Rate of feed degradation in strip-grazing dairy cows

Abstract

Nylon bags containing Persian clover, perennial ryegrass or perennial pasture hay were incubated for 12-h periods in lactating dairy cows grazing Persian clover pasture. The hypothesis was that the rate of dry matter disappearance over the first 12 h of incubation in the rumen would be lower when bags were inserted at a time when the rumen pH was at its lowest point for the day compared with a time when it was at its highest. It was also hypothesised that the reduction in rate of DM loss over 12 h at the initially low and then fluctuating rumen pH would be lower for Persian clover than for perennial ryegrass and the hay. Rumen fistulated cows grazing at 4 different pasture allowances (9, 16, 32 and 53 kg DM/day; 4 cows per treatment) were used in a completely randomised, split-plot design. The nylon bag incubation periods were 0700 to 1900 hours (period 1) and 1900 to 0700 hours (period 2) and rumen fluid pH was measured every 3 h during these periods. The ruminal fluid pH at the time of insertion of the nylon bags was higher (6.3 v. 5.7; P<0.001) in incubation period 1 than in period 2, and there was an interaction between the effects of incubation period and pasture allowance on the average rumen fluid pH in each period. Rate of DM loss was higher (3.47 v. 3.28%/h; P = 0.019) in incubation period 1 than in incubation period 2, highest (P<0.001) for clover, followed by ryegrass and then hay (5.05 v. 3.15 v. 1.93%/h) and higher (P<0.001) in cows grazing at the low (9 and 16 kg DM/cow) compared with high (32 and 53 kg DM/cow) allowances. There was a significant linear relationship (P<0.05) between DM loss rate calculated over 12 h and rumen fluid pH for grass and hay, but for clover this relationship only occurred in incubation period 2. These results suggest that more than 1 factor is important for determining feed degradation rates in the rumen at any particular time and that only using rates of forage DM loss that are obtained from nylon bags inserted prior to the morning feed, may overestimate the extent of rumen degradation of the feed eaten over the whole day in cows strip-grazing highly digestible pastures.