Research data sets have shown conclusively that young cows that calve in thin body condition but regain weight and condition going into the breeding season do not rebreed at the same rate as those that calve in good condition and maintain that condition into the breeding season. The following table from Missouri researchers illustrates the number of days between calving to the return to heat cycles depending on body condition at calving and body condition change after calving. The data was compiled using two-year old Angus first-calf heifers.
Table 1. Predicted number of days from calving to first heat cycle as affected by body condition score at calving and body condition score change after calving in young beef cows. (Body condition score scale: 1 = emaciated; 9 = obese)
This data clearly points out that young cows that calve in thin body condition (BCS=3 or 4) cannot gain enough body condition after calving to return to heat cycles as quickly as cows that calve in moderate body condition (BCS = 5.5) and maintain or lose only a slight amount of condition. Pay particular attention to the heifers that calved in a body condition score of 4 and then were fed enough of a high energy diet to gain 1.5 condition scores by day 90. Compare them with heifers that calved in a body condition score of 5.5 but lost a half score and were 5.0 at 90 days. The heifers that calved in poor body condition and were fed well did not return to estrus as quickly (111 days vs. 102 days) as the heifers that were in good body condition and lost a small amount of body condition after calving. It is very difficult to add body condition on young lactating cows in most range situations. Cows must be rebred by 85 days after calving to calve again at the same time next year. Notice that none of the averages for two-year old cows that calved in thin body condition were recycling in time to maintain a 12 month calving interval. This illustrates why many ranches breed the yearling heifers 2 to 3 weeks ahead of the start of the breeding season for adult cows. It gives these heifers extra days to return to heat cycles and therefore breed at about the same time as the other cows in the herd.
Source: Glenn Selk, Oklahoma State University Emeritus Extension Animal Scientist