Effects of shade on temperate pasture species

Abstract

The increasing world population and limited land resources have encouraged the integration of pasture, livestock and tree growing as a farm production system referred to as "silvopastoralism". A major constraint in the exploitation of plantation lands for pasture production, however, is the changing light environment below the plantation canopy over time. Shade tolerant pasture species are therefore needed to improve and sustain production; the success of silvopastoral systems depends on the identification of suitable shade-tolerant species.

Two experiments were carried out in 4 litre containers to study the effects of shade on temperate pasture species. The objectives were (1) to determine the relative productivity of different pasture species under different levels of shade so that shade-tolerant species could be identified and (2) to measure variates such as tiller populations, plant height, nitrogen percentage and in vitro digestibility which may influence the feeding value of shaded pasture. In the first experiment, 6 grass and 6 legume species were subjected to three different levels of light intensity. Different grades of shade cloth were used to represent 30% full sunlight and 60% full sunlight and compared with 100% full sunlight. There were two harvests in the open area and two harvests in the glasshouse. In a second experiment 6 grass species were subjected to two levels of shade (light and heavy) under a Pinus radiate canopy. The 6 grasses in 4 litre pots were taken from the experiment 1 and placed in the existing agroforestry field trial of the Department of Plant Science, Lincoln University.

Dry matter production and sward density of both grasses and legumes, and tiller number of grasses decreased, while height increased, with increasing levels of shade. Height of legumes was less responsive to shade than the etiolation observed in grasses particularly during the first two harvests during autumn in the open area. Etiolation of the plants had a dominant influence on sward density. Shaded plants had only about half the sward density of the plants growing in full sunlight Digestibility of vegetative production was not affected by shade. Nitrogen content of grasses in both experiments increased with increasing levels of shade while legumes were generally not affected. Mean area per leaf, dry weight of leaves, dry weight of 100mm2 of leaf, number of leaves, stubble and roots of legumes increased with increasing levels of light.

It was concluded that: (1) Cocksfoot of the grasses tested and lotus of the legumes showed greatest tolerance to shade by producing fairly well in shade relative to their production in full sunlight. (2) Digestibility of pasture was not affected by shade but nitrogen content, particularly of grasses, increased with increasing levels of shade. (3) Animal production per head from shaded pastures may be reduced because of decreased sward density as a result of increased height and fewer tillers in grasses and fewer leaves in legumes. These sward characteristics could lead to decreased intake per animal because of smaller bite size. (4) Grasses and legumes produced less under shade which will lead to pasture under trees supporting lower stocking rates than unshaded pastures. (5) Slower pasture production under trees will result in herbage being older for any given pasture mass. (6). Shaded pasture is therefore likely to be of lower feeding value for free grazing livestock even though the nutritive value of grass may benefit from small increases in crude protein levels.