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A short term feeding trial was conducted to assess effects of nitrogen (N) supplementation on growing cattle and the ability of the Cornell Net Carbohydrate and Protein System (CNCPS) to predict responses. Four corn-based diets were used: (1) a 30% ruminal N-deficient control diet; (2) a urea supplemented diet; (3) a urea+isolated soy protein (ISP) diet; and (4) a urea+ISP+blood meal (BM) diet. Four Holstein steers (average initial body weight of 251 kg) were randomly assigned in a 4×4 Latin Square treatment design. Steers were fed 95% of ad libitum intake on an hourly basis, and 500 μg of oestradiol-17β were administered twice daily. Average daily gain (ADG) increased (P <0.05) when urea was added to the diet, but including urea+ISP was not better than the control or urea alone (P >0.05). The urea+ISP+BM diet was better than the control or urea+ISP diets (P <0.05), but itwas not better than urea alone (P >0.05). Addition of ISP or ISP+BMto the urea diet did not increase whole tract true digestibility of neutral detergent fibre, acid detergent fibre, or apparent digestibility of the dietary non-structural carbohydrate fraction (P >0.05). The addition of urea, ISP or ISP+BMto the control diet caused an increase in N intake (P <0.05), but faecal N did not increase (P >0.05). Urinary N excretion increased 2.8-fold compared to the control diet when urea, ISP or ISP+BMwere added (P <0.05), but therewas no difference between these latter three treatments (P >0.05). Urea, ISP and ISP+BM increased N retention (P <0.05), and ISP+BM was higher than urea and ISP (P <0.05). The numerical difference in N retention between urea and urea +ISP was not significant (P >0.05). When urea, ISP or ISP +BM were added, plasma urea N increased (P < 0.05), but these three treatments had no effect on glucose, non-esterified fatty acids or insulin (P > 0.05). Results show that a ruminal N-deficiency can have a negative impact on ADG, and this effect was consistent with predictions made by the CNCPS. Substitution of urea by ISP did not cause an increase in ADG, indicating that ISP may not be as ruminally degradable as urea. Blood meal increased ADG, consistent with CNCPS predictions. Because ISP could have been a source of ruminally undegradable protein rather than ruminal peptides and amino acids, the peptide stimulation predictions of the CNCPS could not be evaluated.