Ruminal large and small particle kinetics in dairy cows fed primary growth and regrowth grass silages harvested at two stages of growth

Passage, breakdown and digestion rates of large and small particles were estimated using the rumen evacuation technique and total faecal collection with four lactating dairy cows in a 4×4 Latin square experiment. Two primary growth grass (PG) and two regrowth grass (RG) silages, each harvested at two different growth stages, were fed as dietary treatments supplemented with 8.0 kg concentrate per day. Ruminal contents and faeces were divided into large (>1.25 mm; LP) and small (1.25-0.04 mm; SP) particles by wet sieving. Indigestible neutral detergent fibre (iNDF) was determined by 12-day ruminal in situ incubation followed by a neutral detergent extraction. Ruminal iNDF content of LP (LP-iNDF) and SP (SP-iNDF) for both PG and RG diets increased with advancing maturity of the grass. The passage rate of SP-iNDF tended to be faster (P<0.10) for PG than for RG diets (0.0478 versus 0.0418/h on average). Particle breakdown rate increased with advancing maturity for PG while it was not affected by maturity for RG diets (P<0.01 for interaction of harvest×maturity). The passage rate of potentially digestible neutral detergent fibre (pdNDF) for SP (SP-pdNDF) of PG decreased (0.0350 versus 0.0284/h) while it increased (0.0284 versus 0.0342/h) for SP-pdNDF of RG with advancing maturity (P<0.01 for interaction of harvest×maturity). The digestion rate of LP-pdNDF and pdNDF in SP (SP-pdNDF) decreased with advancing maturity for both PG and RG diets (P<0.05), and it was faster for SP compared to LP for all diets (P<0.001). Ruminal mean retention times (MRT) of LP-iNDF and SP-iNDF for RG were longer than those for PG diets (P<0.001 and P<0.05, respectively). Ruminal mean turnover time (MTT) of LP-pdNDF was longer for RG than PG diets (P<0.05) and it increased with advancing maturity for both PG and RG (P<0.05).