Physiological attributes as possible selection criteria for milk production 1. Study of metabolites in Friesian calves of high or low genetic merit

The value of indicator traits in the genetic improvement of dairy cattle

The value of indicator traits (7), such as physiological or biochemical traits in the genetic improvement of dairy cattle for milk yield (M) was studied. First, some corrections were made to the base rates of genetic change possible by improvement systems based on progeny testing and on multiple ovulation and embryo transfer (MOET), and on combinations of these. Efficient field progeny-testing systems can be competitive with current adult MOET nucleus herd schemes but juvenile MOET nucleus herd schemes offer substantial increases in rates of response. With high co-heritability, selection for the T alone may allow greater rates of response than those currently considered feasible using progeny testing. However, faster rates are obtained with combined selection. When breeding values are accurately measured by pedigree and performance records on M, as in the progeny test, the extra rates of response with combined selection may be small. Where breeding values are less accurately assessed, as in juvenile MOET nucleus schemes, the extra rates of response can be appreciable. For T with co-heritability (hMrGhT) of 0·27 and the CV for M from 0·15 to 0·20, response rates of 2·0 to 2·7% of the mean per year possible by traditional methods could be increased to 2·2 to 2·9% in progeny testing schemes, 2·3 to 3·1% and to 4·3 to 5·7% for adult and juvenile MOET nucleus schemes respectively.

A possible useful indicator trait is blood urea nitrogen (BUN) measured in young animals after a short fast. Results from four experiments with calves having high or low genetic merit for M were summarized. The pooled co-heritability estimate was —0·27 (s.e. 0·05). With this, or even a more modest effect, BUN would be a useful indicator trait in selection for milk production. Its use in practice in high and low selection lines or in a section of the industry, would allow assessment of the merit of the method.

Concentrations of blood constituents in genetically high and low milk-production lines of British Friesian and Jersey cattle around calving and in early lactation

Differences between Friesians and Jerseys and between progeny of high and low contemporary comparison (CC) sires were sought by examining nine blood constituent concentrations around first and second calvings and during lactation. A total of 1359 samples from animals at first lactation and 1148 samples at second lactation were collected. All animals were individually fed a complete pelleted diet ad libitum. Blood constituents analysed were free fatty acids (FFA), ketones, glucose, calcium, magnesium, copper, phosphorus, albumin and globulin. All nine constituents showed marked changes around parturition and early lactation and several constituents showed changes with age. Jerseys had higher average copper and albumin levels and lower globulin levels than Friesians at both lactations and higher FFA concentrations at second lactation. Changes in plasma concentrations of FFA, ketones and glucose around calving were consistent in suggesting that Jerseys and progeny of high CC sires had a substantially greater energy deficit in early lactation than Friesians and progeny of low CC sires at the first but not the second lactation. Predicted requirements v. recorded intakes suggested that the Jerseys and high CC progeny had the greater energy deficit at both lactations. Although the incidence of hypocalcaemia was nearly zero, changes of calcium and magnesium concentrations were consistent with reports of Jerseys being more susceptible to hypocalcaemia in later life.

Despite large differences in milk yield, differences in plasma concentrations between animals classified as either high or low achieved yielders within their genetic class were generally smaller than differences between progeny of high and low CC sires.

Biochemical and physiological responses to metabolic stimuli in Friesian calves of differing genetic merit for milk production

Changes in blood metabolite and insulin concentrations in response to metabolic stimuli were evaluated as potential predictors of dairy merit. Calves of high or low genetic merit for milk production were subjected to the stress of: simulated feeds by injection of sodium propionate (0·5 mmol/kg body weight), a short fast and short-term cold exposure. Responses of insulin, free fatty acids, urea, glucose and D-β-hydroxybutyrate were determined by serial blood sampling.

Injections of sodium propionate did not greatly affect blood metabolite concentrations but did sharply increase insulin concentrations; no difference in response between high and low lines was observed. During fasting serum free fatty acid and D-β-hydroxybutyrate concentrations were greatly increased but no variation due to line was observed. Urea concentrations were also greatly increased during fasting and in this case animals of high dairy merit showed a considerably smaller increase than animals of low dairy merit. Heart rates decreased by one-third during the fast but no variation due to line was apparent;during refeeding high dairy-merit animals had lower heart rates than low-merit animals.

Short-term cold exposure did not significantly alter plasma insulin or metabolite concentrations.

These results are discussed in relation to previous findings and it is concluded that serum urea concentrations during a fast may provide the basis for a useful and robust predictor of dairy merit in young animals.

The effect of dietary protein on metabolite concentrations during fasting in calves differing genetically in dairy merit

Eighty calves of both sexes, 4 months of age, of two lines, differing in genetic merit for fat plus protein production by 70 kg per 305-day lactation, were given one of four diets differing in protein content to give an estimated range of protein intakes from 299 to 530 g/day. After 21 days the calves were fasted for 72 h. Samples of blood were taken before and after feeding in the morning and afternoon prior to fasting, seven further samples were taken throughout the fast and five samples were taken ranging from 0·5 to 7·5 h after food was re-introduced. The samples were analysed for concentrations of urea, free fatty acids (FFA), glucose, triglyceride and f5-hydroxybutyrate.

No association between metabolite concentrations and predicted breeding value (PBV) were found at any stage of the trial except for the effect of feeding on FFA concentration, the rate of increase of FFA during the fast and its ultimate concentration. These associations with FFA depended on the sex of the calf and were either absent or less marked in males. The results strengthen the possibility that FFA concentrations during fasting are related to PBV in females, and that sexual dimorphism may explain differing conclusions amongst previous trials.

The magnitude of the estimate of coheritability of urea concentration during fasting with PBV, pooled across experiments, is reduced from -0·193 to -0·066 (s.e. 0·037) by inclusion of these results. However, heterogeneity between experiments is evident and remains to be explained. This suggests that urea concentrations are not consistent predictors of genetic merit and cannot be considered to be reliable for use in practice.

Physiological attributes as possible selection criteria for milk production 2. Plasma insulin, tri-iodothyronine and thyroxine in bulls

Insulin, thyroxine and tri-iodothyronine concentrations were measured in samples of blood from 175 Friesian bulls, aged between 2 and 7 years, which were being progeny-tested for milk production. Least squares regression coefficients were estimated for these hormone levels on time of day and time after feed at sampling and age of bull. Correlations between hormone levels and Improved Contemporary Comparisons of the bull, a measure of his daughters' milk production, did not differ significantly from zero, suggesting that simple spot estimates of levels of these hormones in bulls would not supply useful predictors of daughters' production.

Efficiency and performance of genetically high and low milk-producing British Friesian and Jersey cattle

Genetic lines for high and low liquid milk production were established within the British Friesian and British Jersey breeds by random matings of experimental dams to the ‘best’ and ‘worst’ nationally available progeny-tested sires. Some Friesian dams could also be classified as either high or low for genetic potential for milk yield on the basis of previous, but less rigorously controlled, matings to high-or low-production sires. The dams and their high and low potential-production female progeny were reared indoors, and given a single complete pelleted diet ad libitum from weaning until leaving the experiment after their third calving. Height at withers and width at hooks were recorded monthly, cumulate food intake and body weight fortnightly and milk yield, fat and protein concentration weekly, throughout life in the experiment. Measures of lactation production, food intake and efficiency of conversion of food to milk product during the whole calving-to-calving interval were obtained. Yields were about 0-8 times national average yields. Differences between high and low genetic lines appeared similar for the two breeds. High potential-production progeny produced more liquid milk, fat and protein but at a lower fat and protein concentration than did low-potential progeny. High-potential progeny consumed more food from calving to calving and had higher food conversion efficiencies to liquid milk, fat and protein. The response in efficiency accompanying a given change in production was close to that predicted by phenotypic regression of efficiency on yield with a 0-75% increase in efficiency for every 1% increase in yield. The likelihood of smaller returns in efficiency at higher yields is discussed. High potential-production progeny lost more body weight than did low during lactation, suggesting a greater withdrawal of energy from body reserves. High-potential progeny were neither heavier nor taller but were marginally narrower at the hooks than were low-potential progeny, suggesting the possibility of increased calving difficulties as selection for increased yield continues.

Friesians produced 50% more liquid milk, 13% more milk fat and 29% more milk protein and consumed 22% more food from calving to calving than did Jerseys. As a consequence Friesians were markedly more efficient at producing liquid milk (+23%) but essentially no more efficient at producing milk energy (+2%) than were Jerseys. In terms of both biological and commercial application of the results, any biases in the experiment probably operated against the Jersey relative to the Friesian.

Predictions of food intake using accepted feeding standards underestimated observed total food intake during lactation by a factor of 0-8. Linear regression indicated underestimation of maintenance requirement as a likely explanation.

Use of plasma hormone and metabolite levels to predict breeding value of young bulls for butterfat production

The purpose of the experiment was to investigate the possibility of using concentrations of hormones and metabolites in plasma of potential breeding bulls early in life as indirect selection criteria for butterfat production. Sixteen Red Danish bull calves were included in the experiment, 10 were offspring of parents selected for high butterfat production for two or three generations, and six were from parents selected for low butterfat yield. The average estimated relative breeding values of the calves in the two groups were 111 and 91.

At 3·5 and 7 months of age blood samples were collected every 6 h during 2 days of ad libitum feeding, 5 days of fasting and 2 days of refeeding. The samples were analysed for glucose, urea, free fatty acids, insulin and thyroxine. At 3·5 months plasma concentrations of free fatty acids were significantly correlated with breeding value under all feeding conditions examined. The increase in plasma glucose during refeeding at both ages was positively related to breeding value. Plasma insulin during the 1st day of fasting was positively correlated with genetic merit at 3·5 months of age and negatively correlated at 7 months. Plasma thyroxine was negatively correlated with breeding value at both ages, but the correlation was only significant at 7 months. Multiple regression analyses showed that a large part of the variation in breeding value for butterfat production could be described by plasma levels of hormones and metabolites when more than one variable was included in the model. With six variables in the model 0·67 and 0·73 of the variation could be accounted for at 3·5 and 7 months of age respectively.

The results of the experiment have to be considered with caution because only one sire was used per group, the parents were preselected, and the number of animals was small. The results, however, do indicate that plasma levels of hormones and metabolites in the early life of potential breeding bulls, measured after suitable physiological challenges, may be useful as selection criteria for butterfat production of their offspring.

Physiological attributes as possible selection criteria for milk production 3. Plasma hormone concentrations and metabolite and hormonal responses to changes in energy equilibrium

Plasma hormone concentrations and metabolite responses to changes in energy equilibrium were measured in experiments designed to compare a) Hereford × Friesian with Friesian calves and b) Friesian calves sired by bulls with a high level of Improved Contemporary Comparison with those sired by bulls of a low level. The concentration of thyroxine and triiodothyronine tended to be greater in calves of higher dairy merit. The concentration of growth hormone was largely unrelated to dairy merit in the circumstances studied. The concentration of insulin was greater in Hereford × Friesian than in Friesian calves following feeding and following the injection of propionate. Responses to the injection of insulin and of growth hormone (as measured by changes in the plasma concentration of urea nitrogen, free fatty acids, glucose or the other hormone) were little affected by the level of Improved Contemporary Comparison of a calfs sire.

It was concluded that the thyroid system and the response of insulin to perturbation of energy balance may provide criteria of genetic merit for milk production which are neither age- nor sex-limited.

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.

Plasma metabolite and hormone concentrations as predictors of dairy merit in young Friesian bulls: effect of metabolic challenges and fasting

Responses to metabolic challenges were measured in 8-month-old Friesian bull calves with a view to identifying possible predictors of genetic merit for milk fat production. Seven ‘high’ breeding index bulls (mean breeding index for milk fat = 128·9 (s.d. 4·8) percentage units) were compared with six ‘low’ breeding index bulls (mean 107·3 (s.d. 2·4) percentage units). Bulls were from two selection lines and breeding indices calculated as the average of parental breeding index values.

Challenges involved intravenous injection of the following (dose rates per kg body weight): adrenalin (1 μg); glucose (0·17 g); glucagon (0·175 μg); insulin (0·01 mg); and arginine (40 mg). Blood samples were withdrawn prior to and after each challenge (two challenges daily). Bulls were then fasted for 3 days before being refed.

Relative to the low breeding index bulls, those in the high breeding index group exhibited: (a) greater pancreatic sensitivity to circulating glucose as indicated by increased insulin concentrations following the glucose challenge; (b) more rapid clearance of glucose from plasma following the insulin challenge; (c) reduced sensitivity to the glycogenolytic/gluconeogenic effects of glucagon as indicated by lower blood glucose concentrations following intravenous injection of this hormone; (d) elevated plasma insulin and growth hormone concentrations during the fasting period; (e) elevated plasma concentrations of insulin, glucose and urea during refeeding; (f) no difference in responses to intravenous arginine or adrenalin, or in circulating concentrations of Insulin-like Growth Factor-1 or alkaline phosphatase. Results are consistent with previous observations that metabolic differences between cattle in the Massey University high and low breeding index lines are most evident in the metabolism of glucose and insulin. Metabolic challenges offer a potentially useful means of predicting genetic merit for milk fat production but the conditions under which repeatable differences between the lines can best be demonstrated are yet to be determined.

Selection for milk production from a lactation biology viewpoint

The success of selection for increased milk production in dairy cows is apparent. Certainly, many herds now have average production levels that would have only been associated with the best producers in the herd 30 yr ago. There are, of course, many reasons for this success. Among these are improvements in genetic selection methods and associated use of artificial insemination, better fulfillment of nutritional needs and diet formulation, and careful attention to mastitis control and milking management. Development of new management tools (i.e., bovine somatotropin, improved crops, estrus detection devices, estrus synchronization, monitoring of individual animal performance, and disease prevention) should not be forgotten. Although many aspects of a dairy operation determine overall performance and profitability, the focus of this paper is the udder. Information indicates that both the structure and function of the bovine mammary gland have been directly impacted by long-term selection for increased milk production but improved functionality may have been more important. This review also considered studies that attempt to develop techniques and measurements for possible selection of genetically superior animals including measurement of circulating hormones and direct assay of mammary tissue function.

Relationship between age, nutritional status and dairy merit with plasma profiles of growth hormone (GH) and free fatty acids (FFA) in bulls

SUMMARY

Blood plasma levels of growth hormone (bGH) and of free fatty acids (FFA) were measured in repeated samples drawn every 15' for bHG, every 1 h for FFA one day after morning feeding and after 3 days of severe feed restriction of 7 young (< 2 years) and 14 old (> 5 years) bulls. The numerous data were condensated by the PULSAR program of Merriam and Wachter (1982) to mean level, average baseline concentration, number, average amplitude and average duration of secretory peaks. Young bulls had higher values for all of these parameters except peak duration and increased these after fast except for peak duration, baseline and integral while mature bulls reacted little to food restriction. FFA plasma levels were higher in young bulls under normal feeding when mature bulls had more frequent peaks. After feed restriction mature bulls increased the mean plasma level to the increased level of young bulls but decreased the peak frequency. The milk protein yield breeding values are related significantly to mean level, integral and the peak duration of plasma bGH levels but in different directions in young and in mature bulls. ZUSAMMENFASSUNG: Beziehungen zwischen Alter, Futterstatus, Milchzuchtwert und Plasmaspiegel von Wachstumshormon (GH) und Freien Fettsäuren (FFA) bei Stieren Blutplasmaspiegel von Wachstumshormon (GH) und Freien Fettsäuren (FFA) wurden in wiederholten Proben gemessen, die im Falle von GH alle 15 min. und im Falle von FFA jede Stunde einen Tag nach Morgenfütterung, nach drei Tagen starker Futterrestriktion bei sieben jungen (weniger als zwei Jahre) und 14 alten (über fünf Jahre) Stieren gezogen worden waren. Die zahlreichen Daten wurden mit Hilfe des Computerprogrammes von Merriam und Wachter (1982) kondensiert zu durchschnittlichen Plasmaspiegel, durchschnittliche Basiskonzentration, Zahl, durchschnittliche Amplitude und durchschnittliche Dauer von Sekretionsspitzen und Integral. Die Jungstiere hatten für alle diese Parameter höhere Werte, außer der Dauer von Sekretionsspitzen und nahmen mit Ausnahme von dieser, von Basisspiegel und Integral nach Futterrestriktion zu, während Altstiere auf diese sehr wenig reagiert haben. FFA-Plasmaspiegel waren bei jungen Stieren höher unter Normalfütterung, während alte Stiere häufiger Sekretionsspitzen zeigten. Nach Futterrestriktion steigerten Altstiere den mittleren Plasmaspiegel auf den angestiegenen Spiegel junger Stiere, aber verminderten die Häufigkeit der Sekretionsspitzen. Milchprotein-Zuchtwerte sind signifikant mit mittlerem Plasmaspiegelwert, Integral und Dauer der Sekretionsspitzen bei Plasma GH korreliert, aber in verschiedener Richtung bei jungen und bei Altstieren.

Effect of season, genetic line, and sire on growth concentrations of somatotropin in serum of Holstein cows in early lactation

To determine the effect of selection for milk yield on somatotropin concentrations, blood samples were collected from 128 Holstein cows whose sires had either high or average predicted differences for milk. The phenotypic difference in milk yield between the high and average yielding groups was 1726 kg on a 305-d mature equivalent basis. At 37 +/- 7 d postpartum, four blood samples were taken from each cow at 1-h intervals beginning at 0800 h. The statistical model contained genetic line, sires within line, and season as whole-plot effects and time of sampling as a subplot effect. The concentration of growth hormone was significantly higher in the high yielding group (1.89) than in the average yielding group (1.49). Cows sampled in summer had the highest concentrations of growth hormone, whereas cows sampled in spring had the lowest concentrations. Sires did not significantly influence the somatotropin concentration of their progeny. In a second analysis with seven extreme outliers (peak values) deleted, the variation of somatotropin concentrations in the cows was significantly accounted for by their sires. Repeatability of somatotropin concentration was .49 within cows.

Plasma somatotropin and prolactin concentrations in young dairy sires before and after a 24-hour fast

To examine the efficacy of plasma concentrations of bST or prolactin as predictors of expected daughter performance, blood samples were collected from young Holstein sires. Blood samples were collected at 15-min intervals via jugular cannulas from 1000 until 1600 h (d 1), beginning 4 h after morning hay feeding. Bulls were not fed again until after collection of blood samples on d 2. Samples were collected at 15-min intervals from 1000 until 1300 h on d 2. Peak values and frequency of hormonal secretory patterns of each bull were characterized by an iterative process in which values greater than 2 SD from the mean were flagged as peaks and excluded from the subsequent calculation of SD and mean. The process continued until an iteration in which no new peaks were flagged. Imposition of a 24-h fast did not alter mean basal bST or prolactin concentrations, but reduced mean peak and overall concentrations of both hormones. The number of bST peaks on d 1 was inversely related to both USDA and Northeast Artificial Insemination Sire Comparison Pedigree Index for milk yield and both USDA and Northeast Artificial Insemination Sire Comparison sire PD for milk yield, but was positively correlated on d 2 with USDA Pedigree Index for milk yield. Mean peak bST on d 2 was correlated with Northeast Artificial Insemination Sire Comparison Estimated Breeding Value for fat yield and sire USDA PD for fat yield. Prolactin peak frequency on d 1 was negatively related to Northeast Artificial Insemination Sire Comparison Pedigree Index for milk yield and sire PD for fat yield. Difference between mean prolactin on d 1 and 2 was negatively related to Northeast Artificial Insemination Sire Comparison Pedigree Index for milk yield and Estimated Breeding Value for fat yield. Endocrine parameters in young sires may be related to genetic merit for production parameters.

The effect of induced hypomagnesaemia and hypermagnesaemia on the erythrocyte magnesium concentration in cattle

Hypomagnesaemia was induced in ten yearling Friesian heifers by starving them for forty hours while hypermagnesaemia was induced in seven Friesian calves (about two and half months old) by intramuscular magnesium sulphate (MgSO4) injection. The plasma magnesium (PMg) concentration decreased (P less than 0.05) during starvation and increased (P less than 0.05) following MgSO4 administration. However the erythrocyte magnesium (EMg) concentration did not show any significant change either with starvation or following the MgSO4 administration. These results show that sudden and significant changes in PMg concentration do not have any significant effect on EMg concentration. Accordingly EMg concentration may be a better indicator of long-term body Mg status and during hypomagnesaemic tetany could show whether the decrease in PMg concentration was sudden or gradual.

Triiodothyronine and thyroxine during gestation in dairy cattle selected for high and low milk production

Circulating thyroxine, triiodothyronine, and prolactin were analyzed by radioimmunoassay from two groups of pregnant heifers representing genetic populations that differed by 685 kg milk in their first lactation. At 100 days of gestation, blood samples were taken every 15 min for prolactin assay and every 2 h to determine variations of thyroid hormone serum concentration from 0800 to 2000. Basal serum prolactin concentrations in the two genetic groups were not significantly different. A pattern was consistent for both triiodothyronine and thyroxine; concentrations were lower in the morning and higher during the afternoon. Serum triiodothyronine means were not different in the high-producing (1.84 ng/ml) and low-producing heifers (1.86 ng/ml). Serum thyroxine concentrations were significantly higher in the low-producing heifers (67.84 ng/ml) than in high-producing heifers (59.18 ng/ml). The higher thyroxine concentration in pregnant heifers with lower producing ability suggests a negative relationship of thyroxine to milk yield potential.