Abstract
The data analyzed in this investigation were weaning weights of 1,506 lambs, representing the lifetime production of 414 Navajo ewes. These data were collected from 1938 to 1947 as part of the routine records taken at the Southwestern Range and Sheep Breeding Laboratory, Fort Wingate, New Mexico. The unadjusted average weaning weight at approximately 139 days was 59.4 pounds with a standard deviation of 9.8 pounds. Six measurable environmental factors were found to have important effects upon the weaning weights of the lambs: Year of birth, age of ewes, breeding of sire, type of birth and rearing, sex, and age of lamb at weaning. Weaning weights for 1939 and 1946 were 5.2 pounds below average and those in 1941 were 6.9 pounds above average. Two-year-old ewes weaned the smallest lambs; the 4 to 7-year-old group, the heaviest; and the 3-year-old and the 8 to 11-year-old group weaned intermediate weight lambs. Columbia rams sired the heaviest lambs at weaning, followed by Corriedale, crossbred, Navajo, and Romney rams. Single lambs were 11.2 pounds heavier than twins and 2.9 pounds heavier than twins raised as singles at weaning time. Ewe lambs averaged 4.4 pounds less at weaning time than ram lambs. The regression of weight on age at weaning was .37 pound per day. The variance attributed to each of the six factors was statistically significant, the total reduction amounting to 56 percent of the variance in the actual weaning weights. Selection between lambs or between ewes can be made much more accurate by adjusting for differences in their environmental effects. The repeatability coefficient for weaning weights of the lambs raised by Navajo ewes was .217. This estimate represents the degree to which the weaning weights of lambs is a constant characteristic of the ewes. Using averages of 2, 3, 4, and 5 records in practicing selection is respectively 1.28, 1.44, 1.55, or 1.63 times as effective as using only 1 record. Copyright © . .