained monthly for a period of one year from 82 suppliers at two creameries located in southern Chile. Finally, bulk milk urea was measured every two weeks in samples from 24 herds, and the first service conception rate (FSCR) from 2153 dairy cows was determined. Mean bulk urea concentration was highly correlated with mean herd blood urea concentration (r = 0.95; p < 0.01). Mean urea concentration in the bulk milk samples obtained during one year from 82 herds was 4.9 +/- 1.2 mmol/l, with a range of 1.5 to 11.6 mmol/l. The highest values were found during spring and the lowest values during the summer. There was a high seasonal variation (CV = 13-47%) and between-herd variation (CV = 20-31%). Out of a total of 984 samples, 5.4% had urea values > 7.3 mmol/l and 3.8% had values < 2.5 mmol/l. Of the 82 herds, 27% had values outside the reference interval (2.5-7.3 mmol/l) on two or more occasions. FSCR was lower in herds when the bulk milk urea was > 7.3 mmol/l (50.7%) than in cows, where the urea concentration was < 5.0 mmol/l (73.8%) at the time of insemination. The study concluded that bulk milk urea concentrations provided information similar to herd blood urea concentrations in local grazing dairy herds. There was a high frequency of herds with abnormal values, with large variations between herds and between seasons. Increased milk urea concentrations during spring were associated with lower conception rates.

Using milk urea nitrogen to predict nitrogen excretion and utilization efficiency in lactating dairy cows

Because animal agriculture has been identified as a major source of nonpoint N pollution, ways to reduce the excretion of N by production animals must be examined. The objective of this research was to develop and evaluate a mathematical model that integrates milk urea N to predict excretion, intake, and utilization efficiency of N in lactating dairy cows. Three separate digestibility and N balance studies (10 diets, 40 cows, and 70 observations) were used to develop the model, and 19 independent studies (93 diets) were used for evaluation. The driving variables for the model were milk urea N (milligrams per deciliter), milk production (kilograms per day), milk protein (percentage), and dietary crude protein (percentage). For the developmental data set, the model accurately predicted N excretion and efficiency with no significant mean or linear bias for most predictions. Residual analysis revealed that a majority of the unexplained model error was associated with variation among cows. For the independent data set, model prediction error was approximately 15% of mean predictions. A mean of at least 10 cows was determined to be appropriate for model predictions. Target milk urea N concentrations were determined from expected urinary N excretion for cows that were fed according to National Research Council recommendations. Target values calculated in this manner were 10 to 16 mg/dl, depending on milk production. Milk urea N is a simple and noninvasive measurement that can be used to monitor N excretion from lactating dairy cows.

Review: effect of protein nutrition on ovarian and uterine physiology in dairy cattle

Milk production and dry matter intake of dairy cows are stimulated in response to increased intake of dietary protein, but, unfortunately, decreased fertility is often associated with this nutritional strategy. Ruminally degradable protein or ruminally undegradable protein in excess of requirement can contribute to reduced fertility in lactating cows. Dietary protein nutrition or utilization and the associated effects on ovarian or uterine physiology have been monitored with urea nitrogen in plasma or milk; concentrations above 19 mg/dl have been associated with altered uterine pH and reduced fertility in dairy cows. The uterine pH changed dynamically and inversely with plasma urea nitrogen, signaling possible changes in the uterine milieu. Mechanisms for reduced fertility include exacerbation of negative energy balance and reduced plasma progesterone concentrations when cows were fed rations that were high in ruminally degradable intake protein. Alternatively, changes in uterine secretions that are associated with high protein intake and elevated plasma urea nitrogen might be detrimental to embryos. Bovine endometrial cells in culture respond directly to increasing urea concentrations with alteration in pH gradient but respond most notably with increased secretion of prostaglandin F2 alpha (PGF2 alpha). Increased uterine luminal PGF2 alpha interferes with embryo development and survival in cows, thus providing a plausible link between elevated plasma urea nitrogen concentrations and decreased fertility. Poor fertility in high producing dairy cows reflects the combined effects of a uterine environment that is dependent on progesterone and rendered suboptimum by the antecedent effects of negative energy balance or postpartum health problems and that is further compromised by the effects of urea resulting from intake of high dietary protein.

Indications and implications for testing of milk urea in dairy cattle: A quantitative review. Part 2. Effect of dietary protein on reproductive performance

DIETARY PROTEIN AND DAIRY COW FERTILITY: Feeding more dietary protein has been negatively associated with dairy cow fertility in some but not all studies. We used meta-analysis to examine the relationship between dietary crude protein and conception rate. While a higher intake of dietary crude protein significantly lowered conception rate, the potential for feeding less degradable dietary protein to modify this relationship was not demonstrated. MILK UREA CONCENTRATIONS AND DAIRY COW FERTILITY: The use of milk urea as an indicator of dietary energy and protein intake and as an indicator of reproductive performance has been questioned. We found that changes in urea concentration in body fluids explained only 25% (p = 0.08) of the variance in conception rate after conducting a meta-analysis of available studies. INTERPRETATION OF MILK UREA CONCENTRATIONS: High intakes of dietary protein may induce adaptations in urea metabolism, and the negative relationship identified between high intakes of dietary protein and fertility for Northern Hemisphere dairy herds may not necessarily apply in Australasian dairy herds. Because of the potential for cows to adapt to high protein diets, the use of a single milk urea determination on a herd will have limited value as an indicator of nutritional status and little value as a predictor of fertility.

Indications and implications for testing of milk urea in dairy cattle: A quantitative review. Part 1. Dietary protein sources and metabolism

MILK UREA CONCENTRATIONS IN DAIRY CATTLE: There has been increased use of milk urea concentration as an indicator of dietary protein intake and protein metabolism in dairy cattle over recent years. The value of milk urea content data in predicting dietary composition, particularly for pasture-fed cattle, has not been well described. PROTEIN METABOLISM AND UREA SYNTHESIS: Many factors influence the degradation of dietary proteins in the rumen, post-ruminal protein metabolism and urea synthesis in cattle. Strong positive correlations between nitrogenous fertiliser use and the crude protein content of pastures were identified by use of meta-analysis. Similar strong positive correlations were noted between dietary protein intake, rumen ammonia and plasma urea concentrations. The costs of urea synthesis include energy losses, and importantly, the loss of endogenous amino acids, which are deaminated in the synthesis of urea. MILK UREA AS AN INDICATOR OF PROTEIN METABOLISM: Urea concentrations in blood, plasma and milk are strongly correlated. Milk is an adequate indicator of blood and plasma urea content, but non-nutritional factors may significantly influence milk urea concentrations. Recommendations for dietary protein management based on milk urea concentrations must be undertaken with care.

Effect of long-chain fatty acids on triglyceride accumulation, gluconeogenesis, and ureagenesis in bovine hepatocytes

A relationship between increased lipid concentration in the liver and reduced hepatic function has been suggested; however, there is little direct evidence of change in specific hepatic functions. Hepatocytes were obtained from ruminating calves and were incubated as monolayers for 36 h. The media contained a mixture of nonesterified fatty acids (NEFA) at 0, 0.5, 1.0, and 1.5 mM NEFA with molar proportions of 0.435 oleate, 0.319 palmitate, 0.144 stearate, 0.049 linoleate, and 0.053 palmitoleate. Ureagenesis or gluconeogenesis was measured from 48 to 51 h after plating using hepatocytes that had only previous (12 to 48 h), only concurrent (48 to 51 h), or previous and concurrent (12 to 51 h) exposure to NEFA. A previous 36-h exposure to NEFA caused cell triglyceride accumulation, yielding triglyceride concentrations that corresponded with liver that is clinically described as normal to moderately fatty. Previous, prolonged exposure to NEFA reduced ureagenesis and increased gluconeogenesis. Concurrent exposure to NEFA did not significantly affect gluconeogenesis or ureagenesis and did not alter the residual effect of prolonged incubation with NEFA. Reduced ureagenesis was related to increased cell triglyceride accumulation independently of other direct NEFA effects. Decreased ureagenic capacity may play a role in the morbidity associated with periparturient fatty liver in dairy cows.

A statistical evaluation of animal and nutritional factors influencing concentrations of milk urea nitrogen

Data from 35 trials with 482 lactating cows fed 106 diets were used to study the effects of animal and dietary factors on the relationship between milk and blood urea N and the value of milk urea N in the assessment of protein status. In two trials, urea N in whole blood and in blood plasma were closely related (r2 = 0.952); the slope was not significantly different from 1.0, and the intercept was not significantly different from 0. Regression of milk urea N on blood urea N with a mixed effects model using all 2231 observations yielded the equation: milk urea N (milligrams of N per deciliter) = 0.620 x blood urea N (milligrams of N per deciliter) + 4.75 (r2 = 0.842); this model accounted for a significant interaction of cow and blood urea N. Single factors that yielded significant regressions on milk urea N with the mixed effects models were dietary crude protein (CP) (percentage of dry matter; r2 = 0.839), dietary CP per megacalorie of net energy for lactation (NEL) (r2 = 0.833), excess N intake (r2 = 0.772), N efficiency (r2 = 0.626), and ruminal NH3 (r2 = 0.574). When all factors were analyzed at once, 12 were significant in a mixed effects model. Blood urea N, body weight, yield of fat-corrected milk, dietary CP content, excess N intake, dry matter intake, and days in milk were positively related to milk urea N, and parity, milk and fat yield, dietary CP per unit of NEL content, and NEL intake were negatively related to milk urea N. In one trial, the mean urea concentration was 35 times greater in urine than in milk; lower proportions of total urea excretion in milk were observed in the a.m. sampling (1.8%) than in the p.m. sampling (3.3%). Measuring urea N in a composite milk sample from the whole day substantially improved reliability of data. The number of cows fed a specific diet that must be sampled to determine mean milk urea N within 95% confidence intervals with half widths of 1.0 and 2.0 mg of N/dl was estimated to be 16 and 4, respectively.

Dietary excesses of urea influence the viability and metabolism of preimplantation sheep embryos and may affect fetal growth among survivors

In the first of two experiments investigating the effect of dietary urea on the survival and metabolism of ovine embryos, 30 Border Leicester x Scottish Blackface ewes received a maintenance diet (milled hay, molasses, minerals, vitamins) with no urea (control, C; n = 10) or with added urea at 15 g (low urea, LU; n = 10) or 30 g (high urea, HU; n = 10) kg-1 feed for a 12 week period. The degraded nitrogen (N) status relative to estimated rumen microbial N requirements was -2, +9 and +20 g per day, respectively. One week after allocation to diets, progesterone priming (12 days) commenced. Ewes received 800 IU of equine chorionic gonadotrophin at progesterone withdrawal, were inseminated 52 h later (Day 0) and embryos were collected from five ewes per group at Day 4 and from five ewes at Day 11. If available, one embryo was returned to each ewe; the rest were cultured in vitro. There was no effect of treatment on progesterone, luteinizing hormone (LH), or time of oestrus onset C, LU and HU plasma urea (P < 0.001) and ammonia levels (C vs. HU, P < 0.01; LU vs. HU, P < 0.05) differed. Day 4 HU embryos were retarded relative to C and LU embryos. After 3 days of culture, 70%, 66% and 0% of C, LU and HU embryos, respectively, were viable. Mid-term pregnancy rates following transfer were 63%, 43% and 33%. Only one HU lamb (male) was born following embryo transfer, its birthweight (10.1 kg) exceeded that of its C (n = 3; 7.0, 7.0, 7.5 kg) and LU (n = 2; 7.3, 8.2 kg) counterparts (P < 0.025). In the second experiment, C2 (2.5 g urea kg-1; n = 5) and HU2 (30 g kg-1; n = 7) diets which provided similar intakes of degraded N relative to microbial requirements as those for C and HU ewes in Experiment 1 were fed to Border Leicester x Scottish Blackface ewes superovulated with 16 mg of porcine follicle-stimulating hormone. Urea and ammonia levels in utero-oviductal samples were elevated in HU2 ewes (P < 0.05). At collection (Day 3), HU2 embryos used more glucose (P < 0.01) and, following culture, some exhibited up to a 2.8-fold increase in metabolism. In conclusion, excess rumen degradable N in ewe diets elevates urea and ammonia in plasma and in utero, with an associated increase in embryo mortality. Nevertheless, metabolism appears to be up-regulated in some embryos and, among those that survive, fetal growth appears to be enhanced.

Effects of excess degradable protein on postpartum reproduction and energy balance in dairy cattle

The objectives of this study were to observe endocrine and reproductive responses of cows and heifers fed two diets (16 and 19% CP), which met undegradable protein requirements but differed in rumen degradable protein. Cows (n = 33) and heifers (n = 32) were randomly assigned within parity to diets at calving and remained on diets for 20 d after first breeding. Energy balance was determined twice weekly through the first luteal phase. Blood and milk samples were taken three times per week. Diet did not affect average daily energy balance, days to negative energy balance nadir, days to first ovulation, days to first service, or plasma glucose concentrations. First service conception rate was lower (31% vs. 48%) and plasma urea higher in animals fed the high protein diet. Days to energy balance nadir was correlated with days (r = .75) to first ovulation. Luteinizing hormone pulse frequency increased and pulse amplitude decreased in frequent samples (12-min intervals for 8 h) collected at 14 d postpartum versus sampling after the energy balance nadir. These data suggest that energy balance status plays an important role in determining the postpartum return of cyclic ovarian activity. Feeding excess CP as rumen degradable protein elevated plasma urea concentrations and decreased first service conception rate.

Impact of protein nutrition on reproduction in dairy cows

In most but not all published studies, feeding diets with high concentrations of crude protein decreased reproductive efficiency. We were able to resolve some of the inconsistencies among published reports by considering the fate of consumed protein in the rumen and then using logistic regression analysis to identify factors affecting pregnancy. Our models showed that type and amount of protein fed explained much of the variation in conception rate. Age and dietary concentration of energy were identified as modifiers of the impact of protein on reproduction. Protein nutrition can affect reproduction through toxic effects of ammonia and its metabolites on gametes and early embryos, through deficiencies of amino acids, and by exacerbations of negative balances of energy. Alterations in the hypothalamic-hypophyseal-ovarian axis may be responsible for many of the effects of protein on reproduction.

Interactions of nutrition and reproduction

As dairy cattle achieve higher milk production, we are becoming aware of closer relationships between nutrition and fertility. An increasing body of evidence indicates that excessive protein intake in early lactation may be detrimental to postpartum fertility. In addition, protein solubility and degradability are important. Energy restriction in high producing cows may be detrimental to fertility through its effect on hypothalamic and ovarian function. Dairy producers are susceptible to advertisements advocating the addition of specific vitamins and minerals to the rations of high producing cows since inadequate or excessive quantities may be detrimental to fertility. This is particularly true for minerals. However, high producing cows do not usually receive grossly inadequate or excessive dietary sources of vitamins and minerals. Rather, they may be presented with a borderline intake of several vitamins and minerals, which together may be detrimental to fertility. Finally, new methods must be developed to provide a more sensitive indication of fertility. These are some of the research and educational needs of the dairy industry if it is to continue successfully increased milk production. Predictions of herd averages of 16,000 kg milk and individual records exceeding 30,000 kg have been made for the yr 2000.

Influence of protein intake and feeding strategy on reproductive performance of dairy cows

A study was conducted to determine the impact of dietary CP concentration (13 vs. 20%) and feeding strategy (total mixed ration vs. separate feeding of the forage and concentrate) on reproductive performance of 57 early lactation dairy cows. Cows were assigned to one of four treatments in a 2 x 2 factorial arrangement. Rations composed of 40% forage (50% grass-legume silage:50% corn silage) and 60% concentrate (DM basis) were fed from d 5 to 100 postpartum. Cows fed a total mixed ration had lower ruminal ammonia and plasma urea N concentrations compared with cows fed separately. Feeding strategy groups showed no differences in reproductive performance. Cows fed a 20% CP ration had higher CP intake, higher ruminal ammonia, and higher urea N concentration in plasma and vaginal mucus. There were no differences between low and high CP groups in days to first observed estrus (24 vs. 27), days to first service (55 vs. 59), days open (72 vs 82) or services per conception (1.5 vs. 1.8). Days to first estimated ovulation were longer (22 vs. 17) in the high vs. low CP groups, and there was an interaction between protein intake and lactation number. Cows managed with an intensive program for detection of estrus and for reproductive health did not show differences in reproductive efficiency when fed 13 or 20% CP rations.

Effects of nutrition on reproduction in dairy cows

It is clear that nutrition is closely related to reproduction in the dairy cow. In addition to classic nutrient deficiency and excess, dry cow body condition and postpartum energy balance have a major impact on fertility. Dry cow rations must be designed to prevent milk fever, dystocia, retained placenta, other calving problems, and metritis. Most importantly, milk yield and reproductive performance should be regularly monitored to detect nutritionally associated health and performance changes that precede impairment of reproductive function.

Influence of dietary protein on reproductive performance of dairy cows

From 10 +/- 3.5 d after parturition to 149 +/- 3.5 d of lactation, 146 cows (109 Holstein and 37 Ayrshires) were fed either a 15 or 20% crude protein diet of 45% sorghum silage and 55% concentrate. Milk production was enhanced by the 20% protein diet, but percentage of milk fat and protein were unchanged. Average daily intake of DM was unaffected by diet. Actual decreases in body weight and condition were small, although cows on 15% protein lost more condition. Concentration of plasma urea nitrogen increased rapidly during the first 4 wk of the experiment with cows on the 20% protein diet maintaining a 10 mg/dl advantage after the 4th wk on experiment. Diet did not affect the postpartum interval to first normal corpus luteum and standing estrus. Cows were inseminated artificially between 55 and 145 d post-calving with one freeze batch of semen from one bull of each appropriate breed mainly by one inseminator. Overall, days open, services per conception, and percentage cows pregnant on experiment were 80, 1.40, and 85, respectively. Concentration of protein in the diet did not affect these reproductive traits. There was no conflict between the amount of dietary protein provided for lactation and reproduction in this experiment.

Effect of crude protein and solubility on performance and blood constituents of dairy cows

Thirty-six mature Holstein cows were assigned at parturition to isocaloric diets of either 14.4 or 19.0% crude protein from highly soluble or insoluble nitrogen sources. The objective was to determine if elevated dietary crude protein affected milk production or serum metabolites. Cows individually were fed blended rations. Hourly blood samples were taken for a 12-h period during wk 1, 4, 7, and 10 postpartum. Rumen fluid samples were taken during wk 2, 4, and 6 postpartum. Cows fed diets containing 19% crude protein of high nitrogen solubility had larger dry matter intakes and tended to produce more milk than cows fed 14.4% crude protein. High crude protein intakes elevated rumen ammonia and blood urea nitrogen. Cows fed high crude protein, low nitrogen solubility diets had increased acetate to propionate ratio in rumen fluid. Serum insulin was increased in cows consuming high crude protein diets, regardless of the protein source. Serum nonesterified fatty acids decreased and albumin and glucose increased with week postpartum. Body weight change and fat-corrected milk production were not affected significantly by protein intake. Protein intakes in excess of requirements have no adverse effect on the production of early postpartum cows.

Effect of high crude protein on pituitary and ovarian function in Holstein cows

Intact and ovariectomized cows were used to determine the effect of high amounts of dietary crude protein on pituitary regulation of luteinizing hormone. Estrus was synchronized with prostaglandin F2 alpha in 10 intact dry cows fed 15 or 25% crude protein, and serum luteinizing hormone profiles were evaluated during both follicular and luteal phases of the estrous cycle. Serum progesterone, pituitary luteinizing hormone content, and luteinizing hormone-releasing hormone receptors were also measured. Basal concentrations of serum luteinizing hormone tended to be lower during the follicular phase and were significantly higher during the luteal phase in cows fed 25% crude protein. Serum progesterone was not affected by dietary treatment. In 10 ovariectomized cows fed 24% crude protein, amplitude of luteinizing hormone response to luteinizing hormone-releasing hormone challenge tended to be smaller than that of 8 cows fed 16% crude protein, although total luteinizing hormone released in response to luteinizing hormone releasing hormone challenge was not different between dietary treatments. In both intact and ovariectomized cows, pulsatile luteinizing hormone patterns, pituitary luteinizing hormone content, and luteinizing hormone-releasing hormone receptors were not influenced by treatment. High dietary crude protein did not have a primary effect on luteinizing hormone or progesterone in nonlactating cows.