Invited review: Methods to determine body fat reserves in the dairy cow with special regard to ultrasonographic measurement of backfat thickness.
A meta-analysis was conducted to evaluate the effects of body condition score (BCS) at parturition and at first AI, and of body condition change during the early lactation period on the variables pregnancy rate at first AI and number of days open in dairy cattle. Inclusion criteria for the publications were: comparison of at least two groups of animals of different categories of BCS or change (independent variables) and consideration of pregnancy rate at first AI or number of days open (dependent variables). Fifteen papers were selected and 23 excluded. Data corresponding to 7733 cows from 11 studies described in 10 papers were included in the analysis of pregnancy rate at first AI, and those derived from 4529 cows from 11 studies described in 10 papers were used to analyze effects on the number of days open. Analyses were stratified according to study design and milk production characteristics. We defined low, intermediate and high categories of BCS for values lower than 2.5, from 2.5 to 3.5, or higher than 3.5, respectively. The categories of body condition change were increase (gain in score), slight loss (0-0.5 point loss), moderate loss (0.6-1 point loss), and severe loss (over 1 unit loss). Intermediate body condition at parturition and at first AI, and a slight loss during the early lactation period were used as reference categories. The effects of body condition on the variable pregnancy rate at first AI were highly heterogeneous, while all the studies considering the number of days open presented homogeneous results. A clear association between body condition category and pregnancy rate at first AI was detected only when the effect of a low score at parturition was analyzed: pregnancy rate at first AI significantly decreased by about 10% in cows delivering in poor condition. Animals with a high BCS at parturition showed a significant reduction in the number of days open of 5.8 or 11.7 when compared with animals with an intermediate or low body condition, respectively. In animals in the high body condition category at first AI, the variable days open showed a significant drop of 11.9 or 24.1, compared to animals in the intermediate or low categories, respectively. A severe loss in score during early lactation was related to a significant (10.6) increase in the number of days open, while a slight or moderate body condition change, either loss or gain in score, was not significantly related to the variable days open. We found that the number of days open was a good indicator of the effects of BCS or change in score on reproductive performance in dairy cattle. The variable pregnancy rate at first AI, however, yielded heterogeneous results among studies.