Lamb birth weights are reduced and lamb survival is decreased when temperatures remain high throughout pregnancy which is often the case for fall-born lambs. Sperm production by the ram is sensitive to temperature changes. Environmental temperature above 90 F for a prolonged period of time will usually interfere with the production of semen. During prolonged periods of excessive heat, the ram may become sterile. This damage is usually not permanent, but requires approximately 50 days of cooler temperatures before the ram fully recovers.
Heat stress is greater as the relative humidity rises.
Ewes in moderate body condition during the breeding season will produce more lambs than ewes in poor condition or ewes that are overfat. The results from flushing experiments, however, have been variable. In general, ewes in poor or medium body condition will respond to a nutritional flush by producing a higher lamb crop. Response to flushing is also greater either early or late in a breeding season. Ewes should not be grazed on pastures containing a high percentage of legumes, such as alfalfa or clover just before and during the breeding season.