Interactions between pasture species and management and their implications for evaluating perennial ryegrass (Lolium perenne L.) cultivars in dairy systems

An index to rank perennial ryegrass cultivars based on their relative economic benefit to pasture-based dairy systems, has been developed in New Zealand (Chapman et al., 2016; DairyNZ) in recent years. Performance values in this system are calculated for the key trait of seasonal dry matter (DM) production, using data from cultivar evaluation trials conducted using perennial ryegrass monocultures and high nitrogen (N) fertiliser inputs. To determine if the index should account for genotype × management interactions, experiments with a common design were established in four regions of New Zealand in 2012. Results of the first two years of the Canterbury experiment are presented in this thesis. The experiment was conducted at the Lincoln University Research Dairy Farm, Lincoln, Canterbury, and used a split plot design with eight subplots randomly allocated within four main plots each replicated in five blocks. Main plots comprised all combinations of pasture sown with or without white clover receiving either low (100 kg N/ha) or high (325 kg N/ha) rates of nitrogen fertilizer annually, randomized within blocks. In the plus clover treatments, pastures were sown with a 50:50 mixture of two clover cultivars commonly used in dairy pastures. The eight perennial ryegrass cultivars were selected to provide contrasting phenotypes for two traits that may influence competition between grass and clover: morphology, and heading date. The morphological contrast was between high tiller density/fine leaf material (both diploids) and low tiller density/broad leaf material (both tetraploids). The heading date contrast was between mid-season and late-season heading date materials, all of them diploids. Each main plot was grazed by dairy cows following standard farm management practices. Total DM yield was estimated in each subplot before grazing by cutting, using a Haldrup forage harvester. Botanical and pasture nutritive value sampling was conducted pre-grazing in spring, summer and autumn each year. Ryegrass and clover population density were measured in autumn each year. Results of the first two years of the experiment show that seasonal and total annual yield of the High N treatments was greater than from the Low N treatments. With the exception of spring of the establishment year, seasonal and total annual yield of the plus clover treatments was greater than from the minus clover treatments. N and clover interactions were observed in summer of the first year, autumn of both years, and for the total annual yield from both years. In general, the Low N plus clover treatment yielded similarly to the High N treatments, and yielded significantly more DM than the Low N minus clover treatment. The effect of cultivar on DM yield was significant in spring and autumn in both years, in winter 2013, and in the annual total of the second year. The clover content of pastures was always greater in the Low N treatments compared with the High N treatments and was affected by the ryegrass cultivar during spring in both years and in the second summer. There were no significant interactions between N and cultivar for clover content during the two years of the experiment. The heading date contrast affected the white clover content of pastures during summer in both years and in autumn 2013, resulting in mid heading date cultivars having greater white clover content than late heading date cultivars. Despite the effects of cultivar and treatments on DM yield and clover content, no significant interactions were detected between clover inclusion/exclusion and perennial ryegrass cultivar, or between N level and perennial ryegrass cultivar on seasonal or annual total DM yield, with the exception of winter 2013. As a consequence no evidence of re-ranking emerged and therefore performance values in the DairyNZ Forage Value Index (DairyNZ) do not need adjustment to account for grass-clover interactions over time and their effects on total pasture DM yield. The second experiment reported in this thesis was carried out with the objective of analysing how the perennial ryegrass and white clover characteristics affected their competitive ability, their proportion in the sward and the DM yield when grown in mixtures. The experiment was conducted at the Lincoln University Research Dairy Farm, Lincoln, Canterbury, and used a split plot design with four blocks. Main plots were two nitrogen levels (100 and 325 kg N/ha/year), randomised within blocks. Subplots were the pasture types (24), made up of a 4 × 4 factorial of 4 perennial ryegrass cultivars and 4 white clover cultivars (16 subplots) plus monocultures of each cultivar (8 subplots), randomised within main plots. Four perennial ryegrass cultivars were selected to create a range from fine to broader leaved materials and from open to denser cultivars. The four white clover cultivars were selected to create a range in leaf size, from small to large leaved. Total DM yield was estimated in each subplot by cutting, using a Haldrup forage harvester. Botanical composition was determined by dissecting a subsample collected from the harvested herbage at every harvest. Ryegrass and clover population density were measured four times during the experimental period (winter 2014 to autumn 2015). Photosynthetically active radiation (PAR) intercepted by the canopy and canopy height were measured three times during the year. Total annual DM yield of the mixtures was greater in the High than in the Low N treatment. The inclusion of clover increased the total annual DM yield under both N treatments, but the increment was greater under Low than under High N treatment. Only on one occasion was the white clover content of pasture affected by the interaction between perennial ryegrass and white clover cultivar, but not during the rest of the season. There were effects of perennial ryegrass and white clover cultivars on DM yield of the mixtures in some of the harvests during the year, but the total annual DM yield was similar for mixtures sown with different grass or different clover cultivars. With the exception of one occasion, no significant interactions were detected between perennial ryegrass cultivar and white clover cultivar on DM yield of the mixtures, meaning that the inclusion of different white clover phenotypes did not affect the DM yield of the mixture differently when associated to different perennial ryegrass phenotypes.