EFFECTS OF FEEDING HIGH-CONCENTRATE DIETS AT RESTRICTED INTAKES ON DIGESTIBILITY AND NITROGEN-METABOLISM IN GROWING LAMBS

In Trial 1, 12 wether lambs (34.2 +/- .8 kg BW) were used to determine the effects of restricting intake of diets increasing in percentage o f concentrate, to achieve isoenergetic intakes, on digestibility and N metabolism The trial consisted of two 14-d periods, 9 d for;adaptatio n and 5. d for total collection of urine and feces. Treatments were 22 , 39, 61, and 92% concentrate diets fed at ad libitum intake and 90, 8 0 and 70% of ad libitum intake, respectively. Diets were formulated to provide equal daily intakes of ME. Feeding diets increasing in propor tion of concentrate at restricted intakes resulted in linear increases (P < .001) in DM, OM, ADF, and NDF digestibilities. Starch digestibil ity was ndt affected (P > .10) by intake. Apparent N digestion was imp roved (P <.001) with restricted feeding of diets containing greater pr oportions of concentrate. Nitrogen retention was increased (P < .005) for lambs receiving diets containing a greater proportion of concentra te at reduced intakes. In Trial 2, 12 wether lambs (30.6 +/- .6 kg BW) were used to determine the effects of feeding high-concentrate (92%) diets at reduced intakes on digestibility and N metabolism. Diets were fed at ad libitum intake and 96, 80, and 70% of ad libitum intake. Th e trial consisted of two 14-d periods, similar to Trial 1. Restricting the intake of high-concentrate diets improved (P < .001) digestibilit y of DM, OM, ADF, starch, and CP. Digestibility of DM, ADF, CP, and st arch increased .142, .423, .497, and .046 percentage units, respective ly, for each 1% reduction in DM intake. Nitrogen retention was quadrat ic with maximal retention (percentage of N digested) occurring at an i ntake of 89% of ad Libitum. It was concluded that diet digestibility a nd N utilization were enhanced when a restricted feeding program was u sed that provided equal daily intakes of protein, vitamins, and minera ls.