Ruminal degradation characteristics of neutral detergent fibre (NDF), starch and crude protein (CP) in concentrate ingredients were estimated in dairy cows using nylon bag incubations. Soluble fraction (starch, CP), undegradable fraction (NDF, CP), lag time (NDF) and rate of degradation of the insoluble but degradable fraction (NDF, starch, CP) were measured and showed large variation. Based on measured and partly estimated characteristics ratio's were calculated between total rumen available CP and carbohydrates, between soluble CP and soluble carbohydrates and between insoluble rumen available CP and insoluble rumen available carbohydrates. Ratio's varied largely between feeds and between soluble and insoluble fractions. It is concluded that such ratio's can be used to optimize the composition of concentrates with regard to rumen fermentation. (Abstract retrieved from CAB Abstracts by CABI’s permission)

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.

Free AA in plasma and muscle were monitored in 36 dairy cows from 2 wk before until 15 wk after parturition. Cows were kept indoors and fed individually. The AA concentrations in plasma from 6 to 15 wk of lactation were compared with precalving concentrations. Plasma concentrations of Met, Phe, Glu, and Gln decreased by 16, 24, 25, and 25%, respectively. The ranking of the essential AA according to their decrease in plasma corresponded to the order in which they generally appear to be limiting for milk protein synthesis. The decrease of Glu and Gln in plasma exceeded that of the essential AA and contrasted strongly with all other nonessential AA. The change in the AA profile in muscle from pregnancy to lactation resembled that described for the catabolic state in mammals and suggests that muscle protein was degraded for supply of AA to the udder, despite excess protein and energy supply 15 wk into lactation. In muscle, the decrease in the pool of free Gln exceeds 25% and is higher than for any other AA. These observations support our hypothesis that Gln is potentially limiting for milk protein synthesis in the high yielding dairy cow.

Changes in digestion and AA supply in dairy cows were studied when fresh grass was partly replaced by concentrate mixtures based either on corn starch or sugar beet pulp fiber. Treatments were tested in a Latin square utilizing three lactating cows' with ruminal and intestinal cannulas. Partial replacement of grass decreased CP digestibility. When high starch concentrate was fed, NDF digestibility was lower than that of the high fiber diet, mainly because of decreased ruminal digestion of NDF. For the high starch concentrate, 39% of the ingested starch escaped ruminal fermentation. Although less OM was fermented in the forestomachs on high starch concentrate, the duodenal AA N flow was higher than for the high fiber concentrate. The proportion of microbial protein was unaffected; thus, efficiency of microbial synthesis was estimated to be higher when high starch concentrate was fed.

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