THE NUTRITIVE-VALUE OF WHEAT AND OAT SILAGES ENSILED ON 3 CUTTING DATES

The production of biomass and its partitioning into stem, leaf, ear and dead material were monitored weekly in crops, of wheat and oats from 16 May 1990, which corresponded approximately to ear emergence in the wheat, until ensilage on 29 June, 7 July and 19 July 1990 using the Ag-Bag system. An enzyme/inoculant additive was used on both crops on each ensilage date, although a portion of oats was ensiled without additive on the first two cutting dates, giving a total of eight silages. Changes in rumen degradability of dry matter were assessed for both crops throughout the monitoring period using the synthetic fibre bag technique. The digestibilities of gross energy, modified acid detergent fibre, organic matter and crude protein were determined in vivo using sheep. Over the total period of monitoring, the daily rates of increase in dry matter (DM) yield were 15.1 (+/- 1.6) and 16.5 (+/- 1.9) g DM/m2 for wheat and oats respectively. From 29 June to 19 July the rates of increase were 11.3 (+/- 3.1) and 23.1 (+/- 6.0) g DM/m2. The increase in weight of the ears accounted for almost all of this increase and, by the end of the monitoring period, the ears contributed approximately half of the dry matter of each crop. In both crops the portion of dead material was small until 19 July, when there was a rapid increase in the amount of dead stem. The DM content of both crops remained < 30 % up to day 33 (18 June) but increased from c. 33 % to c. 52 % between days 53 and 65 (8-20 July). Rumen degradability of both crops decreased rapidly from c. 66 % on 16 May until 16 June, when it was c. 56 % for wheat and 55 % for oats; it remained constant at these values thereafter. For silages made on the three successive ensilage dates, there was an increase in DM and starch contents but decreases in digestible energy, digestible fibre and digestible crude protein contents. Digestible organic matter was similar for the first two ensilage dates but lower in silage made on the third date. The use of an additive with the oat crop resulted in decreases in the digestible energy, digestible organic matter and digestible crude protein contents of the silages compared with untreated oats. Maximum yield of DM from wheat (18.6 t/ha) was obtained with the third cutting date, but optimum yield of energy (170 GJ/ha) and digestible crude protein (746 kg/ha) were found at the second cutting date. For oats, maximum yield of DM (17.3 t/ha) was again found with the third cut and maximum yield of energy (159 GJ/ha, untreated; 140 GJ/ha with additive) with the second cut. Digestible crude protein yield was greatest with the second cut of oats when no additive was used (708 kg/ha) but with additive it was greatest with the first cut (661 kg/ha). It was concluded that, under these conditions, the optimum date for ensilage of wheat or oats was 7 July and that the use of the additive was detrimental to the quality of the oat silage.