Effect of quantity and source of rumen nitrogen on the efficiency of microbial protein synthesis in steers consuming tropical forage

Abstract

Low values for the efficiency of microbial protein synthesis (EMPS) in cattle consuming tropical forages are related to low rumen degradable crude protein (RDP) intakes. This study examined the effect on the EMPS of the quantity and source of nitrogen (N) supplied to the rumen of eight entire and four rumen-fistulated Brahman steers consuming mature tropical grass hay (57.3 g crude protein/kg DM). Four treatment diets were fed in a Latin square design and included a basal diet of mature pangola grass (Digitaria eriantha) hay (control) and hay plus supplements estimated to provide 150 g RDP/kg digestible organic matter intake (DOMI), as urea or casein, or 300 g RDP/kg DOMI as casein. The EMPS was only increased (P < 0.05) above that for the control diet (167 vs 123 g microbial crude protein (MCP)/kg DOMI) when RDP was provided at the highest rate of 293 g/kg DOMI. This increase was also associated with an ~4-fold increase in the concentration of NH3-N (277 vs 73 mg/L) and of branched-chain volatile fatty acids (44 vs 10 mmol/mol of total volatile fatty acids) in rumen fluid of the steers. However, the source of rumen degradable N (urea or casein) had no effect on the EMPS (109–115 g MCP/kg DOMI) when supplied at ~150 g RDP/kg DOMI. There was no effect of treatment on in vivo neutral detergent fibre digestibility (599 g/kg DM) or the rate (0.037/h) or extent (potential degradable fraction: 636 g/kg OM) of in sacco disappearance of pangola grass hay. In addition, rumen particle dilution rate was unaffected by treatment (0.022/h) and rumen fluid dilution rate, although showing some treatment differences (0.048–0.062/h), was poorly correlated with EMPS. It was concluded that only high amounts of RDP supply to the rumen, in the form of true protein, resulted in increased EMPS whereas at the lower intakes of RDP formulated to achieve EMPS in the range suggested in the feeding standards (130–170 g MCP/kg DOMI) there was no difference in providing the RDP as non-protein N or degradable-protein.

Small differences in biohydrogenation resulted from the similar retention times of fluid in the rumen of cattle grazing wet season C3 and C4 forage species

“…The rumen volumes estimated here have potentially large errors given the non-steady state conditions under which the CrEDTA marker is used but nevertheless they provide a comparative relative estimate. The values are comparable to the values derived by Poppi et al (1981a) under steady state conditions and Bowen et al, (2017) under grazing conditions using a similar procedure. Visual observations, obtained when the cannula was opened for sampling, indicated that the rumens were quite full except for the dry season speargrass, which was also seen in the estimate by marker dilution.…”

“…Insert Table 7 with ryegrass (10.8 h) (Bowen et al, 2017). The important parameter is the actual RTs, which are lower than those suggested from pen studies with tropical grass hays (16-19 h) (Poppi et al, 1981a).…”

“…Therefore, gathering information about the FA profile and the RT in the rumen of cattle grazing various forages commonly used in the tropics and subtropics is important. We hypothesized that C4 grasses with an expected long RT of rumen fluid (RF) (Poppi et al, 1981a;Bowen et al, 2017) would lead to a much more extensive biohydrogenation than C3 grasses, plants that utilize the C3 carbon fixation pathway (Taiz and Zeiger, 2002), with the result of a greater degree of saturated FA and lower concentrations of CLA isomers.…”

“…The addition of a legume in the pasture had no major changes and the RT were similar to wet season grass. These results measure for the first time the RT of CrEDTA in the rumen when animals graze a variety of tropical grasses and may be compared to the only other study, that ofBowen et al (2017), who found RT of 9-10 h for C4 wet season tropical grasses typical of southern Queensland in Australia (black speargrass and forest bluegrass (Bothriochloa bladhii) in a native pasture, and the introduced pasture, creeping bluegrass (Bothriochloa insculpta)), and RT ranging from 8.5-12 h for tropical legumes Lablab purpureus and Clitoria ternatea, and 22 h for RT of the liquid phase (as measured by CrEDTA) in RF of cattle grazing the dry season tropical grass. These RTs are not much greater numerically to that found…”

“…The tropical forages were all grazed in a growing vegetative state representative of wet season pastures when quality is highest and when animals deposit most weight and presumably fat. The exception to this was the grazing of dry season speargrass which resulted in significantly lower total VFA concentration and longer RT of liquid phase marker in the rumen and represents the lowest quality pasture in a seasonal grazing cycle, such as found in tropical regions ofAustralia (Winter et al, 1991) Bowen et al (2017). found low efficiency of microbial synthesis in the rumen (i.e.…”