Effect of Administration of L-Thyroxine 25 and 50% Above Secretion Rate on Lactating Cows

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Abstract
The thyroxine secretion rates (TSR) of 6 mature cows which calved normally, 12 heifers varying in age from 11/2 to 4 yr., which were induced to lactate experimentally, and one 13-yr.-old cow which had been lactating continuously for about 10 yr. had been determined on several occasions. L-thyroxine (T4) was injected daily subcutaneously for 7 wk. at a level 2 5% above their highest TSR, beginning in Sept., 1963. At the end of this period, the T4 was increased to 50% above their TSR for a 2nd period of 7 wk. The mean milk production of the 6 mature cows was 46.9 lb. in their 26th to 39th week of lactation. The increase in production on 125% T4 ranged from 8.5 to 81.2%, with a mean of 27.0%. The mean daily milk yield reached a maximum of 56.1 lb. the 3rd wk, which was 9.2 lb. above the mean pretreatment level. Five 11/2-yr-old heifers were producing 18.3 lb. of milk before treatment. The increase in this group ranged from 8.7 to 62.9%, with a mean of 31%. The mean milk yield reached a maximum of 21.4 lb, an increase of 3.1 lb. The 7 heifers ranging from 2 to 4 yr. old were producing a mean of 24 lb./day. Their increase ranged from 11.6 to 62.0%, with a mean of 34%. The highest mean production of 31.0 lb. was reached the 3rd wk., an increase of 7 lb./day. The old cow showed an 83.2% increase of 13.9 lb. /day.