Heifer calves born the first 21 days of calving will make the best replacements, according to University of Nebraska research.
Heifer calves at the Gudmundsen Sandhills Laboratory born between 1997 and 2009 to females three years and older were classified as being born during the first, second or third 21-day period of calving. Performance and reproductive variables were measured through calving on all heifers kept as replacements.
Heifers born in the first calving period were 16 and 36 days older on average than heifers born in the second and third period, respectively, and had the lowest birth weights. First-born heifers had greater weaning weight (483 vs. 470 and 434 lbs.) and greater pre-breeding weight (653 vs. 644 and 608 lbs.) than second- or third-period heifers, respectively.
Percent of heifers cycling at the beginning of their first breeding season was greatest for the early-born heifers (70% vs. 58% and 39%). Pregnancy rate was also greater in early-born heifers (90% vs. 86% and 78%).
Early-born heifers calved earlier as first-calf replacements and thus had older and heavier calves at weaning with a greater opportunity to rebreed early for their second calf. It appears beneficial to identify first-born heifer calves and select these as replacements accordingly.
http://www.ianrpubs.unl.edu/epublic/live/mp95/build/mp95.pdf, Page 18.
Scott B. Laudert, Ph.D., is a beef cattle technical consultant and former Kansas State University Extension livestock specialist based in Woodland Park, CO. He can be reached at 719-660-4473.