It is commonly assumed that sun-loving tropical pasture grasses are more productive in full sunlight than in shade. However, certain pasture species defy this belief and were shown to flourish in the shade in field experiments in soils where nitrogen was the limiting factor to growth.
The connection between shade and productivity has often been noted for grasses thriving under the canopy of leguminous tree species. This has been attributed to leaf drop, better nutrient cycling, higher soil organic matter, improved soil physical structure, and transfer of fixed nitrogen. But in experiments when the tree was a non-legume, or was replaced with an artificial canopy, the effect of shade alone was still marked. The dry weight of bahia grass (Paspalum notatum) increased as much as 35%, with a 67% increase in nitrogen content when grown in association with Eucalyptus grandis trees. When grown under artificial shade cover, Green panic grass (Panicum maximum) produced 25% more shoots with 76% higher nitrogen content.
The secret may be nitrogen. In comparison trials, whole plant yields of guinea grass, cori grass, and maelani grass grown on nitrogen depleted soils increased by 11, 62 and 26%, when light was reduced by at least 45%. Under a normal nitrogen supply with the shade treatment, yield decreased by 60, 31, and 34% respectively. Preliminary results support the hypothesis that shade increases the availability of soil nitrogen, which leads to better grass growth under N-limited conditions. Three experiments where conducted to investigate this possibility.
The site is the CSIRO Narayen Research Station in central Queensland, Australia. Average annual rainfall is 719 mm. Well-established pastures were chosen that had never received nitrogen fertilizer and were nitrogen deficient. The pastures were located on two quite different soil types, a heavy clay soil and a light sandy soil. On the clay soil, Green panic, buffel (Cenchrus ciliaris) and Rhodes grass (Chloris gayana) pastures in full sun were compared to areas covered by a shade cloth that reduced light by 50%. Green panic, buffel and spear grass (Heteropogon contortus) pastures were similarly investigated on the sandy soil. Treatments were rainfed, but the Green panic plots were also replicated under irrigation. Yield harvests cut at ground level were taken on five different dates. Leaves were analyzed for nitrogen content.
Green panic was grown outdoors in tanks of base nutrient solution. Two nitrogen treatments were established. Some tanks received an optimal N application, and the others received one-quarter of the optimal rate. The solution was monitored daily. Seedlings were raised in sand and transferred into the solution 21 days after germination. Plants from the optimal and the limited N treatments were divided into two groups, some raised in full sun, and others under a shade canopy that reduced light by 50%. Harvests were taken every 10 days during the last 40 days of the experiment.
This trial was conducted at the CSIRO Samford Research Station near Brisbane on red-yellow podzolic soil. Average annual rainfall at the site is 1117 mm. Plots were laid out on an established pasture of bahia grass. Half the pasture area was in the shade (55% light transmission) of Eucalyptus grandis. Of 16 plots in full sun, eight were randomly selected and covered with a shade cloth (50% light transmission). Another eight plots under the trees were selected. In all plots temperature sensors were placed on the soil surface, under the leaf litter, and in the soil at depths of 2.5, 7.5, and 15cm. Temperatures were logged every two hours. Gypsum blocks were placed in the soil at depths of 2.5, 5, 10, 20 and 40 cm to monitor soil water. In situ core samples were taken to measure soil mineralization rate, and changes in nitrate N and ammonia N concentration. Yield harvests were taken.
Irrigated Green panic grown in the shade showed a larger yield increase than the rainfed pasture on the clay soil, but not on sandy soil. Leaf nitrogen content increased at every harvest for all of the shade plots. No consistent differences in ammonium-N concentrations between the treatments were recorded.
The results suggest that the shade phenomena exists regardless of soil type. Green panic grass, known to be moderately shade tolerant, showed the greatest shade response. Rhodes grass gave an intermediate response, and Buffel and spear grass, known to prefer open grasslands, were less responsive.
Generally soil water content remains at a higher level longer under shade than in full sun. But in this experiment the Green panic shade treatments under natural rainfall vs. irrigation did not differ greatly (even though rainfed plots were water stressed). The major response to shade appears to be an increase in available soil nitrogen.
Wilson, J.R. and D.M.W. Wild. 1991. Improvement of Nitrogen Nutrition and Grass Growth under Shading. Forages for Plantation Crops, ACIAR Proc. No. 32. pp 77-82.
John R. Wilson
CSIRO Division of Crops and Pastures Cunningham Laboratory
306 Carmody Road, St Lucia
Qld 4067, AUSTRALIA
Fax: (07) 371 3946