Shade affects the growth and nutritive value of tropical pasture grasses

Abstract

The effect of shading on the growth, yield, and nutritive value of some tropical pasture grasses was investigated in south-east Queensland to improve understanding of the shade response and, hence, aid in selection of grass species suitable for pasture production beneath trees. The work consisted of a series of five experiments.The first experiment was undertaken to establish the effect of reducing incident light by 50% (with shadecloth), compared with full sunlight, on the growth and nutritive value of five tropical pasture grasses at the University of Queensland Mt Cotton Research Station. The grasses used were Brachiaria decumbens cv. Basilisk. Panicum maximum cv. Petrie, Panicum maximum cv. common guinea grass - Riversdale, Pennisetum clandestinum cv. Noonan, and Setaria sphacelata cv. Kazangula. In the second experiment, this material was fed to sheep at the University of Queensland Mt Cotton Research Station. The third experiment involved assessment of growth and nutritive value of seven tropical pasture grasses grown in full sunlight and beneath shadecloths with 30%, 50%, and 68% light transmission at the University of Queensland Redland Bay Research Station. Grasses used in the third experiment were Axonopus compressus, Brachiaria decumbens cv. Basilisk, Cenchrus ciliaris cv. Biloela, Panicum maximum cv. Petrie, Panicum maximum cv. Riversdale, Setaria sphacelata cv. Kazangula, and Stenotaphrum secundatum.In experiments one and three, when the regrowth period allowed was less than 10 weeks, shading decreased herbage yield. However, when the regrowth period was extended to about 17 weeks or more, yields of shaded grasses were higher than, or equivalent to, grasses grown in full sun. Shading affected the proportions of plant parts differently according to the length of the regrowth period, except for the amount of dead material, which was consistently reduced under shade. In the third experiment, shade was associated with an increase in leaf proportion (36.4%) and a decrease in leaf bulk density in the lower strata (15.3%) for the older herbage. This indicated the possibility that light capture and use may be modified in grass swards beneath shade to contribute to greater yields, compared with full sun. The increased yields under shade were also partially attributed to an enhanced nitrogen environment.In the second experiment, the influence of shade on the chemical composition and in vitro dry matter digestibility of the grasses was small but variable. Shading consistently decreased total nonstructural carbohydrate contents of leaf and stem. The small changes in chemical composition had no effect on the nutritive value (intake, digestibility) to sheep. The fourth experiment compared the transmission of shortwave solar radiation and diffuse radiation by 12 commercially available shading materials with a tree canopy at the University of Queensland Gatton College. Shading materials used included shademesh 50%, shademesh 70%, multimesh, birdguard, solarweave, glasshouse film, and lattice. Colour greatly influenced the proportion of shortwave radiation transmitted by commercial shadecloths. In addition, the mean proportion of diffuse radiation increased under pale shadecloth, clear solarweave, dense lattice, and the tree canopy, compared with the incident radiation. The magnitude of the increase in the diffuse component varied according to the composition of the incident radiation. Black shadecloth reduced the total radiation and the diffuse component of radiation beneath to a similar extent.The fifth experiment investigated the effects of reduced levels of incident radiation and increased levels of diffuse radiation on the radiation use efficiency and growth of two tropical pasture grass species at the University of Queensland Gatton College. Lower incident radiation (74%) decreased final pasture yield (10.4%), final leaf area index (21.5%), and final accumulated intercepted radiation (26.2%), but increased radiation use efficiency (16.4%), leaf nitrogen concentration (4.6%), and the overall effectiveness of the canopy to intercept light (35.9%), compared with the full sun treatment. Lower incident radiation (78%) combined with increased proportions of diffuse radiation decreased final accumulated intercepted radiation (21.2%), but increased final pasture yield (18.0%), final leaf area index (10.6%), the overall effectiveness of the canopy to intercept light (64.1%), and radiation use efficiency (47.7%), compared with the full sun treatment. Changes in radiation use efficiency with increased proportions of diffuse radiation were not associated with changes in nitrogen status. Increases in efficiency of utilising incident radiation were most likely due to more efficient use of light by sunlit leaves at lower light levels, and to an increase in the diffuse component of light being spread over the areas of sunlit and shaded leaves.Overall, these studies showed that shading of infrequently defoliated tropical pasture grass swards can result in an increased yield, which is due to enhanced light capture and efficiency of use in the shaded environment. The response is a direct consequence of the nature of the radiation incident on the pasture grass sward, more specifically the increased proportion of diffuse radiation. Dark shadecloth does not adequately simulate a tree canopy regarding the diffuse component of radiation.