ESTIMATION OF INDIVIDUAL AND MATERNAL HETEROSIS, REPEATABILITY AND HERITABILITY FOR EWE PRODUCTIVITY AND ITS COMPONENTS IN SUFFOLK AND TARGHEE SHEEP 1 R. T. E. Long 2, D. L. Thomas 3, L. Femando, J. M. Lewis, U. S. Garrigus and D. F. Waldron University of Illinois'*-s Urbana 61801 ABSTRACT The objectives of this study were to estimate individual and mammal heterosis, repeatability and heritability for ewe productivity and its components in Suffolk and Targhee sheep and to predict the productivity of various mating systems from the performance of breeds and crosses in the study. Suffolks from the North Dakota Experiment Station at Fargo (NDS), Targhees from the Ohio Experiment Station at Wooster (OT) and Suffolks and Targhees (ILS and ILT, respectively) from the University of Illinois Dixon Springs Agricultural Center (DSAC) were assembled at DSAC over a 3- yr period (1959 to 1961) by sampling the respective populations. Reciprocal matings were made between ILS and OT and between ILT and NDS to obtain F1 females. These Ft ewes were back-crossed to a ram from the breed-station flock of the F1 ewe's dam. Records from 747 purebred or crossbred ewes, 150 sires of ewes and 143 service sires over a 7-yr period were included in the data set. Individual heterosis estimates were fertility, .6%; prolificacy, 3.3%; lamb survival, 6.9%; 90-d weight, 2.8% and ewe productivity, 13.8%. Maternal heterosis estimates were fertility, 2.6%; prolificacy, 6.6%; lamb survival, 1.4%; 90-<! weight, 5.4% and ewe productivity, 10.1%. Repeatability estimates were fertility, .11; prolificacy, .11; lamb survival,. 18; 90-d weight, .24 and ewe productivity,. 15. Heritability estimates were fertility, .07; prolificacy and lamb survival, inestimable; 90-d weight, .07 and ewe productivity, .10. Simulated mating systems that produced F1 terminal market lambs were calculated to produce 12.8% more weight of marketable lamb at weaning than mating systems that produced only purebred lambs, and mating systems that produced back-cross terminal market lambs from F1 ewes produced 25.1% and 10.9% more weight of marketable lamb at weaning than mating systems that produced purebred lambs only or F 1 terminal market lambs, respectively. (Key Words: Sheep, Suffolk, Targhee, Heritability, Repeatability, Heterosis.) J. Anita. Sci. 1989. 67:1208--1217 1Project 340 of the Illinois Agric. Exp. Sta. and contributing project to former Regional Project NC-50, "Improvement of Lamb Meat Production Through Breed- ing. I~ 2present address: Dept. of Anim. Sci., Univ. of Nebraska, Marvel Baker Hall, Lincoln 68583-0820. 3"1"o whom reprint requests should be addressed. 4Dept. of Anim. Sci., 126 Anita. Sci. Lab., 1207 W. Gregory Dr. ~Personnel at DSAC are thanked for animal manage- ment and data collection and Annette Hollandand A. Carol Collins for manusctipt preparation. Received May 17, 1988. Introduction Total weight of lamb weaned per year from a flock of sheep is the best single measure of that flock's productivity (Sidwell and Miller, 1971). This measure is related directly to the average production of each ewe in the flock. Ewe productivity can be defined as kilograms of lamb weaned per ewe exposed to breeding per year and is determined by its component traits: fertility (ewes lambing per ewe ex- Accepted October 10, 1988. 1208