Abstract
An in vitro method based on 15N-labelled forage nitrogen (N) was developed to study ruminal N metabolism of soluble N (SN), insoluble N (ISN) and neutral detergent insoluble N (NDIN) fractions of timothy forage. Timothy grass was grown on replicated experimental plots with one plot receiving 15N-labelled and the other unlabelled N fertilizer. Harvested grass was preserved as dried grass or as formic acid treated or untreated silage. The intact forages and their corresponding N fractions were incubated in buffered rumen fluid in vitro to determine degradation parameters based on the 15N fluxes between labelled feed N and ammonia N pools. A high percentage (25–38%) of 15N-labelled ammonia disappeared from ammonia N pool during the first 15 min of incubation. Microbial uptake of dried grass SN fraction was higher than of silage SN fractions. Fractional degradation rates of SN from formic acid treated silage, untreated silage and dried grass during the first 6 hours of incubation were 0.145, 0.125 and 0.115 /h, respectively. By the end of the incubation period (28 h), 69, 66 and 43%, of the SN fraction of formic acid treated silage, untreated silage and dried grass, respectively were recovered as ammonia. The percentage of ISN fractions degraded to ammonia N were 9, 34 and 27%, respectively. Based on the changes in 15N-labelled ammonia N pool in blank incubation and appearance of 15N to ammonia N pool from 15N-labelled NDIN fractions, it was estimated that a significant portion of microbial lysis occurred when incubations were carried out for longer than 20 hours. With dried grass the contribution of ammonia N for microbial N synthesis was greater than with silages. Use of 15N-labelled forages together with this in vitro method is a promising technique for determining soluble N degradation parameters, but it requires further development to be used for determining degradation parameters of insoluble N fractions and work with whole feeds.
Highlights
Grass forage, and especially ensiled grass is the major component of dairy cow diet in Northern Europe, often consisting >50% of diet DM
The CP concentration was higher in the silage soluble N (SN) fraction than in SN from dried grass, whereas the CP concentration was lower in the silage insoluble N (ISN) and neutral detergent insoluble N (NDIN) fractions than in the corresponding fractions in dried grass
The results have indicated that the degradation rate of soluble non-ammonia N (SNAN) fraction to ammonia N is not infinite and that quantitatively an important fraction escapes ruminal degradation
Summary
Grass forage, and especially ensiled grass is the major component of dairy cow diet in Northern Europe, often consisting >50% of diet DM. Microbes in silage use water-soluble carbohydrates during silage fermentation to produce energy for growth This leads to losses of energy and changes in the composition of the fermentable substrate available for ruminal organisms [3,4], and reduced ruminal microbial efficiency, especially in poorly preserved forages [3]. Jaakkola and Huhtanen [5] reported greater microbial nitrogen (N) flow from the rumen with silage-based than with hay based diets, but the differences in total non-ammonia N flow were not significant These conflicting results indicate a need to improve understanding of the effects of forage preservation method on ruminal protein metabolism in dairy cows as total rumen protein degradability alone seems insufficient for evaluating forage protein value. Restricted silage fermentation has been shown to increase production of milk fat and protein, mainly due to increased dry matter intake [7]
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