Particle size reduction of the leaves of a tropical and a temperate grass by cattle: I. Effect of chewing during eating and varying times of digestion

This study investigates forage particle breakdown, which is a factor of potential importance for the low intake of forages, particularly tropical grasses. A comparison was made between fresh leaf blades of a tropical grass, green panic ( Panicum maximum var. trichoglume ) and a temperate grass, Italian ryegrass ( Lolium multiflorum ). Leaf blades were chopped into 50 mm lengths and fed to cattle with oesophageal and rumen fistulae. Particle size was assessed on chewed samples immediately after eating and after digestion for 6, 12, 24, 48, 96 h and 3 weeks in nylon bags in the rumen. Chewing during eating reduced particle size more in length than in width and more in green panic than in ryegrass. Mean length was decreased 9·2‐fold and 4·7‐fold for the two species respectively, and mean width 5·4‐fold and 2·3‐fold. Green panic leaf had a higher cell wall content and higher dry matter content than ryegrass leaf. Digestion caused a substantial reduction in the width of the particles but not in the length. Width reduction occurred more rapidly in the ryegrass which was reduced to narrow fibres within 12–24 h of digestion while in green panic the same extent of degradation took < 48 h. After 96 h digestion, mean width of the chewed material had been reduced 40‐fold in ryegrass compared with only 165‐fold in green panic; all particles of both species were < 1 mm in diameter. The mean length of particles after 96 h digestion was similar to that of the chewed feed. Ryegrass was more digestible than green panic, a difference of 150 g kg DM −1 which was maintained from within the first 6 h of digestion up to 96 h digestion. After three weeks the digestibility of ryegrass was c. 50 g kg DM −1 higher than that of green panic but the proportion of cell wall that was ultimately indigestible was the same (0465) for both species.