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

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.

Growth, hormonal and metabolic response of post-pubertal entire male cattle to trenbolone acetate and hexoestrol

The response of post-pubertal entire male cattle (bulls) to implantation with trenbolone acetate and hexoestrol was studied in two experiments. In experiment 1, eight British Friesian bulls, weighing approximately 359 kg at the beginning of a 90-day experiment, were either sham-implanted or implanted with 45 mg hexoestrol, followed 21 days later with a further implantation of 300 mg trenbolone acetate. In experiment 2, 16 bulls (14 British Friesian and two Simmental < ♂ × (Hereford < 5 × Friesian ♀), weighing approximately 349 kg, were similarly untreated or implanted with 30 mg hexoestrol and 300 mg trenbolone acetate. Treatment with hexoestrol gave a small and non-significant increase in live-weight gain in both experiments. However, live-weight gain and efficiency of conversion of food into live-weight gain were significantly improved in both studies following implantation with trenbolone acetate (P < 0·05). Treated bulls had heavier carcasses and greater mean values for estimated carcass gain (P < 005 in experiment 2). Comparison of mean values for blood metabolites indicated significant differences in concentrations of plasma glucose, free fatty acids and cortisol, and serum total protein, growth hormone, insulin and prolactin. Mean values were significantly lower in treated animals for serum albumin in experiment 2 (P < 001) and for plasma urea in both experiments (P < 0·001; P < 0·05). Treatment with hexoestrol resulted in lower mean concentrations of plasma androgens. These were further reduced following treatment with trenbolone acetate (P < 0·01). The results suggest that the combination of trenbolone acetate plus hexoestrol may improve growth performance, by altering protein metabolism, in post-pubertal entire male cattle. The combination of trenbolone acetate plus hexoestrol appears to have more potent anabolic activity than the endogenous hormones of these animals.

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.

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.

A note on the variability of hormone concentrations in twice-weekly blood samples taken from heifer calves during the first 110 days of life

To assess the variability of hormone values obtained from calves at an early age, plasma concentrations of prolactin, growth hormone, insulin and thyroxine were measured in blood samples taken twice-weekly from seven heifers maintained under commercial conditions during their first 110 days of life. With the possible exception of thyroxine, there was considerable variation in the concentration of each hormone in each animal throughout the period of sampling, thus indicating that twice-weekly sampling was inadequate for investigating differences in hormonal concentrations between cattle of different production potential. Although there were trends in the pattern of plasma hormone concentrations as the calves progressed from the pre-ruminant to ruminant states, few of the changes during the pre-weaning, weaning and post-weaning periods were consistent between animals.

Selection on predicted breeding value for milk production delays ovulation independently of changes in follicular development, milk production and body weight

Two studies were conducted to investigate the effect of selection on predicted breeding value (PBV) for milk production, and its associated metabolic and endocrine milieu, on follicular development and ovulation in the postpartum period. A total of 71 cattle from lines selected to differ in their PBV were used in two consecutive years. In Study 1 the first ovulation and commencement of normal luteal function occurred significantly earlier in low (L) line cows than high (H) line cows. In Study 2 average daily milk production during the study period did not differ (P>0.10) between cows in either the H (=31.0+/-1.5kg/day) or L (=30.2+/-1.7kg/day) PBV lines although, 305 days milk production was significantly different (P<0.01; H=6880+/-164kg versus L=5795+/-317kg). As in Study 1, first ovulation postpartum in Study 2 occurred earlier (P<0.01) in the L (day 19) versus the H line (day 28). Circulating concentrations of glucose and insulin were significantly lower, whilst concentrations of GH and BOHB were higher in cows from the high PBV line. No differences in gonadotrophin concentrations were seen between lines. Both changes in body weight and patterns of follicle development did not differ between lines. By day 15 postpartum all cows had follicles of all three-size categories (small, medium-sized and large). Small (P<0.07) and medium-sized follicle numbers increased (P<0.01) with day postpartum. However, the inclusion of predicted changes in body weight as a covariate in the analysis, demonstrated that changes in number of small and medium-sized follicles were associated with changes in body weight. In conclusion, selection on PBV for milk production is associated with a longer interval from parturition to first ovulation, independent of changes in follicular development, milk production and body weight implicating other factor(s) associated with genetic selection.

Considerations in multiple trait evaluation

Traits of dairy cattle known to contribute to genetic merit are discussed briefly. Focus is on alternative measures and research to reflect more closely genetic pathways of performance because knowledge is expanding about cellular, organ, and animal function. Improved measurement of physiological traits of economic characters portends increased genetic control of animal productivity by considering more alternative traits. Research should clarify multiple trait selection programs to maximize profit. The null hypothesis to test is that selection for milk is optimal index selection for milk income and costs of production.

Direct and correlated responses to selection for plasma thyroxine levels in mice

Selection was carried out in mice for concentration of thyroxine hormone (T4) in plasma of males at 11 weeks of age over seven generations. Selection was practised for high level in two replicate lines and for low level in two replicate lines, and there was an unselected control. There was a response in both directions, and the divergence of 12·4 ng/ml observed in generation seven was equivalent to about 20% of the base population mean or nearly one phenotypic standard deviation. The realised heritability was 9%.

Plasma thyroxine level had a repeatability of 0·54 when two measurements were made 24 h apart. The responses made at 11 weeks in males were also evident in both males and females at 5 weeks. Plasma tri-iodo thyronine (T3) concentrations showed a correlated response almost as large, relative to the mean level, as that in T4.

Positive correlated responses were observed in total weights of the litter at 12 days, and in individual weights at 3, 6 and 9 weeks, the responses in the early weights being greater relative to their mean. The results suggest that the correlated weight changes were due to genetic responses in maternal characteristics, probably milk production, rather than individual growth.