Microbial crude protein (MCP) is the primary source of protein for ruminants grazing protein deficient pastures in northern Australia. Increasing efficiency of MCP (eMCP, g MCP/kg digestible organic matter (DOM)) has the potential to increase daily live weight gain (LWG) of cattle grazing pastures typical of northern Australia by increasing protein supply to the animal. Microbial crude protein production can be indirectly measured using urinary excretion of purine derivatives (PD), a portion of these are of endogenous origin. The experiments reported in this thesis investigated the effect of N source and host on MCP production and eMCP, and to test whether endogenous PD output varied between individual animals.
Experiment 1 (Chapter 4) measured urinary PD excretion with increasing levels of dry matter intake (DMI) at 5, 10, 15 or 20 g DM/kg live weight (LW)/d. Daily urinary PD excretion was regressed against digestible organic matter intake (DOMI kg/d) and the y intercept indicated daily endogenous PD output for each individual animal. This was then compared to the mean endogenous value of 190 8mol/kg W0.75 from Bowen et al. (2006). Individual endogenous PD values ranged from 50 to 466 8mol/kg W0.75. Using the individual endogenous PD excretion value to calculate MCP production for individual Bos indicus steers (mean LW 467 ± 7.0 kg) resulted in MCP values that were not significantly different to those calculated using a single endogenous value in the equation from Bowen et al. (2006). This suggested that it was appropriate to use the constant value of 190 8mol/kg W0.75 for estimating endogenous PD output. The application of this value resulted in group mean values for MCP production not significantly different to those calculated by applying estimated individual endogenous PD excretion in the equation.
Experiment 2 (Chapter 5) measured response curves of MCP production to supplementation of a low (75 g RDP/kg DOM) or a medium (121 g RDP/kg DOM) rumen degradable protein (RDP) hay with supplements of urea, casein or branched chain amino acids + phenylalanine (BCAA) to gauge the response of rumen microbes to protein supplements (casein, BCAA) in comparison to non-protein N supplements (urea). Urea was provided at levels to give a total of 130, 170, 210 or 250 g RDP/kg DOM. The BCAA were fed in conjunction with urea at a level of 170 g RDP/kg DOM to ensure there was adequate N for rumen microbes to respond to BCAA supplements. The BCAA were fed at levels equivalent to the BCAA present in casein fed at 130, 170, 250 or 300 g RDP/kg DOM. Casein was fed at levels of 130, 170, 250 or 300 g RDP/kg DOM. There was no response of MCP (low RDP hay 0.7 g MCP/kg W, medium RDP hay 0.9 g MCP/kg W) or eMCP (low 103 g MCP/kg DOM, medium 96.7 g MCP/kg DOM) to increasing levels of RDP supplements. Supplementation had no effect on hay intake, digestibility or marker retention time or fractional outflow rate. Rumen ammonia concentrations and proportions of individual VFA were affected by supplementation. Rumen ammonia concentrations increased with all supplements as expected. BCFA increased as proportion of total VFA with casein and BCAA supplements.
Experiment 3 (Chapter 6) examined two groups of cattle that were selected on the basis of their growth rate for 90 d post-weaning (-70 g/d and 210 g/d) and measured a variety of rumen parameters to determine if variation in rumen function or voluntary feed intake (VFI) provided an explanation of the variation in growth rate. There were no differences in VFI (g DM/kg W/d) or any rumen parameter (MCP production, eMCP, rumen ammonia concentration, VFA profiles, marker fractional outflow rates and rumen pH) between the two groups. Serum IGF-1 levels were measured 90 d post-weaning and after 28 d of pen feeding and showed a significant difference between the two groups. The cattle were then supplemented with increasing levels of urea and cottonseed meal (CSM) to see if the response to N and protein supplements varied between animals selected on the basis of low or moderate growth rate. There was no difference between the two groups in their response to urea or CSM supplements. Microbial crude protein production and eMCP were not increased by urea supplements, but MCP production in response to CSM was increased by 1.49 ± 0.311 and 1.65 ± 0.109 g MCP/kg W/d for low and medium growth rate groups respectively indicating that the differences in post weaning growth rate were not due to variation in rumen function or VFI.
Fractional outflow rate was not increased in either experiment (Exp. 2 or 3). Efficiency of MCP is closely related to the fractional outflow rate of rumen contents and it was suggested that unless treatments also cause a change in fractional outflow rate then changes in eMCP are not likely to be observed. The variation in serum IGF-1 between the two groups of weaners selected on the basis of post-weaning LWG on N deficient pastures suggests that whole body metabolism rather than rumen function explains the variation in LWG seen in cattle in northern Australia.