Relationships among Weight Change, Body Condition and Reproductive Performance of Range Beef Cows 1

A 5-yr study involving 45 to 78 pregnant Hereford range cows each year evaluated relationships among prepartum nutrition, body condition scores, BW changes and reproductive performance. Four prepartum nutritional treatments were imposed. One group of cows were fed to maintain (M) their November BW until calving in March and April. The other three groups of cows were fed to lose about 5% of their November BW by 8 wk before parturition, then to maintain BW (LM), lose an additional 5% of their BW (LL) or gain 5% of their BW (LG). After calving, all cows were fed to maintain BW. Body condition scores and BW were recorded every 14 d throughout the trial. Linear regression analyses were conducted to examine treatment effects on BW, body condition score and measures of reproductive performance. A discriminant analysis was performed on pregnancy rate and percentage of cows with ovarian luteai activity by 85 d after parturition. The M cows had a greater pregnancy rate (71%) than cows on other treatment groups. The LL cows had a reduced pregnancy rate (42%) compared with LM (51%) and LG (58%) cows. Prepartum nutritional treatment did not affect the days from parturition to conception. Precalving body condition score and November to January BW changes influenced pregnancy rate (P < .001). A cubic response curve described the relationship between pregnancy rate and precalving body condition score for cows with condition scores of 3 through 7. The decrease in body condition score from November to January was correlated (r = . 25 , P < .01) with days to conception. Body condition scores precalving and at the start of the breeding season, along with BW changes between 2 and 4 mo before parturition, are major factors that influence pregnancy rate of range beef cows. (