The value of the improvement of a trait for a breeding organization is determined by its impact on the saleability of its breeding stock. This impact is influenced by the competitive position of the breeding organization, i.e. by the performance of its breeding stock relative to other breeding organizations. A method was developed to take effects of competitive position on the breeding goal into account in designing selection indexes. The main conclusions were as follows.

(1) When the performance level of a trait is equal to the average performance level of other breeding organizations, its economic weight (i.e. its weighing factor in the breeding goal) is equal to its importance on a commercial level. With lower performance, the economic weight increases. With higher performance, it decreases.

(2) The effect of competitive position on the economic weight of a trait depends on the degree of compensation between traits. When a weakness (negative monetary deviation compared with competitors) n i one trait can be totally compensated by the strength (positive monetary deviation of the same order) i n another trait, then competitive position has no influence on the economic weight.

Selection can produce lines of animals which diverge widely for a range of characters, so that the genetic relations among these and other traits can be investigated. Results are reported on chemical analysis of whole body composition of 10-week-old mice from replicated lines selected high or low for either amount of lean, proportion of fat, or food intake corrected for body weight. There were over two-fold differences in proportion of fat in lines so selected for fat, whereas those selected for total lean showed little difference in composition. Lines selected for high food intake were no fatter than those selected for low food intake. It is confirmed that dry matter proportion is an accurate predictor of fat proportion.

An attempt was made to use anal skinfold thickness as a quantitative measurement to estimate body fatness in Boran cattle (Bos indicus). Using the normal single-operator technique the relationship between skinfold thickness and the other measurements was poor but was greatly improved by using two operators. It nevertheless remained inferior to body condition score, which was very highly correlated with estimated body fat proportion, It was concluded that condition scoringprovides a valuable index of the nutritional status of Bos indicus cattle, and that measurements of anal fold thickness are not applicable to such animals.

In order to compare the digestibilities of amino acids in meat, skimmed-milk and fish meals, isonitrogenous, semi-purified diets containing these foodstuffs were given to five castrated male pigs, of 13 kg live weight, with ileo-rectal anastomoses. Apparent digestibilities were calculated as (amino acid consumption —Heal digesta amino acids)Iamino acid consumption. In the order, meat, milk and fish meal, digestibilities were: of N, 0·80, 0·84, 0·88 (s.e.d. 0·021); of lysine, 0·84, 0·94, 0·94 (s.e.d. 0·011); of threonine, 0·81, 0·86, 0·91 (s.e.d. 0·017); of methionine, 0-86, 0·94, 0·94 (s.e.d. 0·006). In vitro solubility of N in 0·2 gll solution of pepsin and hydrochloric acid was 0·93 (s.e. 0·003) for meat, 0·97 (s.e. 0·001) for milk, and 0·95 (s.e. 0·001) for fish meal. In conclusion, the digestibilities of essential amino acids were greater in the milk and fish meals, than in the meat meal.

Seventy-eight gilts in replicates of three litter-mates were allocated at 135 days of age and 65·4 kg live weight among three treatments. Between allocation and mating gilts were given either 1·78 kg/day (LL) or 2·72 kg/day (LH) diet containing 12·3 MJ digestible energy (DE) per kg and 5·3 g lysine per kg or 2·57 kg/day (HH) diet containing 12·6 MJ DE per kg and 8·1 g lysine per kg. More gilts in the LH and HH groups attained puberty within 200 days of age than LL gilts (41/52 v. 72/26, X2 = 8·9, P < 0·01). Live weight at conception was 93·8, 105·3 and 108·9 kg for LL, LH and HH gilts respectively at a mean age of 203 days. Subsequent reproductive efficiency and sow body weight and backfat changes were not significantly affected by treatments imposed during the early stages of reproductive life.