The influence of protein:energy value of the ration and level of feed intake on the energy and nitrogen metabolism of the growing pig. 2. N metabolism at two environmental temperatures.

1. The nitrogen balances of thirty-six individually-housed, entire male pigs (body-weight range 19–50 kg) were measured over 7 d periods when the animals were kept initially at an environmental temperature of 22° and then at 10° while fed on rations containing 153, 201 and 258 g crude protein (N × 6·25; CP)/kg dry matter ( DM ). The respective metabolizable energy ( ME ) contents were 16·29, 16·96 and 17·24 MJ/kg DM . Each ration was given at three levels, 20, 35 and 50 g feed/kg body-weight per d. The animals fed on the 20 and 35 g/kg feeding level were catheterized and blood samples withdrawn on two consecutive days within the N-balance periods for the determination of blood urea (BU) concentration both before and at hourly intervals for 7 h following the morning feed. 2. An increase in feed intake resulted in a significant increase in N retention (NR) at each environmental temperature. However, NR as a proportion of N intake was higher the lower the protein content of the ration. With the exception of the animals fed on the low-protein ration, NR at any given feed intake was lower at 10° than at 22° and these differences were reflected in the animal's body-weight gain. 3. Values for the fasting N metabolism (N f ), calculated from the relationship between NR and intake of digestible N (IDN), were temperature-dependent. At 22°, a constant N f value of 0·255 g N/kg body-weight 0·75 per d was found appropriate, while at 10° N f increased with increase in protein content of the ration from 0·380 on the low protein ration to 0·533 and 0·753 g N/kg body-weight 0·75 per d on the medium- and high-protein rations respectively. 4. The efficiency of N utilization ( k N ) reflected the differences in the relationships between NR and IDN. At 22° the relationship was curvilinear so that k N decreased with increase in both the level of feed intake and the protein content of the ration. At 10° the relationship was linear, hence k N was independent of feed intake within rations. However, it decreased from 0·909 to 0·679 as the protein content of the ration was increased. 5. The concentration of BU attained a maximal value some 3–5 h after the ingestion of the feed, with the values at 10° being higher than those at 22°. BU increased as the level of protein in the ration increased but decreased with the level of feed intake when dietary protein concentration was held constant. There was a significant correlation between BU and k N , indicating that BU is a useful criterion for assessing the efficiency of N utilization.