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Three rumen fermentation studies in combination with three feeding trials were carried out to investigate the effect of partial replacement of heavily fertilized perennial ryegrass by low protein feedstuffs on pH and concentrations of VFA and NH3 N in the rumen and on N excretion in milk, urine, and feces by dairy cows. Feedstuffs tested were high fiber concentrate mixtures based on sugar beet pulp and soybean hulls and high starch concentrate mixtures based on corn (Experiments 1 and 2), corn silage (Experiment 1), dried and ensiled pressed sugar beet pulp and high moisture ear corn silages with or without husks (Experiment 3). In the fermentation studies, N intake ranged between .43 and .57 kg/d. Partial replacement often increased DMI (maximal by 2.6 kg), resulting in minor effects on N intake. Urinary N excretion ranged between 30 and 58% of N intake and decreased by 30 to 40% when grass was partially replaced. Fecal N output was between 25 and 30% of N intake and tended to increase with the low protein feed. The reduction in urinary N excretion corresponded to a decrease of rumen NH3 N. Replacement by concentrate mixtures based on corn reduced milk fat content; for mixtures based on beet pulp, milk fat content was not changed.
Vuuren et al. (Fri,) studied this question.