ADJUSTMENT FACTORS FOR 120-DAY WEANING WEIGHT IN RAMBOUULLET RANGE LAMBS

Additive and multiplicative environmental adjustments for effects of dam age, lamb sex and birth-rearing type on 120-d weight were derived using data from 819 Rambouillet range lambs. The relationship between mean and variance within environmental effect classes was one criterion for evaluating suitability of adjustment methods. Among lamb sexes and birth-rearing types, larger means were associated with greater variability (P \u3c .01). Variances became homogeneous (P \u3e .10) when data were expressed on a logarithmic scale, suggesting that multiplicative adjustments for these effects are needed. Because variances were homogeneous with respect to dam age (P \u3e .50), additive adjustment factors are suggested for this effect. Validation of a proposed combined multiplicative and additive adjustment procedure was based on a double cross-validation procedure and suggested difficulty in developing widely applicable sets of factors. Application of established industry adjustment factors failed to eliminate variation associated with lamb sex and birth-rearing type. Concordance between individual lamb rankings for adjusted weights derived from five sets of adjustment factors was observed. Under more (5%) and less (30%) intensive selection of ram and ewe lambs, respectively, similar animals were retained as replacements irrespective of the factors used. These results suggest that the use of autonomous adjustments has greater impact on the calculation of adjusted weights than on selection decisions among lanabs within sex based on such weights