**2.3 Effects of heat stress**

Heat stress is defined as the rise in air temperature above a threshold level for a significant amount of time to cause damage to normal growth and development [11]. Heat stress limit plant growth and productivity even in the presence of adequate soil moisture. Just like drought, heat stress can happen at any stage of crop growth and its negative effect varies with the onset, intensity, and duration during plant growth. Field crops which give flowers, pollen, tassels, silking, grain filling, storage root formation and bulking are most susceptible to heat stress at reproductive stages due to flower drop. Yield losses can reach 100 percent depending on intensity and duration of the heat. In addition, an increase in air temperature results in raised soil temperatures which can be higher than air temperature when soil moisture is limited. Root development is severely affected in both field crops and forage/pastures for animals.

The vulnerability of livestock to heat stress varies according to species, genetic potential, life stage and nutritional status. The projected warmer temperatures expected in sub-Saharan Africa are likely to cause heat stress in beef cattle raised under extensive production systems. Heat stress in extensive beef cattle production systems will likely reduce foraging time, feed intake, growth performance and carcass quality. Reproductive performance will also be compromised, for example, conception rates will decrease, calving intervals will increase, and spermatogenesis and semen quality will be impaired. Heat-induced reduction in feed intake will result in a decline in milk yields in dairy cattle [12].

The effect of heat stress is not likely to be as adverse in small ruminants, due to the small body weight, well developed water retention in the kidney and lower metabolic rates of smaller ruminant species. Goats are more likely to cope with, and adapt to, the increasingly hot and dry conditions expected in the region compared to sheep and cattle, because of their low feed and water requirements, ability to exploit low quality forage and disease resistance. Goats can survive harsher climates than cattle and require less space [12].
