Controlled grazing studies on Molinia grassland: effects of different seasonal patterns and levels of defoliation on Molinia growth and responses of swards to controlled grazing by cattle.
Abstract
Experiments were carried out at three sites in southern Scotland to determine the suitability of Molina caerulea grassland for sustained use in providing summer grazing for cattle, and to investigate grazing control as a tool to manipulate the species composition of hill pastures in Britain. Cutting experiments showed that frequency and severity of defoliation were more important than timing in their effects on Molinia. Weights of clippings declined in successive years only in response to treatments that involved repeated within-season cutting. Three years of repeated light defoliation (33% lamina length removed each June, July and August), compared with uncut controls, reduced leaf production in a fourth uninterrupted growing season by 40%, while repeated heavy defoliation (66% lamina removal) reduced it by 78%. Reductions in both the numbers and the size of tillers contributed to this result. Single annual cuts only reduced leaf production at 66% lamina removal when they took place late in the season. Plots grazed by cattle at two sites for 6 years compared treatments where 66% rather than 33% of the herbage was removed by grazing. The rates of leaf extension in Molinia were reduced at the higher level of use. In comparison with areas protected from grazing during the final year of treatment only, the biomass of Molinia and other grasses in areas open to grazing showed that the taller Molinia was utilized to a much greater extent than the other grasses. After 6 years of grazing, the biomass of Molinia at 33% utilization was reduced by 46-65% compared with ungrazed exclosures, while at 66% utilization it was reduced by 86%. Basal internode size was greatly reduced in the grazed plots compared with the ungrazed exclosures, with effects on tiller base size being more important than variation in concentrations in determining amounts of starch, total water soluble carbohydrates, N, P and K on a per tiller basis. Site × management interactions for concentrations could be interpreted in terms of variations in stage of maturity, potential for growth and plant/animal nutrient cycling pathways. Floristic diversity was increased on grazed compared with ungrazed areas. The cover of Molinia was decreased and that of other broad-leaved grasses increased by grazing. At 33% utilization, the cover of Molinia appeared to be levelling off (at around 60-65% after 3-5 years) while at 66% utilization a continued downward trend was evident. Species trends were also influenced by site factors, with exclusion of grazing leading to a reduced cover of Festuca ovina only where conditions were favourable for high yields of taller grasses. The responses of Molina to defoliation are discussed in relation to its pattern of growth and low rate of leaf and tiller turnover and to its responses to soil and climatic factors. The effects of grazing on nutrient cycling and sward canopy structure, in influencing competitive relationships with other species, are considered and the wider issues relevant to management protocols are outlined.