Ecology and management of adventive annual clover species in the South Island hill and high country of New Zealand

Increasing legume abundance is an important component of pastoral intensification, in providing increased quality feed and nitrogen inputs to nitrogen deficient, hill and high country grassland. Establishment and persistence of traditionally sown clover species white clover (Trifolium repens L.) and subterranean clover (Trifolium subterraneum L.) is often limited in summer dry areas. The ecology of naturalised adventive (volunteer) annual clovers species Trifolium arvense L., Trifolium dubium Sibth, Trifolium glomeratum L., and Trifolium striatum L. commonly found in the South Island hill and high country areas of New Zealand was examined through a series of site surveys, field trials, and glasshouse experiments. The relationship of these clover species to environmental factors of climate (annual rainfall), topography (slope, altitude, aspect), soil fertility (phosphorus, sulphur, pH), and sheep grazing (spring management, and preference) were investigated. On two contrasting hill and high country stations in the South Island: Glenfalloch in inland Canterbury (1665 mm annual rainfall) and Mt Grand in Central Otago (703 mm annual rainfall), the abundance of these species was quantified in relation to environmental factors. Site surveys were conducted in three hill blocks per farm, with measurements of grassland species cover, slope, aspect, grazing intensity, soil fertility, soil depth, and altitude, made within quadrats along three transects at upper, middle and lower hill slope positions. The single adventive clover present at Glenfalloch was Trifolium dubium, whereas all adventive species were present at Mt Grand. The % cover of adventive annual clovers was greater (30.1) than that of white clover Trifolium repens (3.8) or subterranean clover Trifolium subterraneum (0.1) at Mt Grand. Trifolium repens % cover decreased with increasing altitude. Striated clover Trifolium striatum and cluster clover Trifolium glomeratum % cover was greatest on sunny aspects, while Trifolium repens % cover was greatest on shady aspects. Trifolium repens cover increased and Trifolium striatum cover decreased with increasing available soil phosphorus (P). Annual rainfall appeared to be the main environmental factor influencing the abundance of adventive annual clover species on two geographically and climatically different South Island high country stations. A two year spring grazing management and superphosphate (SP) application field trial was conducted at a typical summer dry pasture site on a mid altitude (~700 m a.s.l.) north-facing moderately steep, hill site at Mt Grand Station. Adventive annual clover biomass, botanical composition and population dynamics were measured from October 2008–May 2010, from low (75 kg/ha) and high (200 kg/ha) SP application split plots of 2.5 x 5 m area within different grazing management main plots (continuous and mid-spring closure) of 5 x 5 m area. Adventive annual clover species contributed the largest proportion of legume sward content at this site, with sown species contributing the least. Collectively, adventive annual clover species accounted for over 90% of the sward clover content present during both years. Trifolium striatum was the dominant adventive species. Large variation in biomass production (668 and 51 kg DM/ha), sward botanical contribution (28 and 2%) and seedling recruitment (9.7 and 24 seedlings/core) by adventive annual clovers was observed between the two years of sampling, which was attributed to high rainfall during spring and early summer of the first year compared to the very dry spring of the second year. Deferred spring grazing resulted in significantly more adventive annual clover biomass in a moist year (1452 kg DM/ha) in comparison to continuous grazing (674–805 kg DM/ha). However, this did not appear to result in significantly more adventive annual clover seed production, seedling recruitment, or biomass in the following year. Superphosphate (SP) application had no significant overall influence on adventive annual clover species biomass. High SP application did have a negative effect on autumn seedling recruitment of the three most prevalent adventive annual clover species (Trifolium striatum, T. dubium, and T. glomeratum). Soil plant-available N sourced from pasture legumes has been shown to be positively associated with long-term use of fertiliser phosphorus (P) and sulphur (S) use in NZ high and hill country. Determining the nutrient requirements of little known naturalised, adventive pasture legume species such as cluster clover (Trifolium glomeratum), haresfoot clover (T. arvense), striated clover (T. striatum) and suckling clover (T. dubium) that exist on low fertility, summer dry hill and high country slopes is important for the productive pastoral sustainability of extensive livestock grazing agroecosystems. The first glasshouse study was conducted to determine the yield response of these four adventive annual species, plus Trifolium subterraneum and T. repens, to increasing levels of available P in a typical, low fertility, acidic NZ high country soil. The order of greatest yield DM response was T. subterraneum > T. arvense > T. repens > T. dubium > T. glomeratum > T. striatum (4.4–0.8 g DM/pot), while the P application rates at which maximum yield was produced varied between the species with T. arvense and T. striatum yielding the most DM at 250 mg P/kg soil, T. subterraneum, T. glomeratum and T. repens yielded the greatest amount at 500 mg P/kg soil, and T. dubium producing its highest yield at 2500 mg P/kg soil. The order of greatest P-response efficiency by species (how much P was required for each gram of DM produced) was T. subterraneum > T. arvense > T. repens > T. glomeratum > T. dubium > T. striatum. Implications for low input, extensive grazing systems in hill and high country areas are discussed. The growth response and nutrient uptake of four adventive annual clovers to applied S or lime (CaCO3), grown in a typical low fertility South Island (S.I.) high country soil, were investigated under glasshouse conditions and compared to white and subterranean clovers as reference species. The annual species had yield responses of 12–17%, or were unresponsive, to S applications. Maximum yields were generally in the order of T. subterraneum ≥ T. arvense > T. striatum ≥ T. dubium > T. glomeratum > T. repens (8.0–5.2 g DM/pot). For lime treatments, yields were strongly driven by P availability, linked to soil pH. Trifolium repens and T. striatum responded to liming at low lime rates (1 t/ha equivalent), while all other species had negative yield responses to liming. The data indicate that the adventive annual clovers are better adapted to low soil fertility (low pH and S) conditions, which in turn may be an important factor contributing to their success under S.I. high country field conditions. Grazing preference of Merino sheep for the adventive annual clover species was investigated in a field trial at a dryland pasture site on the Canterbury Plains at Lincoln University. The quantity of clover species sward biomass removed during grazing and the time spent grazing from different clover species swards was quantified during grazing preferences tests in November and December. Relative preference (Chesson-Manly index) for each clover species sward on offer was calculated from the utilisation of each species relative to the utilisation of all other species on offer. Relative preference decreased and was significantly different between species when they reached a reproductive stage of maturity. This was attributed to an overall decrease in nutritive value of clover herbage, with relative preference significantly correlated with the concentration of acid detergent fibre (ADF), neutral detergent fibre (NDF), protein, and dry matter digestibility (DMD) chemical components. The latest flowering clover species were the most preferred, with Trifolium repens the most preferred clover species overall. Trifolium arvense was the most preferred adventive clover species when most species were predominately vegetative. Implications of key results and further research avenues are discussed.