Differential response from selection for high calving ease vs. low birth weight in American Simmental beef cattle

Abstract

<jats:title>Abstract</jats:title> <jats:p>Data on calving ease (CE) and birth, weaning weight (WW), and yearling weight (YW) were obtained from the American Simmental Association (ASA) and included pedigree and performance information on 11,640,735 animals. Our objective was to quantify differential response from selection for high CE vs. low birth weight (LBW) in first-calf Simmental heifers. We hypothesized that direct selection for CE should be used as the primary approach to reduce dystocia and mitigate losses in growth-related traits. WW and YW were adjusted to 205 and 365 d of age, respectively. Sire and maternal grandsire (co)variance components for CE, birth weight (BW), and 205-d weaning weight (205-d WW), and sire covariance components for 160-d postweaning gain (160-d gain) were estimated using a sire-maternal grandsire model. Direct and maternal expected progeny differences (EPD) for CE, BW, and 205-d WW and direct EPD for 160-d gain and 365-d yearling weight (365-d YW) for first-calf Simmental heifers population (465,710 animals) were estimated using a threshold-linear multivariate maternal animal model. This population was used to estimate genetic trends and as a selection pool (control) for various selection scenarios. Selection scenarios were high CE (HCE), LBW, the all-purpose selection index (API = −1.8 BW + 1.3 CE + 0.10 WW + 0.20 YW) of the ASA and its two derived subindices: (API1 = 1.3 CE + 0.20 YW) and (API2 = −1.8 BW + 0.20 YW), and lastly Dickerson’s selection index (DSI = −3.2 BW + YW). Data for each selection scenario were created by selecting sires with EPD greater than or equal to the average along with the top 75% of dams. Comparison between selection scenarios involved evaluating the direct and maternal genetic trends from these scenarios. Direct heritabilities for CE, BW, 205-d WW, 160-d gain, and 365-d YW of Simmental cattle were 0.23, 0.52, 0.28, 0.21, and 0.33, respectively. The single trait, HCE, selection scenario, as opposed to LBW, increased the intercept for CE by 57.7% and the slopes (P &amp;lt; 0.001) for BW, 205-d WW, 160-d gain, and 365-d YW by 27.9%, 37.5%, 16%, and 28%, respectively. Comparisons of various selection scenarios revealed that the CE-based selection scenarios (HCE, API, and API1) had a greater response for CE and growth traits.</jats:p>

Novos critérios de seleção para avaliação de bovinos de corte

“…A herdabilidade materna obtida pelos autores apresentou média superior à encontrada no presente estudo, no entanto, devemos levar em consideração que o conjunto de dados utilizado por Silva et al (2020) foi composto por apenas 2 rebanhos e os parâmetros genéticos foram obtidos através do modelo touro-pai avô materno, o que pode superestimar as estimativas de herdabilidade. Saad et al (2020) relataram estimativas de magnitudes semelhantes às obtidas nesse estudo, os autores analisaram população de bovino Simental Americano e obtiveram através do modelo touro e touro avô-materno herdabilidade direta de 0,23 e materna de 0,14.…”

“…Estudos envolvendo outras raças de bovinos de corte apresentaram maiores estimativas de herdabilidade para PN. Saad et al (2020) na raça Simental apresentaram herdabilidade direta de 0,52, assim como Herrera-Ojeda et al ( 2019), que para a raça Charolês, obtiveram para PN estimativa de 0,51 para herdabilidade direta e 0,32 para materna.…”

“…Mujibi e Crews (2009) apresentaram, para bovinos da raça Charolês, correlação genética entre PN e FP (transformada em escores de Snell) de -0,93 para efeitos diretos e -0,69 para maternos. Para bovinos da raça Simental, foram relatadas estimativas de correlação genética direta entre PN e FP de 0,67 a -0,85 (JAMROZIK; MILLER, 2014;SAAD et al 2020). A partir das estimativas analisadas na literatura e os resultados obtidos no presente estudo, tem-se a justificativa da adoção da característica PN como critério indicativo de distocias.…”