Effects of shading and nitrogen on phenology and yield of foxtail millet (Setaria italica L.) in competition with white pigweed (Amaranthus albus L.)

Light is a vital component for photosynthesis and plays a significant role in the competitive ability of plants. The nitrogen response of competing plants may be affected by radiation availability and maximum potential growth rate, which determine plant N requirements. Shading reduces the light intensity, which leads to changes in the morphology, physiology, biomass, grain yield, and quality of crops. Moreover, shading stress delays flowering and decreases biomass and grain yield. Since photosynthesis results in dry matter accumulation, and reduced light is known to limit carbon accumulation and nitrogen content, understanding these processes in weeds may provide insight as to their effects on crop production, help to predict their occurrence, and ultimately provide the needed information for their management. In order to evaluate foxtail millet competition with pigweed at different levels of radiation and nitrogen, two separate experiments were conducted in split plot arranged in randomized complete block design with three replications at the Research Farm of Birjand University in 2015. Texturally the soil was loam, with 8.16 pH, 0.03% total N, 12 ppm available P and 250 ppm available K. The experiment was laid out in a split-plot design with three replications having three shade levels (0, 41 and 75% shade) in main plot and three pigweed density (0, 12 and 24 plant per square meter) in subplots in two separate experiments, one under nitrogen application and the other without nitrogen usage. In 0% shade treatment, sunlight was allowed to fall over the millet and pigweed without any barrier. In 41% and 75% treatments, however, the light levels in the form of PAR were reduced using shade nets. At the end of growth stage millet traits including days to panicule emergence, days to maturity, grain filling period, number of grain per panicule, 1000 grain weight, grain yield, forage yield, biomass and harvest index were measured. Data analyses were performed using two-way analysis of variance (ANOVA) with SAS 9.1. Means of treatments were compared between nitrogen, shade treatments and pigweed densities according to protected least significance difference (FLSD) test at the 5% level. Nitrogen had a significant effect (P < 0.01) on days to panicule emergence, days to maturity at 1% probability level and on grain filling period and grain yield at 5% probability level. Application of 150 kg nitrogen per hectare led to improvement of phenological traits and grain yield of millet. Nitrogen can extend vegetative growth stage period of plants and reduce grain filling rate (Nezamzadeh et al., 2011). Also, nitrogen fertilizer can increase grain formation and grain yield through improvement of photosynthesis ability owing to increases in leaf area index and leaf area duration (Moosavi et al., 2015). Shading had a significant (P < 0.01) influence on millet phenological traits and grain yield and had a significant effect (P < 0.05) on number of grain per panicule. Shading at level of 75% dramatically reduced number of grain per panicule and grain yield of millet while increased days to panicule emergence, days to maturity and grain filling period. Nasrollah-zadeh et al., (2011) also reported that grain filling period of faba bean increased by 3-4 days in shaded plants in comparison with control. With shading stress at its highest level, grain yield was significantly reduced by 61% from 3.70 to 1.44 ton per hectare. The effect of pigweed density on grain yield (P < 0.05) and plant height and stem diameter was significant (P < 0.01). The effect of pigweed density was also significant on forage yield (P < 0.01) and days to panicule emergence and grain yield (P < 0.05). Pigweed density of 24 plants per square meter led to 21% reduction in millet grain yield against control. The interaction between nitrogen and pigweed density on millet traits was limited, and only significantly (P < 0.05) affected grain yield. In competition of crops and weeds, the space required for crop to expand the leaf area decreases, and the competition between the species increases to absorb the active photosynthesis radiation. Under these conditions, less assimilates are transmitted to the reproductive organs, which has a profound effect on reduction in grain yield and further on harvest index (Gholamhoseini et al. 2015). The results of this study showed that the grain yield of foxtail millet reduced by 61 percent with increasing of shading intensity contrasting with no shading. This reduction was due to the negative effects of shading on the number of grain per panicule and 1000-grain weight. Harvest index also decreased with increasing shading levels. The reduction in harvest index was due to a greater reduction in grain yield contrasting dry matter production at shading treatments. Also, grain yield at the highest level of pigweed density was 21% lower. These results, considering the importance of weed control, show that proper management of weed control in the field by providing conditions to faster closing canopy in order to reduce the competition of pigweed can be effective in grain yield improvement of foxtail millet.