In situ and in vitro evaluation of crude protein degradation and utilisable crude protein content of barley, rye and triticale grains for ruminants.

Abstract

Rations for dairy cows are comprised of high proportions of cereal grains. Thus, despite their low crude protein (CP) content, grains can contribute considerably to the CP intake of dairy cows. This study was conducted to describe and compare ruminal CP degradation of a broad range of barley, rye and triticale genotypes in situ and in vitro and different methods to estimate the utilisable CP at the duodenum (uCP). Twenty samples each of rye, barley and triticale were incubated in situ and in vitro. Exponential regression analyses were used to estimate in situ degradation parameters. Further, the effective degradability (ED), ruminal undegraded CP (UDP) and uCP for ruminal passage rates of 5% and 8% per hr were estimated. The uCP was estimated in vitro and based on two different approaches using in situ UDP data and estimates of microbial synthesised protein (based on fermented organic matter [fOM] or equations of the Gesellschaft für Ernährungsphysiologie). The degradation rate declined from rye (43% per hr) to triticale (27% per hr) to barley (20% per hr), and it exhibited remarkable variation between the genotypes of a single species. The maximal degradable CP fraction also differed between the species, but was overall very high (94%-99%). The lowest washout fraction (26%) and the highest variation in ED (77%-86% and 69%-80% for a passage rate of 5% and 8% per hr, respectively) were found in barley. The in situ uCP content (estimated using fOM) was lower for barley than for rye and triticale at ruminal passage rates of 5% and 8% per hr (barley: 157g/kg DM at both passage rates; rye and triticale: 168 (at 5% per hr) and 169 (at 8% per hr) g/kg DM). In vitro estimations of uCP did not differ between the grain species and uCP estimated according to GfE was higher for triticale than for barley and rye, which did not differ. The low variation within a single grain species and the weak correlations between ruminal CP degradation and nutrient concentrations suggested that differentiation of ED and uCP between the genotypes of a single grain species is not necessary.