Sheila A. Grant

A model of the grazing of hill vegetation by sheep in the UK. I. The prediction of vegetation biomass

1. A computer model is described which predicts monthly growth, senescence, litterfall and standing biomass of ungrazed herbage in seven dwarf shrub-dominated and five grass-dominated vegetation types commonly found in the hill areas of the UK. 2. Using published data, the annual and monthly dry matter (DM) production of each vegetation type is predicted at sea level for the temperature zone in which the relevant data were collected. These estimates are then adjusted to take account of the effects of altitude and temperature zone on the production of all vegetation types and the effects of fertilizer rate, likely levels of soil nitrogen, summer rainfall and available soil water capacity on reseeded grassland. Empirically derived rules predict senescence and litterfall and, together with predicted DM production, are used to determine standing biomass in each month. The model also predicts the mean undisturbed sward surface height (sward height) of the grass vegetation types. 3. The sensitivity of the model to variations in inputs is described and the reliability of the relationships used in the model an discussed. Despite the limited availability of data suitable for model development, the model predicts DM production to lie between 1 and 11% of the measured values for the few remaining sites with appropriate data. 4. The most significant gaps in knowledge are identified and suggestions are made as to how the model might be further developed and tested.

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

Experiments were carried out at three sites in southern Scotland to determine the suitability of Molina 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 x 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.

Controlled grazing studies on Nardus grassland: effects of between-tussock sward height and species of grazer on Nardus utilization and floristic composition in two fields in Scotland

1. The utilization of the tussock-forming grass Nardus stricta was investigated in controlled grazing experiments in which various sward heights of between-tussock grasses were maintained thoughout the growing season using either sheep, cattle or goats. 2. The utilization of Nardus increased as the height maintained for the between-tussock grasses was reduced. The relationship between sward height and level of utilization of Nardus, however, differed greatly among herbivore species. 3. Under similar sward conditions, cattle and goats utilized more Nardus than did sheep. Utilization of Nardus declined over successive seasons under grazing by sheep but was sustained under grazing by cattle or goats. 4. The severity of defoliation of Nardus achieved by cattle or goats when between-tussock grasses were maintained at 4.5 cm (mean sward surface height above ground) resulted in disappearance of tussocks from the sward and in reduced size, leaf extension growth and nutrient reserves of Nardus tillers. Under cattle grazing, the cover achieved by Nardus decreased from 55.4% to 30.0% over 5 years. 5. Under sheep grazing, both the biomass of individual Nardus tussocks and the cover of Nardus increased. Cover increased from 58% to over 86% at a between-tussock height of 4.5 cm, while at a height of 3.5 cm, after initially declining, it increased to reach 72%. 6. The broad-leaved grasses showed a trend towards increase in cover when between-tussock sward height was maintained at 4.5 cm whether grazing was by cattle or by sheep; the trend was not evident at 3.5 cm (sheep grazing only). 7. Changes in the floristic composition of the sward are discussed in relation to the roles of the inherent growth characteristics of the species present, to selective defoliation and uprooting of the shoots, to altered nutrient cycling pathways under grazing and to the nutrient status of sites.

THE UTILIZATION OF BLANKET BOG VEGETATION BY GRAZING SHEEP

determined by the limits to stocking rates set by the need to provide sheep with a minimum tolerable level of winter nutrition. This leads to low levels of pasture utilisation, particularly during the summer growing period, and is associated with the poor cycle of nutrition which is basic to the problems of low levels of individual animal performance. Eadie & Maxwell (1974) have recently proposed grazing systems to improve animal production from hill land. These systems involve the use of a small area of improved pasture with the remaining unimproved hill. The improved area is enclosed and grazed during lactation and lamb growth and prior to and during mating, when high quality food has the most beneficial effect on individual sheep performance. The unimproved area is used during the rest of the year. In such grazing systems, the utilization of the unimproved bog vegetation is likely to be increased and the seasonal pattern of use substantially changed compared with a setstocked year round system of grazing. However, there must be a limit to this use since blanket bog vegetation appears to be vulnerable to overgrazing, particularly in combination with periodic burning (McVean & Lockie 1969). A prediction of the effects of these changes in grazing management on species composition and productivity of bog communities is not possible on present evidence. This study arose from the need to obtain further information. The results reported here concern the patterns and levels of utilization of the main bog species as affected by season of use and grazing pressure.

Grazing ecology and the conservation of heather moorland: the development of models as aids to management

Loss of heather has occurred as a result of overgrazing by sheep. There is a clear need to develop grazing management protocols to sustain heather.Most areas of heather moorland contain a mosaic of four or more vegetation types, with the precise proportion by area of each vegetation type as well as patch size, shape and distribution being unique to each moor. The vegetation types differ in their dry matter production, feeding value and attractiveness to grazing sheep and other herbivores.The unique nature of the vegetation mosaic on each moor has important consequences for management. In the case of sheep farming, both the impact of the animals on the vegetation and the carrying capacity of a moor for sheep production will vary depending on the vegetational composition. The prescription of a single stocking rate for use on all grazing units containing heather is not sensible in these circumstances.Our knowledge and understanding of the biology of the moorland grazing system, however, has reached the stage where information on the productivity and feeding value of the various vegetation types and on foraging behaviour and diet selection of the sheep is sufficient to build computer models. These models allow examination of different management options.An early model currently allows the determination of the upper limits to stock numbers for year-round grazing systems on particular moors if damage to heather is to be avoided.A more complex model is being developed which will allow examination of the effects of different seasonal patterns and levels of stocking on the offtake levels from the various vegetation types on a particular moor, and on the quality and quantity of the sheeps diet. This model is still being developed and validated, but it provides a promising tool with which to explore management options. Extension of the latter model to include other species of grazer and the effect of different patterns of vegetation is needed to develop multiple land-use models or prescriptions to obtain the optimum biological and economic balance.

Seasonal pattern of leaf growth and senescence of Nardus stricta and responses of tussocks to differing severity, timing and frequency of defoliation

1. A field study of the seasonal growth of Nardus stricta showed that leaf turnover was slow, averaging 2.5 leaves per tiller over the growing season. Periods of lamina extension growth and senescence were generally separated in time; fastest growth, at 4-5 mm per tiller per day, occurred in June July and the senescence rate was low until early autumn when it rose to 6 mm per tiller per day. Sheathing tiller bases, already well developed in May, increased in weight by 27% between May and October; total water-soluble carbohydrate (TWSC) concentration increased by 150% (from 75 to over 165 g per kg dry weight) while N, P and K concentrations declined over this period. 2. A pot study showed that the age class of inter-connected tillers on rhizome tips greatly influenced growth; mean rates of lamina extension and production of daughter tillers were higher in younger tillers while the likelihood of flowering was greater for older (3-4 leaf) tillers. The pattern of growth provided evidence for acropetal movement of assimilate. 3. Field cutting experiments showed that Nardus was relatively insensitive to defoliation in the short term. Removal of 50% or 100% of the length of the leaf lamina in June only, July only or in both June and July resulted in a temporary reduction in lamina extension rate per tiller but did not significantly affect the weight of leaf produced in the first season. There was a non-significant trend for the size of tiller base to be reduced as the severity of cutting was increased. Concentrations of TWSC and N,P, and K in tiller bases showed no variation attributable to cutting. 4. In a long-term cutting experiment lasting 4 years, Nardus was cut so as to leave 1, 4 or 8 cm of lamina in June only, July only, or in June and July. Two seasons elapsed before lamina extension was reduced on cut plants. The weights of clippings were greatly reduced over successive years when tillers were cut to leave only 1 cm oflamina but were little affected when 8 cm of lamina remained, with the 4-cm treatment intermediate; the reduction was greatest with plants cut in both June and July and least with plants cut in June only with those cut only in July being intermediate. 5. Tussock biomass, assessed after 4 years of cutting and a season of uninterrupted growth, was decreased as the height of cut was reduced and by repeated cutting; timing (June compared with July) was unimportant. Both tiller numbers and mean weight per tiller were reduced. Flowering vigour was increased by cutting. Timing was important, with plants cut in July only or in both June and July having more flowers than those cut in June only. The amounts of TWSC and N, P and K reserves were reduced by cutting with reduction in tiller base size being more important than effects on concentration. 6. In a comparative field study, the rate of lamina extension in Nardus averaged less than half that of Agrostis capillaris or A. canina growing in the same sward. This underlines the importance of selective grazing as an influence on species balance in acid grasslands.