The influence of protein:energy value of the ration and level of feed intake on the energy and nitrogen metabolism of the growing pig

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Abstract
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/kgDM. 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50 g feed/kg body-weight per d. The animals fed on the 20 and 35 g/kg feeding level were catheterized and blood samples withdrawn on two consecutive days within the N-balance periods for the determination of blood urea (BU) concentration both before and at hourly intervals for 7 h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 (Nf), calculated from the relationship between NR and intake of digestible N (IDN), were temperature-dependent. At 22°, a constant Nfvalue of 0·255 g N/kg body-weight0·75per d was found appropriate, while at 10° Nfincreased 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-weight0·75per d on the medium- and high-protein rations respectively.4. The efficiency of N utilization (kN) reflected the differences in the relationships between NR and IDN. At 22° the relationship was curvilinear so thatkNdecreased with increase in both the level of feed intake and the protein content of the ration. At 10° the relationship was linear, hencekNwas 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 andkN, indicating that BU is a useful criterion for assessing the efficiency of N utilization.

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