Amount and quality of grass harvested for first-cut silage for differing spring-grazing frequencies of two mixtures of perennial ryegrass cultivars with contrasting heading date

There are potential advantages and disadvantages associated with grazing spring perennial ryegrass swards designated for first‐cut silage. These may differ for intermediate‐heading (0·50 ear emergence in the second half of May) and late‐heading (0·50 ear emergence in the first half of June) cultivars. The interactions between cultivar type, spring‐grazing frequency, silage‐harvest date and year were examined in an experiment with a randomized complete block (n = 4) design with a factorial arrangement of treatments, conducted in Ireland. The factors were (i) two perennial ryegrass mixtures: intermediate‐ vs. late‐heading cultivars, (ii) three spring‐grazing regimes: no grazing, grazing in mid‐March or grazing in both mid‐March and mid‐April, (iii) four first‐cut silage‐harvest dates that were at c. 10‐d intervals from 19 May and (iv) 2 years (1998 and 1999). The effects of cultivar mixture on herbage mass of the swards in spring were small and not statistically significant. The late‐heading cultivars provided lower amounts of herbage dry matter for harvesting for first‐cut silage but herbage with higher in vitro organic digestibility values compared with intermediate‐heading cultivars. To achieve the same amount of herbage for silage, the late‐heading cultivars needed to be harvested 8 d later than the intermediate‐heading cultivars. Even with this delay in harvest date, the late‐heading cultivars had higher in vitro organic digestibility values than the intermediate‐heading cultivars. The late‐heading cultivars could be harvested up to 30 d later and produce a higher amount of herbage for first‐cut silage with similar digestibility values compared with the intermediate‐heading cultivars.