Relationship between body condition score index and fertility in beef cows subjected to timed artificial insemination

Visual evaluation of body condition score (BCS) is an important method for the subjective quantification of body energy reserves in cattle. The objective of this study was to develop an index to estimate the relationship between BCS and fertility, using 2321 lactating Nelore cows from 29 cohorts in Mato Grosso do Sul and Rondonia states, Brazil enrolled in timed artificial insemination (TAI) programs. All females were evaluated according to BCS (scale 1 to 5) and were included in an estradiol-progesterone-based TAI protocol. Estrus scores were evaluated (scale 1-3) for all cows at TAI. The cows in each TAI protocol were categorized according to pregnancy per AI (P/ AI) into low ( 45%, LF), moderate (45%-60%, MF), and high ( 60%, HF) fertility groups. To evaluate whether the BCS of each cow was in accordance with the target BCS (3 to 4) for beef cows to be enrolled in TAI programs, three variables were calculated: (1) adequate BCS rate (ABR), in which cows with BCS on target received 100% and cows with BCS values outside the target range received 0%; (2) the difference between the real BCS and the target BCS (DBCS); and (3) the adjusted DBCS (ADBCS), in which DBCS was transformed into an inverse value. Finally, the average between ABR and ADBCS resulted in a BCS index (BCSi). Analysis of variance was used to compare BCSi, BCS, and estrus scores among fertility groups, and Tukey's test was used to compare means among groups. There were differences in BCSi between fertility groups (P<0.05). Both the BCSi and BCS were highest in the HF groups, which also showed the highest estrus scores. However, the BCSi provides information that takes into account the proportion of cows with or without adequate BCS, and could be more useful to apply in practice. The results demonstrate that determining the BCSi presents a novel approach that is highly associated with fertility in TAI programs.