Effects of nutritional environment and ram effect on breeding season in range sheep.
Effects of season, nutritional environment, and presence of rams on incidence and rate of ovulation in fine-wool sheep in southern New Mexico were investigated over 14 months (February 1983-March 1984). Number of corpora lutea (CL) were observed each month by laparoscopy in random samples from six ewes in each subgroup. Seasonality of ovulation in fine-wool sheep managed on range was much more marked than in animals managed on alfalfa hay in drylot. Incidence of ovulation approached zero in range-managed ewes during May, June, and July (8%, 0%, 4%) but was higher (P<0.05) in the alfalfa-drylot group (42%, 17%, 75%), respectively. Although the incidence remained lower (P<0.05) in August in range-managed than in drylot ewes (50% and 83%, respectively), both groups were ovulating at a high rate during September through January. Incidence of ovulation dropped dramatically both years in the range-managed group in February compared with that of the alfalfa-drylot group (48% and 36% vs 100% and 83%). Mean ovulation rate per ewe ovulating did not differ (P>0.10) between nutrition groups within the 14 months. Body weight was related to CL number during periods of seasonal transition only. Continuous or intermittent presence of a ram had no real effect on either incidence or rate of ovulation during the transitional months. Nutritional environment can affect seasonality of breeding in fine-wool range sheep. This effect may not be consistently modified by presence of rams.
Hulet
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U.S. Department of Agriculture, Agricultural Research Service, Jornada Experimental Range, Las Crusces, NM 88003 USA.
Shupe
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Ross
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Richards
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