Effect of Cover Crops on Vertical Distribution of Leaf Area and Dry Matter of Soybean (Glycine max L.) in Competition with Weeds

Introduction Amount and vertical distribution of leaf area are essential for estimating interception and utilization of solar radiation of crop canopies and, consequently dry matter accumulation (Valentinuz & Tollenaar, 2006). Vertical distribution of leaf area is leaf areas per horizontal layers, based on height (Boedhram et al., 2001). Above-ground biomass is one of the central traits in functional plant ecology and growth analysis. It is a key parameter in many allometric relationships (Niklas & Enquist, 2002). The vertical biomass distribution is considered to be the main determinant of competitive strength in plant species. The presence of weeds intensifies competition for light, with the effect being determined by plant height, position of the branches, and location of the maximum leaf area. So, this experiment was conducted to study the vertical distribution of leaf area and dry matter of soybean canopy in competition with weeds and cover crops.
Materials and methods This experiment was performed based on complete randomized block design with 3 replications in center of Agriculture of Joybar in 2013. Soybean was considered as main crop and soybean and Persian clover (Trifolium resupinatum L.), fenugreek (Trigonella foenum–graecum L.), chickling pea (Lathyrus sativus L.) and winter vetch (Vicia sativa L.) were the cover crops. Treatments were included cover crops (Persian clover, fenugreek, chickling pea and winter vetch) aDid you mean: nd cover crop planting times (simultaneous planting of soybean with cover crops and planting cover crops three weeks after planting of soybeans) and also monoculture of soybeans both in weedy and weed free conditions were considered as controls. Soybean planted in 50 cm row spacing with 5 cm between plants in the same row. Each plot was included 5 rows soybeans. Cover crop inter-seeded simultaneously in the main crop. Crops were planted on 19 May 2013 for simultaneous planting of soybean. The dominant weed species were green foxtail (Setaria viridis L.), foxtail grass (Alopecurus myosuroides Huds), and red root pigweed (Amaranthus retroflexus L.) in the experimental field. The other weed was considered as the others. At the soybean canopy closure stage, a vertical card board frame marked in 20-cm increments was used in the field as a guide to cut standing plants including soybean, cover crops and weeds. In each vertical layer of canopy, leaves and stem samples were separated. The leaf area both crops and weeds were measured with a leaf area meter LICOR-3000A (LI-COR, Lincoln, NE, USA). Stem and leaf samples oven dried.
Results and discussion The results showed that the maximum leaf area and dry matter of soybean was varied in different layers of canopy depending on the type of cover crop and cover crop planting time, while delaying in planting of cover crop causes lower leaf area and dry matter than the plants were planted at the same time with soybean. In most cases, the maximum leaf area index and dry matter accumulation of weeds were obtained in primary layer (0-20 cm). The maximum weed leaf area (290.4 cm2) was observed in simultaneous planting of soybean with fenugreek and maximum of total weed dry matter (12.4 g.plant-1) was observed in soybean without weeding, while the minimum of weed total leaf area and total weed dry matter (0 cm2 and 0 g.plant-1, respectively) was achieved in planting of winter vetch after 21 days of soybean planting. Also delayed planting of winter vetch produced the maximum grain yield (3792.6 kg.ha-1) of soybean in weedy plots, where it had the greatest weed suppressive ability than other cover crops.
Conclusion Winter vetch can suppress weed with competition and allelopathic mechanism, so we can conclude that winter vetch planting21 days after soybean planting is suitable replacement for weed suppression in sustainable agriculture.