Several studies have quantified the different advantages that calves born during the first 21 days of the calving season have over those that are born later. Each day a calf is on the ground, it has the opportunity to gain weight, and the relationship between birth date and actual weaning weight is obvious: Older calves are typically heavier at weaning compared with younger calves. However, these effects extend well beyond the time of weaning.
In calves placed in feedlots, the greater feedlot entry weights (a function of greater weaning weights) are followed by heavier final carcass weights, improvements in carcass quality grade and the proportion of carcasses qualifying for premium beef programs for calves born during the first 21 days of the calving season compared with those born later. In addition, a greater proportion of replacement heifers born during the first 21 days of the calving season were cycling at the start of their first breeding season, and this subsequently led to greater overall pregnancy rates compared with heifers born later in the calving season.
We also begin to see impacts of early calving on the cows themselves. The pattern of late-calving cows becoming perpetually late calving and subsequently not becoming pregnant is familiar to all of us. Early calving cows are more likely to become pregnant early in the next breeding season and a recent report (Kill et al., 2012) began to quantify the impacts of replacement heifers calving within the first 21 days of the calving season on longevity in the cow herd.
The average time early calving heifers remained in the herd was 5.1 years compared with only 3.9 years for heifers that calves after the first 21 days of the calving season in a group of 2,195 South Dakota producer-owned cattle. In a group of 16,549 cattle managed at the U.S. Department of Agriculture’s Meat Animal Research Center, heifers calving in the first 21 days, second 21 days and later had an average longevity of 8.2, 7.6 and 7.2 years, respectively. In both cases, data were confined to cattle culled for nonpregnancy, and other types of culls (conformation, temperament, etc.) were removed for the analysis. Taken together, this work showed that early calving heifers had at least a one-calf lifetime advantage compared with late-calving heifers.
This one-calf lifetime advantage also was complemented by extra weaning weight at the end of the breeding season that accumulated to the weight of an additional calf during the lifetime of the cow. Thus heifers that calved during the first 21 days of the calving season had the equivalent of a two-calf lifetime advantage over those heifers that calved after the first 21 days of the calving season. The moral of this story should be to focus on keeping heifers that become pregnant during the first 21 days of the breeding season.