133 ANTRAL FOLLICLE COUNT, VULVA WIDTH, AND FERTILITY TRAITS IN BOS TAURUS INDICUS CATTLE (TABAPUÃ)

Abstract

The intense selection of characteristics related to animal production may affect negatively the reproductive efficiency of beef cattle. Thus, the search for characteristics that indicate fertility is readily justifiable in the selection of bovine females. The aim was to evaluate possible associations between the antral follicle counts (AFC), external genitalia measurements, and reproduction efficiency in Tabapuã (a Zebu breed from Brazil) females. The AFC was evaluated in Nulliparous heifers (n = 162) and cows (n = 429) by directly counting all follicles ≥3 mm in diameter with ultrasound. From the frequency distribution of the AFC, animals were divided into groups of high (>50 follicles), average (28–50 follicles), and low (<28 follicles), according to Burns et al. (2005). The vulva width was determined by measuring the distance between the lateral borders of the vulva with a digital caliper placed at a 90-degree angle from the half point of the rima vulvae. Two official reproductive efficiency indexes, adopted by the Brazilian Zebu Breeders Association (ABCZ), were used: age at first calving and calving interval. All statistical analyses were performed using SAS (SAS Institute Inc., Cary, NC, USA). An initial complete model was submitted to logistic regression and all body measurements, such as weight, rump height, and width at pins and hooks, were later excluded because they had no relationships with AFC. The AFC class and parity effects of vulva width were submitted to the PROCGENMOD procedure and the reproductive efficiency indexes were regressed onto age, vulva width and the AFC using the PROCREG procedure. Vulva width was greater (P = 0.05) in the high-AFC class cows (8.81 ± 0.12 cm) in comparison to those of the low (8.38 ± 0.13 cm) and average (8.42 ± 0.11 cm) classes, and was not influenced (P = 0.08) by parity (8.37 ± 0.11 and 8.77 ± 0.14 cm for heifers and cows, respectively). Vulva width was larger in cows with lower calving intervals (r² = –0.21 P = 0.0008), but was not correlated with age at first calving (P = 0.78). Antral follicle counts were similar (P = 0.71) between heifers (38.6 ± 23.96) and cows (38.9 ± 28.00). Calving intervals were shorter in females with higher AFC (r² = –0.17 P = 0.0064). Age was slightly related to a decrease in the AFC (r² = –0.066 P < 0.0001). Vulva width and antral follicle count were correlated and were indicators of reproductive efficiency in females of the Tabapuã breed under the present experimental conditions. To our knowledge, this is the first report where vulva width, a fairly simple to detectable phenotype, was correlated with fertility.