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WS Influence of first calving date on stayability in Bos indicus crossbred cows
Longevity is one of the most important, complex, and difficult to improve traits sought by cow-calf producers. Consequently, a measurement or tool that could be utilized early in a cow’s life to predict her future reproductive performance would be advantageous to producers and researchers alike. In this study, we sought to determine the effect of first calving season period on stayability in Nellore-crossbred females through 5 yr, 6 yr, and 7yr of age. Stayability through each age was scored as a binary trait, with 1 indicating the cow remained in the herd and 0 indicating she was culled, given either a perfect calving or weaning record. Each female was assigned a value of 1, 2, 3, or 4 corresponding to the respective 21-d period of her first calving season (for first, second, or third 21-d period, or > 63 d, respectively). Cow stayability models were evaluated through mixed model procedures (PROC MIXED in SAS). Of the cows with perfect calving records, more (P < 0.05) females that calved in the first 21-d period remained in the herd than those that calved in the second 21-d period through 5 yr (66.9% vs. 53.6%), 6 yr (60.0% vs. 45.9%), and 7 yrs of age (56.7% vs. 39.3%). They also differed (P < 0.005) from females whose first calf was born 63 d or later into the calving season through ages 5 (66.9% vs. 36.0%), 6 (60.0% vs. 29.5%), and 7 (56.7% vs. 27.2%). Of the cows with a perfect weaning record, more (P < 0.05) of the females that calved in the first 21-d period of the calving season remained in the herd through 5 yr (56.1% vs. 31.0%) and 6 yrs of age (48.3% vs. 26.0%) than heifers whose calf was born at the end of the calving season. These results document that regardless of the culling criteria, Bos indicus crossbred heifers that calve early in their first calving season are more likely to maintain a perfect calving or weaning record later in life than females that calve late in the first calving season. Consequently, there is potential that the heifer's first calving season period may be used as valuation or culling criteria when selecting for stayability and longevity, or when merchandizing beef replacements.
Keywords:
beef cows, calving season, stayability