Forage protein value. New estimation of the rumen protein degradability and of the true digestibility of dietary protein in the small intestine, and consequences on the PDI values

Abstract

The PDI system expresses the protein value of ruminant feeds in digestible proteins entering the small intestine. To calculate the PDI value of a forage, we must know its nitrogen ruminal degradability (DT in %) and the true digestibility of dietary proteins in the small intestine (dr in %). Due to the lack of experimental data, the DT values for forages was set to fixed values in 1988, for example 73% of crude protein (CP) content for all fresh forages. However, it was shown that DT is affected by a botanical family, plant species, growth cycle and level of nitrogen fertilisation, and decreases with plant maturity stage. We present the new estimation methods of DT and dr for natural grasslands, grasses and legume forages. The new estimations of DT and dr were integrated in the calculations of the forage PDI values in the renewed INRA 2007 feed value tables.

To improve DT estimation, a database of 375 measurements for fresh forages was built. From the analysis of the database using a mixed model to take into account laboratory effects, DT can be predicted from the CP content of the forage, with a quadratic effect, and from a botanical family and vegetation cycle effects (r²;= 0.87; RSD = 3.5). Furthermore, DT values of conserved herbage can be predicted from the corresponding DT values of fresh herbage for hays (n = 29; r²;= 0.71; RSD = 3.6), as well as fermented herbage (direct cut, wilted or big bale silage) in combination with dry matter content (n = 32; r²;= 0.53; RSD = 4.2). Forage dr values are calculated from the dietary proteins entering the intestine (1.11 x CP x (1-DT)) and from the truly indigestible dietary protein (IDN). We predict IDN from forage characteristics (CP, botanical family, vegetation cycle) using an equation calculated on data of mobile intestinal nylon bags (n=63; r²=0.50; RSD=2.1).

Estimating DT and dr values from forace CP contents allow taking better into account, the effects of plant maturity stage and level of nitrogen fertilisation on herbage protein values. The effects of botanical family, vegetation cycle and the different conservation techniques are also better taken into account. Thus, the consistency of forage PDI values in the INRA 2007 tables is improved. The most important modifications induced by the new estimations of forage DT and dr concern the PDIE values that decrease for forages used at early vegetation stages during the first growth cycle, and that slightly increase for the re-growth cycles. For conserved forages, PDIE values of hays slightly increase, and those of direct cut silages decrease, whereas the values of wilted silages remain unchanged.