A comparative study between the Damara and the German merino with respect to the influence of certain environmental stress factors

Abstract

Abstract taken from Agricultural Research summaries of theses, 1973, p. 208:

Two breeds of sheep, the Damara and the German Merino, were compared in relation to the effects of high temperature and temperature control, while an investigation was also made of the water and electrolyte metabolism during dehydration and saline water intake. The results indicated that the German Merino had a higher respiratory frequency both in a temperate and a hot environment. However, the percentage increase in actual frequency per unit increase in ambient temperature did not differ between breeds. When dehydrated in a hot atmosphere both breeds increased their rectal temperature and decreased food consumption. Heat production was therefore decreased and respiratory rate declined as a result. This mechanism was considered advantageous to the animals when deprived of water in a hot environment

The coats of both breeds were found effective against solar insolation with the German Merino coat (woolly) slightly more effective. Clipping of the coat was not advantageous as it caused panting in both breeds when temperatures were high. Morning and midday Damara rectal temperatures were slightly lower and higher respectively, than those of the German Merino which suggested a diurnal cycle with an adaptive hyperthermia. However, this observation needs confirmation. Thermal polypnoea in both breeds caused an excessive elimination of blood carbon dioxide resulting in respiratory alkalosis with the subsequent elevation of blood pH and plasma bicarbonate. Furthermore dehydration caused an increase in plasma bicarbonate as well as the carbon dioxide concentration in blood. Water intake was initially lower in the Damara in a temperate environment but, probably due to acclimatization, this difference eventually disappeared. With reduction in body mass as criterion, dehydration was more detrimental in a hot than in a moderate environment

In the German Merino the faecal and urine water dropped more effectively during dehydration in a temperate climate whereas in the Damara it dropped more effectively in a hot environment. However, since the plasma variables were more concentrated in the German Merino in both instances, it appeared that the Damara was not unduly stressed by dehydration in a temperate environment. Because the German Merino concentrated plasma variables and haematocrit to a greater extent than the Damara, it appeared that the former, when sufficiently dehydrated, suffered circulatory failure at an earlier stage. The kidney manipulation of salt water, resembling that from brackish underground sources, was similar in both breeds. Thus it seemed that saline water would not affect the production of sheep adapted to the salt-water areas but in areas of high temperature and aridity the production of the German Merino would be suboptimal