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

1. Experiments were carried out at three sites in southern Scotland to determine the suitability of Molina grassland for sustained use in pr oviding summer grazing for cattle, and to investigate grazing control as a tool to manipulate the species composition of hill pastures in Br itain. 2. Cutting experiments showed that frequency and severity of de foliation 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. 3. Three yea rs of repeated light defoliation (33% lamina length removed each June, July and August), compared with uncut controls, reduced leaf producti on in a fourth uninterrupted growing season by 40%, while repeated hea vy defoliation (66% lamina removal) reduced it by 78%. Reductions in b oth the numbers and the size of tillers contributed to this result. Si ngle annual cuts only reduced leaf production at 66% lamina removal wh en they took place late in the season. 4. Plots grazed by cattle at tw o sites for 6 years compared treatments where 66% rather than 33% of t he herbage was removed by grazing. The rates of leaf extension in Moli nia were reduced at the higher level of use. In comparison with areas protected from grazing during the final year of treatment only, the bi omass of Molinia and other grasses in areas open to grazing showed tha t the taller Molinia was utilized to a much greater extent than the ot her grasses. After 6 years of grazing, the biomass of Molinia at 33% u tilization was reduced by 46-65% compared with ungrazed exclosures, wh ile at 66% utilization it was reduced by 86%. 5. Basal internode size was greatly reduced in the grazed plots compared with the ungrazed exc losures, with effects on tiller base size being more important than va riation in concentrations in determining amounts of starch, total wate r soluble carbohydrates, N, P and K on a per tiller basis. Site x mana gement interactions for concentrations could be interpreted in terms o f variations in stage of maturity, potential for growth and plant/anim al nutrient cycling pathways. 6. Floristic diversity was increased on grazed compared with ungrazed areas. The cover of Molinia was decrease d and that of other broad-leaved grasses increased by grazing. At 33% utilization, the cover of Molinia appeared to be levelling off (at aro und 60-65% after 3-5 years) while at 66% utilization a continued downw ard trend was evident. Species trends were also influenced by site fac tors, with exclusion of grazing leading to a reduced cover of Festuca ovina only where conditions were favourable for high yields of taller grasses. 7. The responses of Molina to defoliation are discussed in re lation to its pattern of growth and low rate of leaf and tiller turnov er and to its responses to soil and climatic factors. The effects of g razing on nutrient cycling and sward canopy structure, in influencing competitive relationships with other species, are considered and the w ider issues relevant to management protocols are outlined.