Implications of grass–clover interactions in dairy pastures for forage indexing systems. 3. Manawatu

The core treatments in the national experimental design were implemented at a dryland Manawatu site representative of the lower North Island, and monitored for four full years from 2012/13 to 2015/16. There were main effects of nitrogen (N) fertiliser level in the annual total dry matter (DM) yields and in 11 out of 18 seasonal totals, in each case yield being significantly higher for high N than for low N. There were no significant main effects of clover (in two seasons out of 18, summer 2013/14 and early spring 2014 there was a trend (P<0.1) to higher yield in the clover than in the minus clover treatments), and there were no interactions between clover and N level. With the exception of autumn 2016, there were significant differences among cultivars in all seasons, but no clover × cultivar or N × cultivar interactions. The proportions of clover in the mixed-species pastures reached 15% and 5% for the low N and high N treatments, respectively, during the first summer after sowing. The Manawatu site was characterised by low annual total and summer rainfall during the first three years that severely reduced the presence of white clover and thereafter proportions never exceeded 4%. While there were no clover × cultivar interactions, although occasions where the proportion of clover was higher for the dense, fine-leaved compared with the open, broad-leaved cultivars, and higher for the mid-season compared with the late-season cultivars. However, with the low mean proportions of clover for all cultivar × clover combinations these significant contrasts were not considered influential. Thus, the hypothesis that relative herbage accumulation rankings of ryegrass cultivars do not differ when ryegrass is grown in monoculture or in mixtures with white clover is supported. Further evidence of interactions, perhaps under conditions of more abundant white clover, would be required before considering a scaling factor for ryegrass herbage accumulation performance values in the Forage Value Index. Significant main effects of N consisting of higher crude protein (CP) and metabolisable energy (ME) and lower neutral detergent fibre (NDF) in the high N compared with low N treatments were observed in about half of the measurement occasions. Consistent cultivar differences were largely attributable to lower CP and NDF, but higher ME in the late-season compared with the early-season cultivars. With the exception of higher CP in the clover compared with the minus clover treatment in summer 2012/13 and late-winter 2014, there were no main effects of clover and no clover × cultivar interactions in nutritive value variables indicating that the relative rankings for nutritive value of ryegrass cultivars were similar when grown as monocultures or in mixtures with white clover.