Estimating utilisable crude protein at the duodenum, a precursor to metabolisable protein for ruminants, from forages using a modified gas test

As protein evaluation systems are evolving, they are increasing in their sophistication and complexity. In almost all systems an estimate of microbial crude protein (MCP) and ruminally undegraded feed CP (RUP) must be available. A problem lies in the accuracy of these measurements, especially RUP, which is often estimated by the controversial in situ technique. A new in vitro method has been developed which provides a direct estimate of combined MCP and RUP. The modified Hohenheim gas test (modHGT) involves incubation of a feedstuff in rumen fluid. The non-ammonia N content after incubation is used to determine utilisable CP at the duodenum (uCP) which is defined as the sum of MCP and RUP at the duodenum. In our study, 23 forages were examined using the modHGT and shown at three assumed rates of rumen passage (Kp: 0.02, 0.04, 0.06/h). Results were regressed against uCP values calculated using the standard procedure in Germany. Calculated uCP correlated with determined uCP at all rates of passage (Kp2: P<0.038, r2 = 0.19; Kp4: P<0.0001, r2 = 0.56; Kp6: P<0.0001, r2 = 0.67). Due to the simplicity of the reference method it is possible that the modHGT provides more accurate results. Although the new method is also simple, it considers interactions between carbohydrate and protein degradation by rumen microbes and uCP is estimated from the fermentation end product, ammonia. Utilisable CP may then be used to calculate metabolisable protein.