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

Growth hormone release in calves selected for milk fat yield

In two experiments Red Danish dairy calves of both sexes, and of two lines selected for high (H) and low (L) milk fat production (proportional difference 0·22), were tested at 4 months and again at 10 months of age for their growth hormone (GH) release following intravenous administration of either thyrotropin releasing hormone (TRH, experiment 1: 0·15 μig/kg live weight, no. = 16 H + 10 L) or arginine hydrochloride (ARG, experiment 2: 0·10 g/kg live weight, no. = 19 H + 10 L). The GH response was measured in serial blood samples for 0·5 h prior to and for 2 h following intravenous injections. The response peak, measured as the geometric mean of the 5-, 10- and 15-min samples following TRH was greater in the line selected for high yield, at 10 months (H, 42·4 μg/l; L, 20·6 μig/l; P < 0.01) but not at 4 months of age (H, 25·4 μg/l; L, 18·6 μg/l; P > 0·05). The response peak following arginine measured in the 20-, 30- and 45-min samples was smaller than the peak following TRH and did not differ between selection lines, although there was a tendency for H calves to have a larger release at 10 months of age. After puberty (10 months) male calves responded more to both secretagogues than females, while there was no difference before puberty (4 months). These results suggest that GH release may be useful as a juvenile predictor of dairy merit, but results need to be confirmed in further and larger studies.

Pancreatic insulin responses to exogenous glucose in Friesian heifers of low or high genetic merit for milk-fat yield

Two studies were conducted to examine the effects of selection for milk-fat yield on pancreatic responses to exogenous glucose, as measured by circulating insulin concentrations following an intravenous glucose load, in heifer calves. The first study involved 6-month-old Friesian heifers, eight each from the Massey University high breeding index (HBI) and low breeding index (LBI) lines. Average breeding indices (BI) for milk-fat yield of the calves based on ancestry BI were 136 (s.d. 2) and 112 (s.d. 2), respectively. Oestrous cycling of the heifers was controlled by intravaginal insertion of progesterone-impregnated controlled internal drug-releasing devices (CIDRs). Challenges of glucose (0, 75, 150 and 300 mg/kg live weight) and insulin (0, 0·1, 1, and 10 fig/kg live weight) were administered via indwelling jugular cannulas and were followed by serial blood sampling to measure responses. Plasma insulin concentrations immediately following the glucose challenge were significantly (P < 0·05) greater in HBI than in LBI heifers, peak insulin concentrations (above baseline) in the HBI group being approximately twice those in the LBI group. Plasma glucose concentrations were marginally lower in HBI heifers, probably due to the secondary effect of the relatively greater hyperinsulinaemia in this group. Although the exogenous insulin challenge significantly reduced circulating glucose concentrations in all heifers, there was no difference between the lines in the magnitude of insulin-induced hypoglycaemia.

The second study was designed to determine whether insulin responses to an exogenous glucose load were influenced by stage of the oestrous cycle in heifers of the two lines. In this study, eight yearling HBI heifers and eight LBI heifers (with BI of 135 (s.d. 4) and 111 (s.d. 1) respectively) were administered an intravenous glucose load (150 mg/kg live weight) on tivo occasions, the first being after 17 days of CIDR treatment (simulating the luteal phase of the oestrous cycle) and the second 46 h after withdrawal of the CIDRs (when the heifers were expected to be in oestrus). HBI heifers again showed significantly greater circulating insulin concentrations than LBI heifers following the glucose challenge but the magnitude of this effect was not influenced by time relative to CIDR withdrawal. It is concluded that heifers selected for high milk-fat yield have an increased pancreatic response to exogenous glucose which is apparently independent of the stage of the oestrous cycle. This difference may provide a useful means of identifying heifers of superior genetic merit for milk-fat production prior to their first lactation.

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.

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.

Influence of feed intake and source of dietary carbohydrate on the metabolic response to propionate and glucose challenges in lactating goats

The influence of type of nutrients in the diet (rapidly degraded starch plus rapidly degraded N or highly digestible fiber plus slowly degraded N) and amount of feed offered (2.40 +/- 0.10 or 2.00 +/- 0.10 kg of dry matter/d) on plasma constituents after a glucose tolerance test or a propionate challenge test were studied using 12 multiparous dairy goats. Type of diet and interaction between type of diet and amount of feed offered had no effect on the concentrations of any plasma metabolite measured during propionate challenge. During propionate challenge, goats fed at the high dry matter intake (DMI) had a higher clearance of propionate and glucose and a higher ratio of plasma insulin to glucose. During glucose challenges, goats fed diets that were rich in highly digestible fiber tended to have lower peak concentrations of glucose, and goats fed at the high DMI had a higher clearance of glucose. The plasma insulin response to the glucose test was not altered by treatment. The ratio of plasma insulin to glucose was numerically, although nonsignificantly, higher for goats fed at the high rate of DMI. Most of the differences concerning glucose metabolism, insulin response, and tissue responsiveness to insulin appeared to be primarily due to the amount of feed offered and not to the type of diets fed to dairy goats in midlactation.

Knowledge for animal breeding

The opportunities and knowledge required to improve the technology of animal breeding are assessed relative to the baseline of ca. 2 % annual genetic improvement that is possible at present. Present knowledge of the control of reproduction is such that multiple ovulation and embryo transfer (m.o.e.t.) schemes could virtually double the rate of genetic change in species such as cattle where the annual selection intensity is limited by the reproductive rate. There are direct indications from research that new knowledge to facilitate recognition of genetic merit could have a similar impact. Physical techniques such as computerized tomography could aid the assessment of carcass characteristics. Biological knowledge indicates that while biochemical techniques (enzymes or restriction fragment length polymorphism, r.f.l.p.), based on the importance of single alleles may make specific contributions, physiological techniques may have a more general role to play in the recognition of genetic merit. Research in molecular biology points the way to gene transfer, the original genetics of extra specific genes and the production of novel products such as blood clotting factors by domestic animals. Genetic improvement is welfare and ‘Green’ positive; once achieved, it is maintained with no further input but, as a long-term investment, there is the need for a social framework to ensure the application of knowledge to technology.

Enzyme immunoassay for the determination of bovine growth hormone using avidin-biotin-peroxidase complexes

The development of a competitive enzyme immunoassay for bovine growth hormone (bGH) is described. Antiserum, raised in rabbits and diluted 1/250,000, was preincubated with samples and free antibodies from the reaction mixture were immobilized using a microtiter plates coated with bGH. Bound antibodies remaining from the preincubation were visualized using a biotinylated second antibody as a bridge for subsequent amplification by an avidin-biotin-peroxidase complex. The measuring range was between 0.5 and 100 ng/ml. Cross-reactivity with other pituitary hormones was < 0.1%. The intra- and interassay coefficients of variations were 8.1 and 12.7%, respectively, and the recovery of added bGH was 110%. To validate the assay, two bull calves were treated with bGH-releasing factor. The response showed that there was an immediate rise in immunoreactive bGH which peaked after 5 and 15 min at 200 ng/ml and 60 ng/ml, respectively. This enzyme immunoassay is an economic and sensitive alternative to the established radioimmunoassay, and the first competitive enzyme immunoassay described for bGH.

Somatotropin and lactation

Somatotropin, which can now be produced by biotechnology, could have an enormous impact on the dairy industry. Milk yield has been increased up to 40% with daily injections of somatotropin. Cows adjust their nutrient intake to support this increase. Somatotropin does not adversely affect cows' health, although all studies to date have been for less than one complete lactation. The search for a single biochemical or physiological event to account for the effects of somatotropin on milk production is elusive. Coordinated changes in many tissues and physiological processes occur to support the increases in the synthesis of lactose, fat, and protein in the mammary gland. Changes in the irreversible loss and oxidation rates of two key metabolites, glucose and free fatty acids, can quantitatively account for increases in lactose and milk fat during the short-term administration of somatotropin. Similarly, feed intake and live weight changes can account for increases in milk production in the longer experiments. Parallels between physiological changes that occur during somatotropin administration and differences between genetically high and lower yielding cows are highlighted, and the rates of improvement that can be expected from various new technologies are quantified. Existing data on the safety of somatotropin to both the consumer and the animal are evaluated.