Energy utilization in laying hens. III. Effect of dietary protein level at 21 and 32 C

White Leghorn pullets housed at either 21 or 32 C were used to investigate the effects of dietary protein level on energy utilization. Protein levels of 12, 14, 16, 18, and 20% were employed in this study with three feeding levels of each (two restricted levels and ad lib.). All diets were isocaloric, and the duration of the study was 21 days. Maintenance metabolizable energy (ME) was estimated as 134 and 121 kcal/kg physiological body weight (BW.75) at 21 and 32 C, respectively. Fasting heat production/BW.75 was 21% higher at the lower temperature (89 vs. 70 kcal). Estimates of energetic efficiencies at 21 C varied from 60.9% for the 12% protein diet to 72.4% for the 18% protein diets. At 32 C, similar diets showed lower energetic efficiencies for ME conversion to net energy (50.6% for the 12% protein diet and 70.6% at 20% protein) due to lower feed intakes. Egg weights were significantly higher at the lower environmental temperature, and at each temperature they were increased with the feeding of the higher protein diets. The average feed efficiency (g egg/g feed) was significantly better at the higher environmental temperature (.46 vs. .53). Calculations of heat increment (HI) plus activity from a parabolic regression developed in relation to protein intakes showed higher (HI plus activity) values with either low or high protein intakes. Thus, the feeding of dietary protein levels which result in intakes commensurate with needs should result in maximum energetic efficiencies and dietary net energy.