Ewes that come into heat before or after the normal breeding season demonstrate erratic cycle lengths. The ram does not show a restricted breeding season common in the ewe, but seasonal variations in semen production and characteristics are evident. Sexual activity of the ram tends to be highest in the fall and often declines in winter, spring and summer. Aspects of semen quality, such as total sperm, motility, proportion of live and normal sperm and metabolic activity of the sperm are highest in the fall. The role of temperature in reproduction of sheep is important.
Most breeds begin cycling in cooler weather.
Other breeds, such as Dorset and Merino, tend to cycle year-round and appear to be little affected by higher temperatures. Embryo death losses are increased when environmental temperatures remain 90 F or higher during breeding and early gestation. 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.