Disruption of Leaf Tissues by Rumen Microorganisms: An Approach to Breeding Bloat‐safe Forage Legumes

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Abstract
To develop the basic knowledge for breeding a bloat‐safe alfalfa (Medicago saliva L.) cultivar, the extent of leaf tissue disruption by rumen microbes was measured using a modified nylon bag technique. Only green leaf fragments were retained for determination of undigested dry matter. Among six legume forages, the relative rates of leaf tissue disruption were: alfalfa = red clover (Trifolium pratense L.) = white clover (Trifolium repens L.) > birdsfoot trefoil (Lotus corniculatus L.) = cicer milkvetch (Astragalus cicer L.) > sainfoin (Onobrychis viciifolia Scop.). Thus the bloat‐causing legumes (alfalfa, red clover, and white clover) were disrupted more rapidly than the bloat‐safe legumes (birdsfoot trefoil, cicer milkvetch, and sainfoin). These results are consistent with the cell rupture theory of legume pasture bloat.Six alfalfa cultivars (‘DuPuits’, ‘Ladak’, ‘Lahontan’, ‘Ranger’, ‘Vernal’, ‘Uinta’) had similar rates of leaf tissue disruption, and there were no differences between high‐ and low‐saponin strains of these cultivars. However, consistent differences were detected among 11 clones (genotypes) of ‘Beaver’ alfalfa. This finding encourages further development of nylon bag digestion as a method for breeding a bloat‐safe alfalfa cultivar by selection for low leaf tissue disruption. A 25 to 30% reduction in dry matter loss at 6 to 8 hours of digestion would be required to reach the bloat‐safe threshold.