Influence of oscillating dietary crude protein concentration on performance, acid-base balance, and nitrogen excretion of steers1,2,3

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Abstract
Decreasing dietary N inputs into beef cattle feeding operations could potentially decrease environmental concerns relating to air and water quality. Previous studies with sheep suggest that oscillating dietary CP concentrations may improve N use efficiency and thereby decrease dietary N requirements. Therefore, two studies were conducted to determine the effects of oscillating dietary CP concentrations on performance, acid-base balance, and manure characteristics of steers fed high-concentrate diets. Steers were fed to a constant backfat thickness in both studies. In the first trial, 92 steers (mean BW = 408 ± 2.8 kg; four pens/treatment) were fed the following diets: 1) constant 12% CP, 2) constant 14% CP, and 3) 10 and 14% CP oscillated at 2-d intervals. Steer performance and carcass characteristics were measured. In the second trial, 27 steers were individually fed the same three experimental dietary regimens (nine steers/treatment). Animal performance, arterial acid-base balance, plasma metabolites, and fecal characteristics were measured. In both trials, steers fed the 14% CP diet tended (P < 0.10) to have greater ADG and gain:feed than steers fed the 12% CP diet. Steers fed the oscillating CP regimen had intermediate performance. In Trial 1, steers fed the 14% CP diet tended (P = 0.09) to have smaller longissimus area and higher quality grades than steers fed the oscillating CP regimen. Protein retentions (g/d) calculated from NRC (2000) equations were greater (P = 0.04) for steers fed the 14% CP diet than steers fed the 12% CP diet. Steers fed the oscillating CP regimen tended (P = 0.08) to have greater calculated protein retention (g/d) than steers fed the 12% CP diet. Steers fed the 14% CP diet had greater (P < 0.05) calculated urinary N excretion than steers fed the 12% CP or oscillating CP regimens. Venous plasma concentrations of urea N were greater (P < 0.001) in steers fed the 14% CP diet than in steers fed the 12% CP diet; steers fed the oscillating CP regimen were intermediate but fluctuated over days. Based on arterial blood gas concentrations, acid-base balance was not significantly affected by dietary CP regimen. Results of these trials suggest that the CP requirement of steers in these studies was greater than 12% of the diet DM, and/or that the degradable CP requirement was greater than 6.3% of diet DM. However, the effects of oscillating dietary CP were minimal.

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