Effect of lock up and harvest dates on dairy pasture dry matter yield and quality for silage in south-western Victoria

Summary. At 2 sites in south-western Victoria, 132 plots of predominantly perennial ryegrass pasture were randomly allocated, within 4 replicate blocks, to each of 3 lock up dates (L1, L2, L3) by 12, 12 or 9 harvest times. Harvesting commenced 2 weeks after initial treatment lock up with L1 and L2 being harvested 12 times (weekly intervals) and L3, 9 times. Lock up dates were 15 August (L1), 5 September (L2) and 26 September (L3) at site 1 and 17 August (L1), 7 September (L2) and 28 September (L3) at site 2. For each treatment and harvest date, dry matter yield and botanical composition were determined and samples of total pasture and the ryegrass fraction were collected and assessed for dry matter digestibility, crude protein and neutral detergent fibre. Dry matter yield was measured from the start of L1 (site 1, 15 August; site 2, 17 August) until the final harvest date of L3 (site 1, 12 December; site 2, 14 December). At site 1, L3 produced higher dry matter yields than L1 and L2 at comparable lengths of lock up time, whilst there were no differences at site 2. Over the total experimental period (site 1, 15 August–12 December; site 2, 17 August–14 December) there were no differences in total dry matter yield (t/ha) between treatments at either site (site 1—L1 5.79, L2 6.43, L3 5.94; site 2—L1 6.68, L2 5.07, L3 5.73). Treatments had little effect on botanical composition at either site when compared at the same time after lock up, both during the harvesting period or in the subsequent autumn. Pasture metabolisable energy and crude protein all declined with increasing length of lock up whilst neutral detergent fibre content increased, changes which were similar for both the total pasture and the ryegrass fraction. The metabolisable energy of pasture in L1 and L2 was higher than that of L3 at least until week 8 at both sites. Initial crude protein values were higher for L1 and L2 than for L3 at site 1, whilst at site 2, L1 had higher values than either L2 or L3. Although longer lock up periods produced more herbage, if conserving forage is to be an integral component of managing surplus spring pasture, then dairy farmers should aim to produce high quality pasture for forage conservation. This will be achieved through shorter lock up periods and harvesting pasture no later than early ear emergence in the ryegrass fraction of the sward. This management will reduce dry matter yields, but allow more flexibility for maintaining intensive grazing practices through the spring period. The decision about when to lock up pasture will depend on both plant growth rates and animal feed requirements.