Utilization of energy for maintenance and gain in broilers and Leghorns at two ages.

Regression equations describing the relationship between energy gain and metabolizable energy intake per unit metabolic weight were derived for 8- to 22-day-old and 28- to 42-day-old broilers and 14- to 28-day-old and 28- to 42-day-old Leghorns. The estimated maintenance energy requirements were lower in the 28- to 42-day-old birds than in the younger birds, but the decreases were small. Maintenance energy requirements of Leghorns were 36% less than those of broilers. Age had no effect on the efficiency of energy utilization above maintenance in broilers. However, the efficiency of energy utilization above maintenance in Leghorns was 43% lower in the older birds. This was correlated with a lower percentage fat gain in the older Leghorns. The percentage fat gain in broilers was substantially higher in the older birds, but this was not correlated with changes in energetic efficiency.

Relationship of sex and body growth rate with daily accretion rates of fat, protein and ash in chickens.

A series of serial slaughter experiments was conducted with three strains of chickens of greatly different rates of body growth through 49 days of age. During the first week of life, the body weight of the rapidly growing strain increased from hatching at a rate twice that observed in either the medium or slowly growing strain of chicken. The percentage increase in body weight which occurred during the seventh week of life (i.e., gain from day 42 to day 49 as a percentage of 42-day body weight) was similar for all three strains. Daily gains in crude protein and ash, expressed as percentages of daily carcass weight gain, were similar among strains. However, daily fat gain, expressed as a percentage of daily carcass weight gain, decreased slightly in the slow growing strain during the last two weeks of the study, remained relatively constant in the medium growing strain, but increased linearly and substantially in the rapid growing strain. It is hypothesized that the biological factors which facilitate the very rapid early rate of growth in the rapid growing breed also result in excessive energy consumption as the bird ages and the rate of proportional increase in body weight declines.