| Phillip Lancaster, MS, PhD Ruminant nutritionist Beef Cattle Institute Kansas State University palancaster@vet.k-state.edu |
As we begin spring calving season, focus is often on the calf – making sure it gets colostrum, is healthy, nursing, and thriving – but the dam may get less attention. The focus on the dam is not primarily about the calf by her side, but rather about the calf she will have next year or more specifically when she will have that calf. Some good targets for reproductive success include a 365-day calving interval, 80% of cows calving in first 42 days of the calving season, and < 5% open cows. Achieving these targets revolves around managing the postpartum interval, which is that time period between calving and resumption of estrous cycles.
Reproductive function in cows is highly related to the amount of body fat and the plane of nutrition, most likely the amount of blood glucose supported by the diet. Cows with less than body condition score (BCS) of 5 at start of breeding have longer postpartum intervals and thus longer calving intervals. One of the goals then is to have cows at BCS 5 at calving because getting cows to gain body fat during early lactation is difficult and not cost effective. However, even cows at BCS 5 at calving but lose weight between calving and start of the breeding season have longer postpartum intervals (Figure 1).
To maintain a 365-day calving interval, the cow must rebreed by 80 days postpartum. Previous research indicates that cows maintaining body weight after calving, resume estrous cycles 30 to 40 days after calving, but cows losing body weight after calving don’t resume estrous cycles until 40 to 60 days after calving. Thus, cows maintaining body weight have 2 estrous cycles to get rebred, whereas, cows losing body weight have only 1 estrous cycle to get rebred to maintain the 365-day calving interval.
First-calf heifers are a unique subset of the cow herd when it comes to the postpartum interval. First-calf heifers take about 20 days longer to resume estrous cycles after calving than mature cows and should calve at least 3 weeks prior to the mature cows. But also, first-calf heifers need a amount of body fat to minimize the postpartum interval. First-calf heifers have shorter postpartum intervals when calving at BCS 6 than BCS 5, whereas, BCS 5 is optimum for mature cows. Additionally, since first-calf heifers are still growing themselves, they need a higher plane of nutrition after calving to resume estrous cycles – maintaining body weight is not satisfactory, first-calf heifers need to be gaining body weight.
The amount of supplement needed for mature cows and first-calf heifers to maintain or gain weight is presented in Table 1. Mature cows would need to be fed 1.4 to 3.4 lb/day of a supplement to maintain body weight, and first-calf heifers would need to be fed 5.4 to 6.6 lb/day of a supplement to gain 1 lb/day. These calculations are based on 1300-lb mature cow weight consuming hay with 57% TDN and supplement with 80% TDN, and assuming protein requirements are met. Typical TDN values of common feedstuffs used in supplements such as soybean hulls (74%), wheat midds (75%), dried distiller’s grains (88%), and corn (88%) straddle this value and can be used to create a supplement.
