Further Studies in the Induction of Estrus in Lactating Sows with Equine Gonadotrophin

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Abstract
Seven uninjected, lactating sows were observed for postpartum heat, and all evidenced estrus with an average interval of 2.57±.30 days following parturition. Ten uninjected lactating sows were tested daily for estrus from day-8 to day-56 of lactation without showing symptoms of estrus. A single intramuscular injection of equine gonadotrophin was given to 101 lactating sows of 4 breeds. The injection induced estrus in 76% of the sows injected between the 20th and 39th days of lactation and in 86% of those injected between the 40th and 50th days. Following breeding at this time 44% farrowed when injected between day-20 and day-39 while 66% farrowed in the later injected group. Results were not affected by number of suckling young at time of injection or length of interval from time of injection to estrus. Previous use of gonadotrophin did not appear to effect the efficacy in subsequent breeding seasons. Breeding sows a second time, 24 hours after the first, resulted in a higher percentage of sows farrowing than breeding only once on the first day of estrus. A field trial with 75 injections gave results somewhat less effective than those of the University herd. The reason for this difference is discussed. The mean interval between injection and estrus was 4.81±.14 days in the University herd and 3.98±.13 days in the field trial.

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