Effects of toasting temperature and duration on in vitro ruminal gas production kinetics and post-ruminal crude protein from field pea (Pisum sativum) legume grain silages

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Field peas provide valuable starch and protein, but especially the latter is highly soluble in the rumen. Toasting of ensiled pea grains might reduce ruminal starch and protein degradation. The objective was to test this by measuring 24 h total gas production, gas production kinetics, and post-ruminal crude protein (PRCP) in an in vitro batch culture system. First, toasting temperature was varied with 120, 140, 160, 180, and 200 °C stages, while keeping a constant treatment duration of 30 min, and second, toasting temperature was kept constant at 160 °C, and the duration was varied with 10, 20, and 30 min. Rumen-undegraded protein (RUP) was measured using the Streptomyces griseus protease test. Moreover, ensiling and toasting effects on the morphology of pea starches were visually examined using scanning electron microscopy. In vitro gas production decreased with increasing toasting temperature or duration of heat exposure (P < 0.05). Morphological changes of starch granules and embedding matrices became visible at a toasting temperature of at least 180 °C. Post-ruminal crude protein was less affected by ensiling and ensiling + toasting. However, ensiling + toasting decreased the amount of soluble protein, but increased the insoluble protein fractions. Furthermore, protein solubility decreased with increasing temperature (by at most 41% of crude protein) or duration of toasting (by at most 21% of crude protein), and RUP increased by approximately 10 g/kg dry matter (DM) through ensiling, and by maximally 40 to 50 g/kg DM through toasting (P < 0.05). The results indicate that ensiling and subsequent toasting of field peas might elevate the concentration of ruminally stable carbohydrates and protein, which, however, do not necessarily result in elevated PRCP concentrations. Whether we achieve protein protection or protein damage after heat exposure depends on the temperature and duration of the toasting process.