Calving sub-season and reproductive efficiency of beef cows

[摘要]

It was evaluatedin this work the effect of calving sub-season on the pregnancy rate of 7,726multiparous Hereford beef cows grazing on natural pastures in southern Brazil,from 1994 to 2007. Calving sub-season periods were divided in 20-day intervalsfrom August 12^th to 31^st; from September 1^stto 20^th; from September 21^st to October 10^th;from October 11^th to 31^st; from November 1^stto 20^th. Calving in each sub-season and pregnancy rate (PR) wereanalyzed by the Chi-square test. Overall means of each sub-season were 92.7%(from August 12^th to 31^st), 90.6% (from September 1^stto 20^th), 82.1% (from September 21^st to October 10^th),77.7% (from October 11^th to 31^st), and 70.6% (from November1^st to 20^th). Calving sub-season significantly influencedpregnancy rate. Cows which calved in the initial sub-season showed higher pregnancyrates than cows which calved from the fourth calving sub-season. However thisdifference did not occur in 1999 and in 2007 since calving rates did not statisticallydiffer among the five sub-seasons. In 1998 and 2004, calving rates were lowerand the effects of calving sub-season on pregnancy rates were higher, possiblydue to climate variations. In general, pregnancy rate decreases as calving occurslater in the year, particularly in years when the weather adversely affectspasture growth.

[学科分类] 兽医学