In situ digestibility of protein in barley grain (Hordeum vulgare) and peas (Pisum sativum L.) in dairy cows: influence of heat treatment and glucose addition

Rumen degradation and total tract indigestibility of protein and amino acids (AA) in untreated and heat-treated (at 100, 125 and 150 °C for 5, 15 and 30 min) barley grain (Hordeum vulgare) and peas (Pisum sativum L.) was studied in non-lactating cows. The treated samples, and the control, were incubated in the rumen for 4, 8, and 16 h using an in situ method. Total tract indigestible protein fraction (IPF) was measured on the 16 h rumen residues using an intestine mobile nylon bag method. In both barley and peas, heat treatment reduced (P<0.05) rumen degradation of protein (RDP). In barley, most of the effect was obtained at the lowest heat treatment (100 °C/5 min), and treatment time was significant (P<0.05) only at 100 °C. In peas, the RDP decreased with increasing temperature and time, and the lowest RDP was obtained with the harshest treatment (150 °C/30 min). The content of cysteine, lysine and arginine decreased after treatment at 150 °C for 30 min in both feeds, and lysine in barley and cysteine in peas additionally decreased (P<0.05) after treatment at 125 °C for 30 min. Rumen degradation of total AA (TAA; measured only in control and samples treated for 30 min and without glucose addition), decreased to the same extent as that of CP with increasing treatment temperature, but degradation varied among individual AA. In barley, it was lysine, methionine, alanine and valine that appeared least degradable after treatment at 125 and 150 °C for 30 min. In peas, cysteine was the most ruminally protected AA. Additional glucose did not affect the RDP in heat-treated barley, but it reduced (P<0.05) RDP in peas. In barley, total tract IPF and the indigestible fraction of TAA was increased (P<0.05) by the harshest heat treatment only (150 °C/30 min). Indigestible fractions of cysteine, methionine, threonine, serine, phenylalanine, histidine and lysine of barley were also increased (P<0.05) after treatment at 125 °C for 30 min. In peas, the IPF increased with high heat input and addition of glucose. The indigestible fraction of all individual AA increased at 150 °C for 30 min. Optimal protection of barley protein from rumen degradation is achieved at temperatures below 125 °C, while in peas higher temperatures are needed. Rumen crude protein (CP) degradation is not an accurate predictor of rumen degradation of individual AA.