ALBERT

Description: SummaryCalves from three breeds, Brahman, Hereford × Shorthorn (HS) and Brahman × HS (BX), were divided equally into two groups, one of which was treated every 3 weeks from birth onwards to control ticks and gastrointestinal helminths, and one of which was untreated. Mortalities, growth rates and levels of resistance to environmental stresses that affected both mortality and growth under grazing conditions were recorded for all animals up to weaning (6 months) and for all males up to 15 months of age. The Brahmans were the most and the HS were the least resistant to environmental stresses, each of which was shown to depress growth in proportion to its magnitude and to contribute to the high mortalities of the HS. All breeds responded positively to parasite control with the greatest response in both survival and growth in the HS breed and the least response in the Brahman breed.Samples of males from the various breed-treatment groups were taken into pens where they were protected from environmental stresses and fed both low-quality pasture hay and high-quality lucerne hay ad libitum. Measurements were made of fasting metabolism, maintenance requirement, voluntary food intake and gain, variables related to the growth potential of each animal. The HS animals had the highest whilst the Brahmans had the lowest values for each variable.However, despite their low growth potential, the Brahmans had the highest growtli rate, and the HS, despite their high growth potential, had the lowest growth rate, when growth was measured in the presence of all environmental stresses. When parasites were controlled, growth rates were highest for the BX, the breed with intermediate growtli potential, and did not differ between the HS and Brahmans. These interactions arose because of the different contributions of resistance to environmental stresses and growth potential to growth rate measured at the different levels of environmental stresses. The relevance of these interactions to breed evaluation and cross-breeding is considered.Growth potential and resistance to environmental stresses were negatively correlated both between and within breeds, though the latter was biased by the effects of compensation. The influence of these relationships on the likely outcome of selection for increased growth rate, both between and within breeds, is discussed.

Description: SUMMARYSince 1966 a closed line of Hereford × Shorthorn cattle (selected line) has been selected principally for growth rate under conditions of moderate to high environmental stress. Responses in growth rate were measured over the years 1970–5 as differences from an unselected control line. Birth weight declined but live weight at all other ages increased.Groups of animals were taken from each line in 1976 and 1977 and used to determine the reasons for the differences in growth rate. Growth rate of the groups was measured when they were exposed to several different levels of stress, namely, the plane of nutrition, high ambient temperature, infection with bovine infectious keratoconjunctivitis(BIK) or gastro-intestinal helminths. The selected line was shown to be more heat tolerant, to have lower maintenance requirements and greater resistance to infection with BIK and helminths and, in consequence, always had higher growth rates in the presence of these stresses. However, they did not have superior growth rate at low levels of these stresses. Thus, selection had not increased the components associated with growth potential, and the improvement in growth rate was achieved entirely through increases in resistance to environmental stresses which affected growth rate.The application of these findings to selection for growth rate, particularly in harsh environments, is discussed.

Description: SummaryThe incidence of lantana poisoning and its effect on mortality, growth rate and fertility were recorded from 1975 to 1982 in seven lines of Bos taurus and Bos indicus cattle and their crosses.Two Bos taurus Hereford × Shorthorn (HS) lines had higher incidences of poisoning and higher mortalities after poisoning than the Bos indicus based breeds.During the period of poisoning, live–weight gains of affected animals of all breeds were reduced but subsequent recovery was rapid. Lantana poisoning had no detectable effect on the fertility of animals affected at least 10 months prior to their first mating, irrespective of breed.Parasite control influenced the incidence of poisoning and mortality in genetically similar groups of HS cattle. This observation, when combined with the between-breed relationships between the incidence of lantana poisoning and susceptibility to environmental stresses suggests that the marked breed differences in susceptibility to lantana poisoning stem from differences in resistance to other environmental stresses, including cattle ticks. This offers little scope for selection of animals that are resistant to lantana poisoning per se but has practical significance in lantanainfested regions.