Modelling the long-term consequences of undernutrition of cows grazing semi-arid range for the growth of their progeny

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South African Journal of Animal Science

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University of Cape Town

Cattle grazing semi-arid range may be subjected to low intakes of metabolisable energy and protein as a result of drought, high stocking rates, or during the dry season low digestibility and low protein content of the forage (Butterworth, 1984). The growth and milk yield of cattle are reduced during such periods of undernutrition. Growing animals respond in different ways to nutritional restriction because of differences in age at the start of undernutrition, the severity and duration of undernutrition and the availability and composition of food during rehabilitation (O'Donovan, 1984). Severe chronic undernutrition of cattle in early life reduces growth and leads to smaller animals at any age (Morgan, 1972; Greenwood & Café, 2007). Undernutrition of breeding cows during the last trimester of gestation leads to a reduction in birth weight (Richardson et al., 1979) and in the cow's milk yield (Richardson et al., 1977) as a result of the effect of undernutrition on udder development (Mellor & Murray, 1985) so that pre-weaning growth is also reduced. On the other hand, compensatory growth may occur in rangeland cattle subjected to undernutrition after weaning. For example, Ainslie (1958) reported that when steers were subjected to different planes of nutrition between weaning at 8 - 9 months of age and 12 months the difference of 23 kg between group means was eliminated when all animals grazed together without being given any supplementary food. In this paper a mechanistic model of a rangeland production system has been used to explain the different responses to retardation of growth that have been recorded in the literature.