Calving sub-season and reproductive efficiency of beef cows

It was evaluated in this work the effect of calving sub-season on the pregnancy rate of 7,726 multiparous Hereford beef cows grazing on natural pastures in southern Brazil, from 1994 to 2007. Calving sub-season periods were divided in 20-day intervals from August 12(th) to 31(st), from September 1(st) to 20(th), from September 21(st) to October 10(th), from October 11(th) to 31(st), from November 1(st) to 20(th). Calving in each sub-season and pregnancy rate (PR) were analyzed 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(st) to 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 November 1(st) to 20(th)). Calving sub-season significantly influenced pregnancy rate. Cows which calved in the initial sub-season showed higher pregnancy rates than cows which calved from the fourth calving sub-season. However this difference did not occur in 1999 and in 2007 since calving rates did not statistically differ among the five sub-seasons. In 1998 and 2004, calving rates were lower and the effects of calving sub-season on pregnancy rates were higher, possibly due to climate variations. In general, pregnancy rate decreases as calving occurs later in the year, particularly in years when the weather adversely affects pasture growth.