During the recent Cattlemen’s College at the Cattle Industry Convention, University of Florida animal scientist Cliff Lamb, PhD, discussed the close connection between nutrition and reproductive efficiency in beef cows. He also stressed the value of using BCS as a tool for monitoring the nutritional status of cows and thus their likelihood to breed back in a timely manner.
Research has shown, Lamb says, that cows at BCS 3 will, on average, return to estrus in 88 days, while cows at BCS 4 return to estrus in 70 days and BCS 4 cows do so in 51 days. Cows at BCS 4, he adds, will consistently have lower pregnancy rates at 20, 40 and 60 days into the breeding season compared with cows at BCS 6.
While the actual BCS is important, the trend – whether a cow is gaining or losing condition at calving – probably has a greater effect on fertility. Between calving and re-breeding, Lamb says, cows need to meet their nutritional needs for maintenance, plus for lactation, gaining weight lost in calving and repairing the reproductive tract.
Feeding cows to regain condition during that period usually turns out futile, or cost-prohibitive. Ideally, cows would remain in the BCS range of 5 to 6 year around. With that not being realistic in most production systems, the best bet is to manage cows such that they are gaining condition, or certainly not losing condition, as they head into calving season.
Lamb cites a Florida study in which researchers examined the estrous cyclicity response of heifers to changes in BCS. They developed one group of heifers to BCS 5 and another group to BCS 6, then put both on severely restricted diets to measure at what condition they would stop cycling and how long it would take them to resume cycling once feed was resumed. Both groups stopped cycling once they declined to BCS 3, although, not surprisingly, it took the BCS 7 heifers 90 days longer to become that thin. The heifers that started out at BCS 5 had to return to BCS 5 to return to cycling, and did so in 68 days. The heifers that started out at BCS 7 had to reach BCS 6 to return to cycling, which took, on average, 79 days. Lamb concludes that it is better to maintain females at BCS 5 than to let them decline from BCS 7.