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

Highlights

• Cows subjected to TAI protocols with greater pregnancy per AI have greater Body condition score index.

• Body condition score index presents a novel approach in dealing with the BCS categorical variable.

• BCS data were transformed into an index that is highly associated with the fertility of cows.

Abstract

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.

Introduction

Visual evaluation of body condition score (BCS) is an important method for the subjective quantification of endogenous energy reserves in cattle. This is an easy and inexpensive method, and provides a sufficiently reliable estimation of body energy reserves in Bos indicus cows (Ayres et al., 2009; Silveira et al., 2015). Moreover, it is well adapted to large-scale surveys with large datasets in an environment where animals are subject to considerable variations in body fat (Ezanno et al., 2003). In tropical beef operation areas, such as in the Pantanal wetlands, Cerrado, and Amazon biomes of Brazil, cattle performance under extensive range management is greatly limited by feeding resource availability and quality, especially during the dry season (Ezanno et al., 2003; Baruselli et al., 2004). Thus, the decrease in BCS in postpartum cows raised in these tropical regions is one of the most important factors impairing fertility (Robinson et al., 2006). The profitability of beef farms is highly associated with the capacity of cows to resume ovarian activity soon after parturition and to respond adequately to timed artificial insemination (TAI) protocols to conceive within the first trimester postpartum. Additionally, BCS at calving and during puerperium is known to be associated with the likelihood of early pregnancy in the breeding season (Richards et al., 1986; Moraes et al., 2007).

Evaluation of BCS may be an important method in selecting cows eligible for reproductive management including TAI. Previous studies involving BCS and reproduction in B. indicus cattle (Nelore cows) have used a BCS scoring system with a scale of 1 to 5 points (Ayres et al., 2009; Torres et al., 2014), and have demonstrated that BCS, upon breeding, is highly associated with reproductive performance. Other studies have reported a positive influence of adequate BCS on pregnancy outcomes after TAI in suckled beef cows (Sales et al., 2011; Sa Filho et al., 2013; Pfeifer et al., 2017). For instance, Nelore cows with low BCS (<2.75) had lower pregnancy per AI (P/AI) than cows with BCS >2.75 at breeding (Sa Filho et al., 2010; Sales et al., 2011). Similarly, Nelore cows with a BCS of 2.5 had lower P/AI than cows with a BCS of 3.5 (Meneghetti et al., 2009).

On the other hand, obese cows can also experience suboptimal reproductive performance (McCann & Reimers 1986; Kawashima et al., 2008). Thus, it is generally accepted that cows with a good BCS (3.0 ≤ BCS ≤ 4.0) are more likely to be fertile than thin or fat cows. Consequently, optimal pregnancy outcomes may be achieved by evaluating the BCS to determine which females are suitable for enrollment in a TAI program.

In view of the importance of managing the BCS of beef cows that will be enrolled in a TAI protocol, it is necessary to create systems for BCS assessment that farmers can use to make appropriate decisions regarding the nutritional and reproductive management of their herds. Although it is important to evaluate BCS in postpartum beef cows, this approach provides only categorical data. Consequently, the use of average BCS may not represent the best way to interpret and make inferences about the effects of BCS on the outcomes of TAI protocols. Therefore, a new method of dealing with BCS data, which takes into account the differences between the BCS of each cow and the target BCS, may provide more accurate information about nutritional status and the likelihood of pregnancy of cows included in TAI programs. In this study, based on the abovementioned considerations, we evaluated the use of a BCS index (BCSi) to estimate the association between BCS and fertility of postpartum beef cows subjected to TAI protocols. The objective of this study was to develop an index to better estimate the relationship between BCS and fertility.