A comparison of perennial ryegrass cultivars differing in heading date and grass ploidy with spring calving dairy cows grazed at two different stocking rates

Milk production systems in Ireland are based on the efficient conversion of grazed grass to milk. Increasing cow performance at pasture maybe possible with the use of grass cultivars better suited to the system. The objective of this study was to examine the effects on milk production performance, dry matter intake and sward characteristics of grass cultivars with contrasting heading date (HD; intermediate or late) and grass ploidy (PL; diploid or tetraploid) when grazed at two different stocking rates (SR; low or high), according to a 2 x 2 x 2 factorial design. The study took place over a 2 year period. Eighty Holstein spring calving cows were balanced into eight individual groups (10 cows per treatment) and were assigned randomly to one of eight grazing treatments. The stocking rate imposed from April to early June in year 1 was 4.8 and 4.3 cows per ha for high and low stocking rates and 3.9 and 3.5 cows per ha for the remainder of the study. Stocking rates imposed in year 2, were 5.1 and 4.2 cows per ha from April to early June and 4.3 and 3.5 cows per ha thereafter. The experiment began in April and finished in late September in both years. On average during the two years 144 kg concentrate per cow was offered. Cows grazing late HD cultivars had significantly (P < 0.01) higher milk yield, lactose concentration and solid corrected milk yield (P < 0.09). Cows grazing at low SR had significantly (P < 0.001) higher milk, solid corrected milk, fat, protein, lactose yield and concentration and bodyweight than cows grazing at the high SR. Grass dry matter intake (GDMI) was significantly higher (1.0 kg, P < 0.001) for cows grazing late HD cultivars. Cows grazing at the lower SR had significantly (+1.2 kg, P < 0.001) higher GDMI. Late HD cultivars produced swards with higher ingestibility, better digestibility coefficients, sward characteristics and grass quality. These improvements in sward quality allowed cows grazing later heading cultivars to improve their milk production performance.