A model of individual variation in the food conversion efficiency and other growth parameters of grazing beef cattle in the tropics

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Abstract
Methods to genetically improve gross food efficiency (GFE) in cattle usually rely on a high genetic correlation between GFE and growth rate. Estimating this genetic correlation for cattle grazing tropical pastures is extremely difficult, so a model of individual variation in growth has been developed based on component traits which are known to affect growth rate. The model assumes that there is individual variation in mature size, maintenance requirement, intake to maintenance ratio, and susceptibility to heat stress and to tick and worm infestation. Nutritional level is assumed to interact with maintenance requirement, so that under good nutrition a high level of maintenance requirement maximises growth rate, whereas under a low level of nutrition a low maintenance requirement leads to maximum growth rate. The model predicts that in the presence of environmental stress the correlation between growth rate and mature size is only about 0.5, and there is no correlation between mature size and GFE. Growth rate and GFE are correlated with resistance to parasite and heat stress and to intake:maintenance ratio. Consequently growth rate and GFE are highly correlated, especially under stressful environmental conditions. The model predicts that growth rate in one environment will be highly genetically correlated with growth rate in other similar environments.