Relationship of rate of lean tissue growth and other factors to concentration of urea in plasma of pigs.

The objectives of this study were 1) to investigate the relationship between plasma urea N concentrations (PUN) and lean tissue growth and 2) to compare the value of different variables, related to lean growth and renal function, to correct the relationship between dietary lysine concentration and PUN response for variation not related to amino acid adequacy. Forty-eight gilts (64.8 kg BW) were individually penned (blocks based on initial BW) for 50 d: a 10-d adjustment, a 35-d pretreatment, and a 5-d treatment period. During the pretreatment period, ADFI, urine specific gravity (SG), serum creatinine (SC), PUN, and daily fat-free carcass lean (DFFCL), empty body protein (DEBP), total carcass fat (DCF), and empty body lipid (DEBLI) depositions were measured. Partial correlation coefficients (ADFI effect removed) indicated a strong and inverse relationship between PUN and lean growth (DFFCL and DEBP) (r = -.88 and -.91, respectively, P < .01) and a positive relationship between PUN and fat deposition (DCF and DEBLI) (r = .66 and .54; respectively, P < .22). Treatments consisted of six dietary lysine concentrations (.475, .550, .625, .700, .775, and .850%). Initial and final BW in the treatment period were 103.3 and 107.7 kg, respectively. Pretreatment PUN (PUNO) was the pretreatment variable with the greatest R2 and the smallest MSE when used in the model describing the response of PUN (PUN1) to dietary lysine. The estimated lysine requirements from the PUN1 response corrected with either PUN0 or with the combination of PUN0, ADFI, DFFCL, DCF, SG, and SC were not (P > .05) different (.656 vs > 678%, respectively). We conclude that 1) PUN concentrations have a potential value as an indicator of the efficiency of lean tissue growth and 2) pretreatment PUN is a useful variable to correct treatment PUN for variation not related to amino acid adequacy.