Effects of November–April grazing pressure on hill country pastures 3. Interrelationship with soil and pasture variation

Abstract

Variations in soil moisture content, nutrient levels, and temperature were measured for a range of micro-sites located on a steep, northfacing hillside. Species distribution and plant responses to changing moisture conditions and different grazing treatments were related to these physical conditions. Soil nutrient levels (organic carbon; total, inorganic, and mineralised nitrogen; Olsen phosphorus; potassium) were highest in camp/old track areas reflecting nutrient transfer by grazing animals. Variation in nutrient level between other flat and sloping areas was low. Soil moisture holding capacities were similar for most areas, but sloping areas were consistently drier in the surface horizon (0-50 mm) because of less effective rewetting compared with flatter areas. Maximum soil surface temperatures of 45-50°C were reached on clear days where low pasture cover existed on sloping areas. The effects of grazing treatments on ryegrass and browntop tiller number, white clover leaf number, and seedling establishment were similar across a wide range of micro-sites. However, pasture species content varied between micro-sites: ryegrass was dominant on camps and tracks; brown top increased with steepness of slope; white clover peaked on rolling and easy sloping areas; grass seedlings were most numerous on camps and tracks; and legume seedlings were most numerous on steeper slopes. Pasture type would appear to be determined more by nutrient fluxes (e.g., mineralisation rate, excreta return) and summer rewetting of the soil surface than by nutrient pool size and inherent water-holding capacity of the soil.

Application of canonical discriminant analysis, principal component analysis, and canonical correlation analysis as tools for evaluating differences in pasture botanical composition

“…That smaller scale variation certainly does exist, even on lowland pastures, is evidenced by the pasture composition differences among microsites (Table 2) of Matthew et al (1988). On the other hand, observations for 100 mm quadrats as used by Sheath & Boom (1985) are likely to be more variable than observations averaged over a larger area. We suggest that averaging tiller counts for cores collected over a visually homogeneous area (Matthew et al 1988) may be a good compromise between the two extremes.…”

“…For example, Lambert et al (1986) analysed densities or incidence in hill country farmlets of ryegrass (Lolium perenne L.), other grasses, white clover, other clovers, and moss as separate univariate analyses using an unbalanced factorial design. Sheath & Boom (1985) used as microsites, 100 mm diameter rings placed on seven selected land classes in each of 12 paddocks in a hill country farmlet, and also performed separate univariate analyses of variance for each species counted. Matthew et al (1988) also performed separate analyses for each pasture species when analysing fertility or stocking rate effects on abundance of several pasture species in a North Island lowland pasture.…”

A soil water balance model for sloping land

“…The remainder of the available data were used to evaluate (validate) the model. For the purpose of simulation, a pasture rooting depth of 150 mm with root proportions of 0.55, 0.25,0.15, and 0.05 by weight in each off our 37.5 mm deep layers of soil respectively was assumed (Sheath & Boom 1985). Saturation capacities and field capacities and minimums of the soils were estimated from 0-75 mm gravimetric moisture measurements (Table 1).…”

Effects of November–April grazing pressure on hill country pastures 1. Pasture structure and net accumulation rates

“…In that instance, the pasture predominantly consisted of open, erect species (Lotium perenne X Lotium multiflorum; Dactylis glomerata; Trifolium pratense) which would be more vulnerable to overgrazing than the species of this current experimental hill site. Certainly, hard grazing during the dry or post-dry phases did not lead to species composition degeneration during the 3 years studied (Sheath & Boom 1985a). However, in the context of animal production, pasture residuals of this level during summer-autumn will satisfy only maintenance nutrition and are well below those that will maximise animal production within a hill farming system (Smeaton et al 1981).…”