Goat Production under Harsh Environmental Conditions: the Physiological Basis and the Challenge

Goats living in harsh environments represent a climax in the capacity of domestic ruminants to adjust to such areas. This ability is multifactorial. Low body mass and low metabolic requirements of goats can be regarded as important assets in minimising their maintenance and water requirements in areas where water sources are widely distributed and food sources are limited by their quantity and quality. An ability to reduce metabolism allows goats to survive even after prolonged periods of severe limited food availability. A skilful grazing behaviour and efficient digestive system enable goats to attain maximal food intake and maximal food utilisation in a given condition. There is a positive interaction between the better recycling rate of urea and a better digestion of such food in desert goats. The rumen plays an important role in the evolved adaptations by serving as a huge fermentation vat and water reservoir. The water stored in the rumen is utilised during dehydration, and the rumen serves as a container which accommodates the ingested water upon rehydration. The rumen, salivary glands and kidney coordinate functions in the regulation of water intake and water distribution following acute dehydration and rapid rehydration. Goats in the tropics base their diet when possible on tree-leaves and shrubs (browse) which ensure a reliable and steady supply of food all year around, albeit, of a low to medium quality. Some of the physiological features of ruminants defined as intermediate feeders are large salivary glands, large absorptive area of their rumen epithelium, and a capacity to rapidly change the volume of the foregut in response to environmental changes. These features are most likely responsible for the goat's superior digestion capacity. Although goats and sheep are mixed feeders, under mixed forage conditions goats consume a larger proportion of browse than sheep and use it more efficiently. Unlike sheep and cattle, which do not eat leafy material during the green season, browse constitutes at least 40% of the forage selected by goats at all times. This pattern of diet selection, however, is not compatible with maximizing milk yield. Indeed, selection of goats by men in harsh Mediterranean environments was most certainly based on breeding success and lifetime performance. Most browse species in the Mediterranean are dicotyledons that are high in tanninferous phenolic substances. Recent studies have shown that polyethylene glycol (PEG), a polymer that can bind tannins irreversibly over a wide range of pH, is very efficient in neutralising the negative effects of tannins in ruminant feedstuffs. PEG has improved the performance of grazing goats and sheep in a seemingly