Effects of recombinant bovine somatotropin on fatty acid composition of milk from cows in late lactation

To investigate the effects on milk fatty acid composition of recombinant bovine somatotropin (rBST) administered under South African conditions, 26 dairy cows were paired into two groups 84-112 days after calving. At twoweekly intervals the cows in the experimental group were injected with 500mg of rBST in a slow-release formulation, while cows in the control group were injected with the excipient. Milk specimens taken at two-weekly intervals from 36-40 weeks post partum until drying off were analysed for fatty acid composition. The cows given rBST produced milk with less short-chain (C4), more medium-chain (CS-CI6) and less long-chain (CIS) acids. Hexanoic acid content was unaffected. rBST administration did not affect the proportion of unsaturated fats in the milk. Differences in milk composition between cows and through time were larger than those due to rBST administration. Treated and control cows showed synchronized fluctuations in milk fatty acid composition that were larger than the differences due to rBST.