Protein Value of Forages and Nitrogen Balans in Wethers


1 Institute of Forage Crops, BG-5800 Pleven, Bulgaria
2Unite de Recherches sur les herbivores, INRA, Clermont Ferrand, France


KIRILOV, A., JEAN-PIERRE DULPHY, M. JAILLER and M. DUDILLEU, 2005. Protein value of forages and nitrogen balans in wethers. Bulg. J. Agric. Sci., 11: 87-92

The replacement of the old system of protein estimation "digestible protein" by the new one "protein digested in the small intestine" PDI necessitated a complex procedure requiring determination of theoretical degradability of protein, its real indigestible part and the quantity of the microbial proteinsynthesized in the rumen and having pessed into the small intestene. The hypothesis of relation between the forage protein value and the nitrogen balance in the animal having ingested this forage and whether this relation can be used for total estimation of protein feeding value of forages was verified by this study.
The effect of three different forages (straw, hay from the first cut of a natural sward of grasses and from aftergrass), the wether age (one _ or two-years old) and the different sequence of fedding of the forages was studied for that purpose. It was found that when the animals had passed from feeding with a forage having low nitrogen retention to a forage having higer one the increase of assimilated CP was 31.7 g/day and in case of reverse sequence of the forages the retention was smaller, 21.1 g/day. For the same quantity of ingested protein there was 8.2 g higher nitrogen retention in the young wethers of one year, as compared to those of two years, but the retention coefficient was lower, 0.35 as compared to the accepted one, 0.65. The difference between the theoretically calculated retention and the true one was surprisingly great and it could not be used as an accurate predictor in all cases when determining the forage PDI value. Probably when using wethers for determination of the protein feeding value of a given forage it will be necessary to standardize the animales, i.e. to know at the present moment their capacity of N retention using a control forage or a standard forage.

Key words: nitrogen retention, nitrogen balance, PDI, straw, hay, intake, wethers