Effects of shading on the growth and nitrogen content of green panic and Siratro in pure and mixed swards defoliated at two frequencies

Abstract

A study was made of the effect of illumination at 100, 60 and 40% of sunlight on the growth of Macroptilium atropurpureum cv. Siratro and Panicum maximum var. trichoglume cv. Petrie (green panic) in pure and 50150 mixture swards, dcfoliated every 4 (D4) 01 8 (D8) weeks. The plants were grown without nitrogen fertilizer on a soil of moderate nitrogen status. Shading to 60 and 40% of full sunlight increased the shoot yield of green panic in pure sward by 30 and 27% respectively in the D8, but reduced it in the D4 treatment by 3 and 14%. Shade (40% full sunlight) reduced the shoot yield of Siratro in pure swards by 38 and 33 % in the D4 and D8 treatments. Nitrogen accumulation in green panic was markedly improved by shading: the increase in shoot nitrogen yield in pure sward under 60 and 40% light levels was 29 and 32% for the D4 and 45 and 76 % for the D8 treatment. All plant fractions of green panic increased in percentage nitrogen with increasing shade. The nitrogen yield of Siratro in pure sward declined with shading in proportion to dry weight. Shaded green panic swards had a higher leaf area index, better distribution of leaf area with height, and lower light extinction coefficients. The individual leaves had greater photosynthetic activity than those from the full sunlight swards. Shade-grown Siratro swards had a lower leaf area index and their leaves had a lower photosynthetic potential than in the full sunlight treatment. Nodulation was reduced under shade. Competition between green panic and Siratro was most severe in relation to shoot yield, and was accentuated by shading, frequent defoliation, and time. The proportion of Siratro in the mixture declined from 40 % initially to as low as 4-6 %. The extra competitiveness of the tropical grass, green panic, under shade in this experiment was due to a substantially increased ability to accumulate nitrogen and also to changes in canopy structure. In soils of markedly different nitrogen status or with nitrogen fertilizer the shade response might be modified.