Abstract
Ruminant agriculture suffers from inefficient capture of forage protein and consequential release of N pollutants to land. This is due to proteolysis in the rumen catalyzed by both microbial but initially endogenous plant proteases. Plant breeding‐based solutions are sought to minimize these negative environmental impacts. The aim of this study was to perform an integrated study of rumen N metabolism using semi‐continuous rumen simulation fermenters (Rusitec) to explore the extent to which swards containing Festulolium populations (interspecific hybrids between Lolium and Festuca grass species) with decreased rates of endogenous protein degradation conferred advantageous protein utilization in comparison with a National Listed perennial ryegrass. An in vitro experiment was conducted using three Festulolium hybrids (Lolium perenne × Festuca arundinacea var. glaucescens, LpFg; Lolium perenne × Festuca mairei, LpFm; and Lolium multiflorum × Festuca arundinacea var. glaucescens, LmFg) and a Lolium perenne, Lp control. LpFm and LmFg demonstrated significantly lower plant‐mediated proteolysis than the control. Fresh forage was incubated in Rusitec with rumen fluid from four donor cows. Feed disappearance and production of gas, methane, and volatile fatty acids were similar across cultivars. Whereas no differences in microbial protein synthesis were noted across treatments during early fermentation (0–6 hr after feeding), an increased microbial N flow in LpFm (+30%) and LmFg hybrids (+41%) was observed during late fermentation (6–24 hr after feeding), with higher overall microbial N flows (+13.5% and + 20.2%, respectively) compared with the control (Lp). We propose an underpinning mechanism involving the partitioning of amino acid catabolism toward branched‐chain amino acids and microbial protein synthesis in grasses with slow plant‐mediated proteolysis instead of accumulation of rumen ammonia in grasses with fast plant‐mediated proteolysis. These observations indicate the potential of Festulolium hybrids with a slow plant‐mediated proteolysis trait to improve the efficiency of capture of forage protein and decrease the release of N pollutants onto the land.
Highlights
Ryegrass (Lolium spp.) is often the forage grass of choice in mild temperate climates due to its high yield and nutritious value (Capstaff & Miller, 2018; Humphreys, Yadav, et al, 2006)
We propose an underpinning mechanism involving the partitioning of amino acid catabolism toward branched-chain amino acids and microbial protein synthesis in grasses with slow plant-mediated proteolysis instead of accumulation of rumen ammonia in grasses with fast plant-mediated proteolysis
Milk production is limited by lower crude protein content and dry matter intake compared with other forages such as legumes (Dewhurst, Fisher, Tweed, & Wilkins, 2003b)
Summary
Ryegrass (Lolium spp.) is often the forage grass of choice in mild temperate climates due to its high yield and nutritious value (Capstaff & Miller, 2018; Humphreys, Yadav, et al, 2006). Given that ingested forage undergoes stress-mediated metabolism on entering the rumen (Zhu et al, 1999; Kingston-Smith et al, 2012; Kingston-Smith et al, 2005), and that Lolium and Festuca species can show significantly different protein degradation it follows that Festulolium forage grass hybrids could have the potential to improve ruminal N use efficiency in a fresh feeding system. We hypothesize that by supplying the rumen with a fresh forage feed containing the slow plant-mediated proteolysis phenotype, during digestion the supply of protein breakdown products will better match availability of energy (generated by fiber breakdown) In consequence, this will limit the substrate availability for the hyperammonia-producing bacteria thereby decreasing (but not removing completely) the extent of ammonia production. A semi-continuous flow fermentation system was used to establish whether the presence of a slowed proteolysis phenotype in Festulolium hybrids, in comparison with the rate of proteolysis typical in a National List variety of L. perenne, could be used to enhance efficient microbial protein synthesis
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