Effects of insulin, recombinant bovine somatotropin (rbST) and their interaction on DMI and milk fat production in dairy cows

Dose response of lactating dairy ewes during suckling and milking to bovine somatotropin

The objectives were to determine the effect of administering recombinant bovine ST (bST) every 14 d on milk yield and milk composition in dairy ewes and to assess the possible effects of treatment on milk yield loss that occurs at lamb weaning. Seventy-two lactating dairy ewes were separated into 4 groups at lambing, and each group received no bST (control) or 40, 80, or 120 mg of bST every 14 d until wk 20 of lactation. During the first 5 wk of lactation, ewes suckled their lambs during the night and were milked once daily (period 1). After weaning, from 6 to 22 wk of lactation, ewes were exclusively milked twice daily (period 2). Actual milk yield, potential milk yield, and milk component percentages were recorded weekly throughout lactation. In period 1, bST-treated groups increased yields of potential milk (P = 0.04) and the corresponding 6% fat-corrected milk (FCM; P = 0.04) but not actual milk yield (P = 0.42) compared with the untreated group. In period 2, treated groups increased potential (P < 0.01) and actual (P < 0.01) milk yields as well as their corresponding 6% FCM (potential, P < 0.01; actual, P < 0.01), in comparison with the untreated group. The group treated with 120 mg of bST every 14 d yielded 39% more actual milk and 44% more 6% FCM than the control group for period 2. At weaning, between the last week of period 1 and the first week of period 2, treated groups showed similar absolute (P = 0.15) and relative (P = 0.33) values for the potential milk losses compared with the control group. Treatments increased (P = 0.04) actual milk fat percentages during period 1, but did not affect the other milk components or somatic cell count. In summary, bST increased potential milk yield throughout lactation and actual milk yield only after weaning in dairy ewes. In dairy ewes, bST was not useful for reducing the milk yield loss that occurred at weaning.

Short communication: Regulation of milk fat yield and fatty acid composition by insulin

Diet-induced milk fat depression in dairy cows has been known for many years and several theories have been proposed. One that continues to receive support is the glucogenic-insulin theory. Previous studies testing this theory using a hyperinsulinemic-euglycemic clamp have had variable results attributable to variability in the use of body fat reserves as a source of milk fatty acids. Our objective was to test the glucogenic-insulin theory using cows immediately postpartum, a period when the use of body fat for milk fat synthesis is greatest. During wk 2 postpartum, 5 cows were given a 2-d baseline period and then clamped for 4 d. Insulin was increased more than 2-fold during the clamp while the blood glucose concentration was maintained. Milk yield was not altered by administration of the clamp (38.7 vs. 39.0 +/- 1.4 kg/d); however, the milk fat percentage and yield were reduced by 27% and plasma nonesterified fatty acids were reduced by 68%. Analysis of the milk fatty acid composition revealed that the decrease in milk fat yield during use of the clamp was almost exclusively due to reductions in preformed fatty acids; this is the exact opposite of what is observed with diet-induced milk fat depression. Therefore, our results do not support the glucogenic-insulin theory of diet-induced milk fat depression. The results further indicated that reductions in milk fat observed previously with hyperinsulinemic-euglycemic clamps or with glucose or propionate infusions were most likely consequences of the ability of insulin to inhibit lipolysis, thereby limiting the mammary availability of preformed fatty acids mobilized from body reserves.