THE EFFECT OF GROWTH HORMONE ON THE YIELD AND COMPOSITION OF COWS' MILK

Abstract

SUMMARY Using a single injection technique, a study has been made of the effect of growth hormone (GH) on the yield and composition of cows' milk. Six lactating cows comprising three sets of identical twins each received six different injection levels during a 78-day period. Quantities of GH injected ranged from 6·25 to 200 mg in geometric progression, and responses were measured for milk yield (lb.), fat percentage, and solids-not-fat percentage, together with the derivatives of these three. Quantities of food consumed by the animals were calculated from the weights of food offered and refused, and total dry-matter intakes were derived from dry-matter estimates obtained on every 3rd day. Results from this experiment show that there is a highly significant linear relationship between log weight of GH injected (mg) and the increase in milk yield (lb.) obtained. Significant linear relationships have also been found between log weight of GH injected (mg) and the degree of change obtained in each of the following milk components: water (1b.); fat (1b.); and solids-not-fat (lb.). Further analysis of the data has shown that by increasing the quantity of GH injected, significant increases are produced in the yield of fat (lb.) relative to the yield of non-fatty-solids (lb.). This is attributed to a significant negative relationship between log weight of GH injected (mg) and the extent of the change in the solids-not-fat percentage. Administration of GH produces increases in the efficiency of conversion of food dry matter to milk and the extent of these changes is closely related to those of milk yield. The results are discussed in relation to previously published findings, and possible explanations for some of the phenomena noted are advanced.