Ruminant Nutrition: Growing Cattle and Beef Breeding Herd

Data from an experiment utilizing 78 individually fed heifers (407 ± 32 kg) were used to determine relationships of G: F and residual feed intake (RFI) to metabolizable protein balance (MPB), purine derivative excretion to creatinine excretion (PD:C) and 3-methyl histidine excretion to creatinine excretion(3MH:C). Heifers were fed steam-flaked corn based diets containing either 0 (NEG) or 1.5% (POS) urea for 95 d, resulting in CP levels of 9.6 and 13.7% for NEG and POS, respectively. Animal BW and urine samples were collected at 3 different times (28, 56, and 84 d) and urine was analyzed for PD: C and 3MH: C. To determine MPB, ADG, DMI, and final BW adjusted to equal (28%) empty body fat were used as inputs for the 1996 NRC model. Data were analyzed from 3 periods: early (d 1 to 55; urine 28 d), late (d 56 to 95; urine 84 d), and overall (d 1 to 95; urine d 28, 56, and 84). Relationships between MPB and RFI were inconsistent and poor. A positive relationship between PD: C and G: F was observed within both NEG (r2 = 0.54; P < 0.01) and POS (r2 = 0.38; P = 0.02) treatments during the early period. No relationship between 3MH: C and G: F was observed (P > 0.10). A negative relationship between MPB and G: F was observed for both POS (r2 = -0.79; P < 0.01) and NEG (r2 = -0.65; P < 0.01) in all periods. The more negative MPB was primarily due to greater ADG for cattle with greater G: F. The positive relationship between early period PD: C and G: F suggests that increased microbial protein leads to greater G: F. The lack of a relationship between 3MH: C and G: F suggests that protein turnover does not impact G: F and perhaps differences in G: F related to protein metabolism are due to differences in microbial efficiency.