Increased Lactation Response in Cattle with Thyroxine 50% above Thyroxine Secretion Rate

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Summary Nine lactating dairy cows were injected with thyroxine daily at a level 50% above their individual winter thyroxine secretion rates during the spring and summer to determine the effect upon milk yield, body temperature, and body weight. The body pool of thyroxine was raised to its ultimate level by one injection of thyroxine at the initiation of the experiment. One cow reached a new peak of milk yield (weekly average) during the 2nd wk, one the 3rd wk, and the seven other cows the 4th wk. The mean increase in milk yield was 27.6%, with a range from 12.1 to 67.9%. The effect of thyroxine at this level during the summer appeared to pose no problem in heat dissipation. The elevation in rectal temperature of the experimental cows was very small, with fluctuations in treated and nontreated cows quite comparable. The mean body weight loss was 9.7%, with a range of 3.7 to 14.2%. This loss was regained within 2 wk after cessation of thyroxine treatment. On the basis of the observed range in thyroxine secretion rate, and the beneficial effect of thyroxine upon milk yield administered at 50% above their secretion rate, it was suggested that the level of feeding thyroprotein might be reduced from 15g to about 10 g/1,000lb body weight/day in the great majority of dairy cows.