Effect of stage of maturity and chop length on the chemical composition and utilization of formic acid‐treated ryegrass and formic acid silage by sheep

Two areas of an early‐heading perennial ryegrass cv. Cropper were harvested by either a precision‐chop or a flail harvester at around 50% ear emergence (15 May 1978) and 14d later (29 May). Formic acid (85%) was applied at the rate of 2·2 litres t‐1. Mature crossbred wethers were used in a 2 × 2 factorial design to determine the effect of stage of maturity and method of harvesting (chop length) on the in vivo digestibilities of formic acid‐treated grass in experiment 1 and formic acid silage in experiment 2. Apparent digestibility coefficients were determined at a fixed level of feeding for both grass and silage and at ad libitum access to feed for silage only. There were no significant differences in the concentrations of crude protein (CP), ether extract (EE), acid‐detergent fibre (ADF) or acid‐detergent lignin (ADL) in grass or silages of differing chop lengths but the later cut forages had significantly higher ADF and ADL concentrations and lower CP concentrations than the early‐cut forages. The ADF and ADL concentrations were also higher in the silages than in the corresponding grasses. In general, the fermentation characteristics of precision‐chopped silage were better than for the corresponding flail‐cut silage but date of harvest was a more important determinant of quality and the late, flail‐cut silage had the highest butyrate and ammonia N concentrations and the highest pH (411) of any treatment. There was a significantly higher intake of precision‐chopped as compared with flail‐cut silage with both the early‐and the late‐cut silage but there were no significant differences attributable to stage of maturity (i.e. date of harvest) or significant interaction between chop length and maturity. The slightly increased intake of early harvested, precision‐chopped silage as compared with late precision‐chopped silage was not significant. Dry matter digestibility (DMD) of the grass decreased at a rate of 0004 units d‐1 post 50% ear emergence. The results of experiment 2 indicated a decrease of 0·207 units d‐1 in silage fed at a similar level. The late‐cut silage (DMD 0·292, mean of both harvesting treatments) thus had a significantly lower digestibility than the corresponding grass (mean DMD 0·247). Chop length had a variable influence on the DMD of both grass and silage fed at a fixed level but treatment differences were non‐significant. However, a trend towards higher digestibility of flail‐cut as compared with precision‐chopped silage was apparent and this became statistically significant when the animals were allowed ad libitum access to feed. This may be a response to the generally lower intake of flail‐cut silage.