Energy and nitrogen metabolism of broilers selected over ten generations for increased growth rate, food consumption and conversion of food to gain.

Energy and nitrogen (N) metabolism were studied in 6-week-old male birds taken from 4 lines of chickens selected for 10 generations for increased weight gain (line W), increased food consumption (line F), increased conversion of food to gain (line E) or at random (controls, line C). Calorimetric measurements were made 8 times on each line while fed ad libitum in large open-circuit respiration chambers for 3 d, and 11 to 13 times without food in smaller closed-circuit respiration chambers for 24 h. The F line ate 60% more food, produced 90% more excreta and 34% more heat and retained 80% more energy and 35% more N in their bodies than lines E and C. Line W was intermediate. When differences in body weight were taken into account, the E and W lines had lower heat production than the C line, while the F line ate 40% more food, produced 30% more heat and retained 70% more energy and 30% more N than the E line. In lines W, F, E and C respectively, the mean metabolisability of dietary energy (%) was 69.4, 62.9, 70.1 and 67.8; the fasting heat production (mean +/- SE) was 481 +/- 9, 569 +/- 10, 485 +/- 9, and 508 +/- 9 kJ/kgW d; the net availability of metabolisable energy (NAME) was 0.68 +/- 0.05, 0.76 +/- 0.04, 0.85 +/- 0.06 and 0.73 +/- 0.04; the estimated daily maintenance energy requirements were 671 +/- 15,866 +/- 14,701 +/- 13, and 742 +/- 11 kJ ME/kgW; and the proportion of N retained per unit increase in N intake was 0.38 +/- 0.08, 0.50 +/- 0.06, 0.56 +/- 0.10 and 0.53 +/- 0.06. The contribution of line differences in the above traits to large line differences in efficiency of food utilisation is discussed.

Production responses of layer strain hens to food restriction during rearing.

Production responses of laying hens to food restriction during rearing were measured in two trials. Food was restricted by two methods: time restriction (TR, food continuously available for 24 to 30 h each 72 h) and proportion restriction (PR, daily allocations of about 60 to 70% of that consumed by ad libitum fed birds (F]. The rearing treatments were applied from 42 to 162 d and from 56 to 168 d of age in experiments 1 and 2 respectively. All birds were fed ad libitum in the laying period to 437 d and 476 d of age in experiments 1 and 2 respectively. Body weight was reduced significantly in both experiments by both methods of restriction, but more so by TR. Marked compensatory growth accompanied the high food intake following the end of food restriction. There was a significant increase in average egg weight concommitant with a change in the relationship between egg weight and age. Possibly this is due to an altered pattern of food intake at onset of lay.