Influence of Fertility Level and Treatment of Semen on Nonreturn Decline from 29 to 180 Days Following Artificial Service

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Abstract
Semen from 40 bulls used for over 50,000 artificial services was analyzed for bull differences in nonreturn decline for each day following service up to 180-days post-service. Bulls were divided into 4 fertility groups on the basis of their 180-day nonreturns. Semen from low fertility bulls was associated with a higher than average number of returns to service during the 25- to 29-day period. The decline from 30 to 59 days was significantly higher for low fertility bulls, with no significant difference between fertility groups after 60 days. The high correlation for among-bulls between the 29- and 180-day nonreturn rates (r = 10.87; P< 0.01) makes the 29-day nonreturn a reliable early measure of breeding efficiency. The 180-day nonreturn was negatively correlated with the 30- to 59-day nonreturn decline (r = -0.70; P< 0.01) and with the 60- to 89-day decline (r = -0.47; P < 0.01). The 29-day nonreturn was negatively correlated with the 30- to 59-day nonreturn decline (r = -0.32; P < 0.05) but not with the 60- to 89-day decline. Combinations of penicillin-streptomycin and penicillin-streptomycin-sulfanilamide reduced nonreturn decline for all 4 fertility groups, with semen of low fertility showing a significantly greater reduction. Penicillin alone reduced nonreturn decline only for low-fertility bulls.