Early rather than late scrotal circumference measurements better reflect female precocity in beef cattle

In cow-calf operations the moment the heifer achieves her first pregnancy set up her future productivity. Although puberty itself is a yes-or-no condition, time to the event has a quantitative genetic basis and thus is suitable for genetic selection. However, the trait is difficult to measure directly and proxies such as age at first calving (AFC), or scrotal circumference (SC), are typically used. In genetic evaluations, the age at which SC is measured usually corresponds to last part of pubertal development phase. But given that initiation of puberty in both sexes is controlled by the same neuroendocrine mechanisms, we argue that an earlier measurement, taken instead at the start of the pubertal development phase, is probably a better indicator of female precocity. To support the hypothesis, we fitted a multiple-trait animal model on AFC records and SC measurements taken at 300, 400, and 630 days of age and estimated heritabilities and genetic correlations by REML. Importantly, usually AFC data is afflicted by the problem that when breeding season starts most heifers are already cycling (which is, of course, the desired condition) and thus the record poorly reflects precocity. To avoid the problem, in this study we used records collected from an Angus herd in which heifers receive an early natural first service at 375 days of age. Genetic correlation between AFC and SC300 was twice as large as the one corresponding to SC400 (-0.478 +/- 0.13 vs -0.244 +/- 0.11) and three times larger than to SC630 (-0.478 +/- 0.13 vs -0.152 +/- 0.12), a result that supports our hypothesis. Heritabilities for SC300, SC400 and SC630 were 0.429 +/- 0.07, 0.704 +/- 0.07 and 0.576 +/- 0.08, respectively, and 0.371 +/- 0.05 for AFC. Our results have an important implication in the age at which SC, as an indirect trait for improving precocity, is typically measured in beef cattle breeding programs. Indeed, they indicate that measurements should be taken earlier.