Response to selection for milk yield and metabolic challenges in primiparous dairy cows

Metabolic Responses to Epinephrine by Periparturient Dairy Cows Fed Prepartum Diets Differing in Predicted Metabolizable Protein Supply

We determined the effects of prepartum dietary metabolizable protein (MP) supply on lipolytic and glucose response to epinephrine stimulus during the periparturient period. Twelve non-lactating cows in second or greater pregnancies were assigned to a low MP (LMP) diet formulated to provide ~900 g/day MP or a high MP (HMP) diet to provide ~1,100 g/day MP. Cows received prepartum diets from 28 days before expected parturition to day of parturition and then received a common lactation diet until 35 days postpartum. Blood was sampled weekly to day −7, daily to day 7, and weekly through day 35 relative to parturition for analysis of glucose, non-esterified fatty acids (NEFAs), 3-hydroxybutyrate (BHB), urea N, and total protein concentrations. Epinephrine challenges were conducted on day −10 before expected parturition and days 7 and 14 postpartum. Epinephrine [1.4 μg/kg body weight (BW)] was administered via jugular vein and blood was collected from the opposite jugular vein from −45 min to +120 min relative to epinephrine administration. Plasma was analyzed for glucose and NEFA concentrations. Prepartum dry matter intake (DMI) (14.7 and 12.6 ± 1.5 kg/day for LMP and HMP, respectively), postpartum DMI (21.4 and 19.4 ± 1.3 kg/day for LMP and HMP, respectively), and 4% fat-corrected milk (FCM) (37.0 and 36.8 ± 1.5 kg/day for LMP and HMP, respectively) did not differ significantly between diets. Cows fed HMP had higher plasma concentrations of urea N prepartum than cows fed LMP (14.0 vs. 7.9 ± 0.8 mg/dl). Cows fed HMP tended to have greater prepartum concentrations of total protein in plasma than those fed LMP (7.5 vs. 7.0 ± 0.2 g/dl). The area under the curve (AUC) for NEFA response to epinephrine did not differ between diets, but differed by day relative to parturition [8,284, 29,018, and 18,219 ± 2,302 min × (μeq/l) for days −10, 7, and 14, respectively]. Maximal response of NEFA concentration to epinephrine was greater for HMP than for LMP (744 vs. 438 ± 72 μeq/l). The glucose AUC did not differ between diets or among days. Lipolytic response, but not glucose response, to epinephrine was enhanced during the early postpartum period relative to the late dry period. However, those responses were not affected by prepartum MP supply.

The effect of breed, parity and body fatness on the lipolytic response of dairy cows

The objective of this study was to examine the effect of breed, parity and body fatness on the lipolytic response of dairy cows. The lipolytic response was estimated as the plasma non-esterified fatty acid (NEFA) response to an adrenergic challenge. Four challenges per lactation were carried out on 124 Danish Holsteins, 101 Danish Red and 82 Jerseys through consecutive lactations. Within breed, there were two genetic lines. Cows were equally distributed across two feeding treatments, a normal and a low energy total mixed ration. Diet composition was constant throughout lactation. The lipolytic response was significantly affected by breed, parity and stage of lactation. Lipolytic response was greater in early lactation than other stages in lactation (P 0·001). The larger breeds had a higher lipolytic response than Jerseys (P 0·05), and showed an increase in lipolytic response between first and second parity (P 0·05), but not between second and third lactation. Lipolytic response in Jerseys was not significantly affected by parity. No line or feeding treatment effects were observed on the lipolytic response. Ultrasound measurement of the area of subcutaneous backfat was used to estimate effect of body fatness on the lipolytic response. There was an increase in lipolytic response with increasing body fatness (P 0·05) in mid lactation and the dry period. The slope of this was not affected by breed or parity. It was concluded that the lipolytic response of dairy cows, and by implication the responsiveness of the lipid reserves, varies according to breed, parity and physiological state.

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.

Short Communication: Composition of Milk Protein and Milk Fatty Acids Is Stable for Cows Differing in Genetic Merit for Milk Production

Changing the composition of milk protein and of milk fatty acids alters nutritional and physical properties of dairy products and their consumer appeal. Genetic selection for milk yield decreases concentrations of milk protein and of milk fat. Little is known, however, about how the decrease affects composition of milk protein and milk fatty acids. The objective of this study was to quantify changes in composition of milk protein and of milk fatty acids in cows differing in genetic merit for milk production. Three measures of genetic merit for milk production were used for each cow: genetic line, parent average predicted transmitting ability (PTA) for milk, and cow milk PTA. Composition of milk protein and milk fatty acids were compared in 448 milk samples from 178 cows representing 2 divergent lines of Holsteins that were bred for high or average PTA for milk and combined milk protein and fat yield. High-line cows (n = 97) produced more milk that contained less fat and had higher proportions of alphaS1-casein in milk protein than did average-line cows (n = 81). We additionally obtained from 233 cows (178 cows representing the 2 genetic lines and 55 cows with ancestors from both genetic lines) the parent average milk PTA and cow milk PTA and compared composition of milk protein and of milk fatty acids in 592 milk samples. Cows whose parent average milk PTA was above or equal to the median of the 233 cows produced more milk that contained less protein and less fat and that tended to have greater proportions of alphaS1-casein in milk protein than cows whose average milk PTA was below the median. Similarly, cows with above or equal median milk PTA of the 233 cows produced more milk that contained less protein and less fat and had greater proportions of alphaS1-casein in milk protein than did cows with below-median milk PTA. Milk fatty acid composition was not consistently different between groups. Therefore, selection for milk yield decreased concentrations of milk protein and milk fat but had little effect on composition of milk protein and milk fatty acids.

Pre-pubertal measurements of the somatotrophic axis as predictors of milk production in Holstein-Friesian dairy cows

This study investigated possible relationships between measurements of the somatotrophic axis in pre-pubertal dairy calves and subsequent milk yields. Endogenous growth hormone (GH) release was measured through a fed and fasted period in fifty 6-month-old Holstein-Friesian heifers and they were then challenged with growth hormone-releasing factor (GRF) to assess their GH release pattern. Insulin-like growth factor-I (IGF-I), insulin and glucose concentrations were measured in relation to time of feeding. Cows were subsequently monitored through their first three lactations to record peak and 305-day milk yields. In the first lactation, milk energy output for the first 120 days of lactation was also calculated. The mean 305-day milk yield increased from 7417 +/- 191 kg in the first lactation (n=37) to 8749 +/- 252 kg in the third (n=25). There were no significant relationships between any measures of GH secretion and peak or 305-day yield in any lactation. A highly significant positive relationship was established between the GH peak measured 10 min post-GRF challenge and 120-day milk energy values in the first lactation. This relationship was, however, only present in the sub-population of 12 cows culled after one or two lactations and was absent in the 25 animals remaining for the third lactation. There were no significant relationships between pre-pubertal IGF-I and fed or fasted insulin or glucose concentrations and any subsequent measurement of yield. The usefulness of GH secretagogue challenges in calves as a predictive test for future milk production is thus limited but may have some bearing on nutrient partitioning and longevity.

Expression of Growth Hormone Receptor 1A mRNA is Decreased in Dairy Cows but not in Beef Cows at Parturition

The promoter controlling expression of a major bovine growth hormone (GH) receptor (GHR) mRNA variant, GHR 1A, contains a common DNA element for transcription factors hepatocyte nuclear factor 4alpha (HNF-4alpha), hepatocyte nuclear factor 4gamma (HNF-4gamma), and chicken ovalbumin transcription factor II (COUP-TFII). Expression of GHR 1A mRNA is decreased in the liver of dairy cows at parturition. The objective of this study was to determine whether reduced expression of GHR 1A mRNA in dairy cows at parturition is associated with changed expression of HNF-4alpha, HNF-4gamma, or COUP-TFII mRNA. Liver biopsy samples were taken from multiparous Holstein cows 7 to 23 d before parturition, within 24 h after parturition (i.e., at parturition), and 8 to 18 d after parturition, and the relative amounts of GHR 1A, insulin-like growth factor-I (IGF-I), HNF-4alpha, HNF-4gamma, and COUP-TFII mRNA in these samples were measured by ribonuclease protection assays. As expected, expression of GHR 1A, total GHR, and IGF-I mRNA was decreased at parturition, compared with that detected prepartum or during the postpartum period. Expression of HNF-4alpha and COUP-TFII mRNA was unchanged, but that of HNF-4gamma mRNA was increased at parturition. The same study was also conducted in multiparous Angus cows 7 to 23 d before parturition, at parturition, and 8 to 18 d after parturition. Neither expression of GHR 1A, total GHR, or IGF-I mRNA, nor expression of HNF-4alpha, COUP-TFII, or HNF-4gamma mRNA was changed in the liver of beef cows at parturition. These results together suggest that, at the molecular level, decreased expression of GHR 1A mRNA in the liver of dairy cows at parturition may involve increased expression of HNF-4gamma mRNA and that, at the systemic level, decreased expression of GHR 1A mRNA is not a direct result of the end of pregnancy, parturition, or the initiation of lactation.

Effects of genetic merit and varying dietary protein degradability on lactating dairy cows

Eighty two multiparous Holstein cows were blocked by genetic merit (high vs. low) and assigned to one of two treatments [high rumen-undegradable protein (RUP): rumen-degradable protein (RDP) vs. low RUP: RDP] from d 21 before to d 150 after calving to study the effects of these treatments on production and reproductive performance. Diets were isonitrogenous (dry cow 10.5% crude protein; lactating cow 19.3%), isoenergetic (dry cow 10.0 MJ of metabolizable energy (ME); lactating cow 10.9 MJ of ME) and fed as total mixed rations. Feeding more RUP significantly increased dry matter intake and milk yield, reduced body tissue mobilization, and lowered concentrations of serum nonesterified fatty acids (NEFA) and plasma urea. Expression of estrus at first ovulation was improved, first service conception rate was higher, and calving to conception interval was shorter for the high RUP group. Cows of high genetic merit produced more milk, mobilized more body tissue, and had higher concentrations of plasma growth hormone. The dry matter intake and concentrations of blood metabolites did not significantly differ with genetic merit. Expression of estrus at first ovulation was significantly lower for cows of high genetic merit. Serum NEFA concentrations were significantly higher, and estrus was not observed at first ovulation for cows of higher genetic merit fed the low RUP diet. The interaction between dietary RUP and genetic merit was not significant for other measures of performance or fertility. Feeding a low RUP: high RDP diet had negative effects on some aspects of production and reproductive performance. The effects of diet on NEFA concentrations and estrus display were greater in cows of high genetic merit, indicating that potential interactions should be evaluated in future reproductive studies involving protein and fertility.

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.

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.