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Levels of total nitrogen and total volatile fatty acids in the rumen fluid of sheep under different grazing management systems


, : Levels of total nitrogen and total volatile fatty acids in the rumen fluid of sheep under different grazing management systems. Aust J Exp Agr Anim Husb 7(28): 434-440

Concentrations of total N and total volatile fatty acids (VFA) in the rumen fluid of sheep grazing improved pastures were measured for 10 months in a dry year on the southern tableland of New South Wales, and the concentrations of N and VFA were related to season, wool production, and grazing management. The observations were made during 2 long-term grazing management experiments. In the 1st experiment 4 groups of breeding Merino ewes grazed a Wimmera ryegrass-subterranean clover (Lolium rigidum Gaud.-Trifolium sub-terraneum Lpasture. A deferred grazing system (autumn saving) of pasture management was compared with continuous grazing at stocking rate treatments equivalent to 7.0 and 3.5 ewes to the acre. In the 2nd experiment 2 groups of Merino weaners grazed a Phalaris tuberosa-subterranean clover pasture at a stocking rate equivalent to 8.6 sheep to the acre. One group received a hay supplement, the other did not. In both experiments N values were low between late autumn and mid-winter and again between late spring and summer, and high in early autumn and again in spring (P< 0.05). The low levels were about 55% of peak autumn and spring levels in (experiment 1) and 60% of peak autumn and spring levels in (experiment 2). Total VFA did not vary significantly between seasons in either experiment. At the higher stocking rate the ewes had lower levels of rumen total N than at the lower stocking rate. VFA did not vary consistently between stocking rates. At both stocking rates ewes on the autumn saving system of grazing management had more N in the rumen fluid during late pregnancy and early lactation than did those on the continuous grazing system (P< 0.001). After the ewes had access to the saved pasture, autumn saving resulted in a higher concentration of VFA than continuous grazing (P> 0.05). Although feeding a hay supplement benefited the weaners the concentrations of total N in the rumen fluid of the 2 groups of sheep were similar. There was a fairly consistent tendency for the group receiving hay to have lower concentrations of VFA in their rumen fluid. Possible reasons for these effects are discussed.

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