Metabolisable protein supply to lactating dairy cows increased with increasing dry matter concentration in grass-clover silage

The aim of this experiment was to study the effect of increased dry matter (DM) concentration in grass-clover silage, obtained by extending the pre-wilting period before ensiling, on the amount of metabolisable protein (MP) supplied to lactating dairy cows. Spring growth and first regrowth of grass-clover swards grown by two Danish organic farmers were cut and pre-wilted to a planned DM concentration of 350 and 700 g/kg, respectively, giving in total eight silages with DM concentrations ranging from 283 to 725 g/kg. Four Holstein dairy cows in late lactation with fistulae in rumen, duodenum and ileum were included in a crossover design, with five periods of 21 d. The cows were fed ad libitum with the experimental silages without any concentrate, but with daily supply of minerals and vitamins. Feed intake was registered daily and in the last week of each period 12 subsamples of duodenal and ileal chyme and faeces, respectively, were collected over 94 h to cover the diurnal variation, pooled, and subsequently analysed. Rumen fluid was collected in same sampling procedure. To estimate the duodenal flow of microbial protein, microbes were isolated from the rumen and analysed for amino acids (AA) and purines. Methane (CH4) production was measured the last two days in each period in open-circuit respiration chambers. Results were analysed using a linear random regression model with DM concentration as fixed effect, cow and cut number x farmer as random intercepts and with a cut number x farmer random slope. The amount of AA digested in the small intestine increased (P = 0.024) by 5.59 g/kg DM intake with each increase in silage DM concentration of 100 g/kg. The increased digestion of AA in the small intestine was caused by a higher small intestinal digestibility of AA and a tendency towards a higher duodenal flow of AA. The higher duodenal flow of AA derived from a lower rumen degradation of feed protein and a tendency towards a higher microbial synthesis in the rumen. Fibre digestibility and CH4 production were not affected by silage DM concentration. In conclusion, MP concentration in grass-clover silage can be improved by pre-wilting to a higher DM concentration before ensiling.