Two successive grazing experiments were conducted over 12 weeks on perennial ryegrass pastures with 50 and 44 young cattle to study the effect of N fertilizer when applied at a daily rate of 1 or 3 kg N/ha. At each level of N two stocking rates differing hy 20% were imposed. At the liigher N level and stocking rate, three frequencies of grazing were imposed.At the stocking rates imposed N tended to reduce the daily liveweight gain per head, hut increased the total liveweight gain per ha hy from 0-79 to 1-58 kg/kg N. A 20% increase in stocking rate depressed gain per head in hoth years. In the first year it did not improve gain/ha, hut in the second year gain/ha was increased hy 12-17%.There was a tendency for performance per animal and per ha to increase as the grazing cycle was lengthened. In 1969 the highest yield in the whole season was 1880 grazing days and 1260 kg gain/ha. The overall response to N fertilizer was similar to that recorded in other reports, hut it is possihle that a lack of K had limited pasture growth.

With the objective of determining genetic × environmental interaction for beef production under direct grazing conditions, measured as both individual average daily gain (ADG) and per ha (PROD), 160 steers were utilized through 2 years, from two breeds of different growth potential, and four stocking rates (SR) tending to establish different nutritional environments. The breeds were Aberdeen Angus (A) and high-grade Limousin crossbred (L) and the SRs established were 2·25, 2·87, 3-50 and 4·13 steers per ha. The pasture where the steers grazed was Festuca arundinacea. Response variables were analysed by least-squares using a fixed model of year, breed, SR and their two-way interactions. There were highly significant effects (P < 0·01) of the interaction of breed × SR for ADG and PROD. Quadratic and linear regressions (P < 0·01) were adjusted for PROD and ADG on SR respectively using the least-squares means. The proportional superiority of L on A at the lowest SR was 0·27, while at the highest SR the situation was reversed, A gained proportionately 0·32 more weight than L. These results indicated that under limiting conditions of nutrition as generated at the highest SR in this experiment, the small-sized individuals tended to produce meat more efficiently both individually and per ha, the opposite situation being true when nutritional conditions were not restrictive.