Abstract
There are few studies on weight covering the full life cycle of Zebu cattle, and there is no entire growth description or mean growth pattern for animals belonging to this breed. In order to provide such data, 1,158 Nelore females born between 1985 and 1995 were weighed 14,563 times from birth to full growth maturity, in ten herds spread over seven Brazilian states. The Von Bertalanffy, Brody, logistic and Gompertz non-linear models were used to obtain the asymptotic weights (A) and the maturation rates (K). The (co)variance and breeding value components for A and K were obtained by using the multiple trait derivative free restricted maximum likelihood method under the animal model. Genetic trends were calculated in function of the mean expected progeny differences (EPD) for the trait (A or K) divided by the number of animals according to their year of birth. The genetic trends of the expected progeny difference with reference to the date of birth of the cows were, on average, -6.5g y-1 for A and 2.0g y-1 for K, close to zero as confirmed by the low (0.0023 to 0.003) coefficient of regression values. The curve parameters are recommended as a selection criterion to reach precocity and avoid adult weight increase in the female herd.
Highlights
A selection aimed at increasing the rate of weight gain or weight converted in beef cattle upon slaughter will result in a weight increase in adult animals and, greater herd maintenance costs
The data set consisted of 14,563 weights and ages describing 1,158 Nelore females born between 1985 and 1995 in ten herds that were included in the Nelore Genetic Breeding Program (PMGRN)
The results were the opposite of those estimated by Rosa et al (2001). These authors established the adult weight based on the first time the cow was weighed, they reached their limits from 4 to 12 years of age, not on the growth curve
Summary
A selection aimed at increasing the rate of weight gain or weight converted in beef cattle upon slaughter will result in a weight increase in adult animals and, greater herd maintenance costs. All these aspects together can reduce the economic advantages of weight increase in slaughter cattle (Cartwright, 1970; Dickerson, 1978). Studies on growth and maturity traits deriving from studies on growth curves should be used as additional information in genetic breeding programs (Fitzhugh, 1976; Kaps et al, 2000; Arango and Van Vleck, 2002). Some summaries only provide the adult weight EPD, e.g. the 2002 US Angus sire summary (http:// www.angussiresearch.com/index.html) and the Nelore sire summary under the Nelore Genetic Breeding Program (http://www.ancp.org.br/ sumarios/index.asp)
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