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Simulated Genotype, Environment and Interaction Effects on Performance Characters of Sheep¹

Texas Agricultural Experiment Station, College Station 77843

Abstract

A computer simulation model was used to evaluate productivity of sheep varying in genetic potential for size and milk production in a changing arid environment. Size potentials were characterized by mature weights (WMA) of 30, 35 or 45 kg and milk potentials by peak day lactation (GMLKL) of .90, 1.30 or 1.75 kg. The environment consisted of a base year (BY), which was an average year; a drought year (DY), which resulted in a reduction of forage quality and quantity; and two recovery years (RY1 and RY2), which consisted of the same forage parameters as BY. By exposing all nine genotypes (3 WMA x 3 GMLKL) to the perturbation of the DY, their performance in a good year, a bad year, and during recovery from the bad year were quantified. Ewe body condition influenced reproductive performance and milk production, indicating that minimum levels must be attained before other production characters can begin recovery from DY. Time-lag effects of DY were observed in RY1 for live lambs born per ewe (LLB), weaning weight (WNWT), yearling weight (YRWT), and degrees of maturity at weaning and at 1 yr of age. By RY2 the flocks had almost fully recovered from the DY for the production characters. Two genotypic combinations, 30/1.75 (WMA/GMLKL) and 35/1.75, were slow in recovering from the drought; for these genotypes, lambs weaned per ewe did not increase as rapidly in recovery years as other genotypes and yearling weights remained depressed. The importance of matching body size and milk production became apparent in terms of LLB, WNWT, YRWT and milk production. Within each WMA, the optimal combinations simulated were 45/1.75, 35/1.30 and 30/.90.

¹ Actual data were collected by Dr. C. R. Field of the Integrated Project on Arid Lands, part of UNESCO's Man and the Biosphere Program. Model Development and Analysis were supported in part by funds from the United States Agency for International Development, Title XII, Small Ruminants Collaborative Research Support Program under grant No. AID/DSAN/XII-G-0049 in collaboration with the Kenya Ministry of Agriculture and Livestock Development.

² Anim. Sci. Dept.