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The high vulnerability of sweet corn (Zea mays L. var. saccharata) to weed competition and the urgent reduction of the dependence on chemical herbicides are major challenges facing the agricultural community. To investigate the effects of plastic mulches on weed control and yields in sweet corn under different sowing dates, a 2-year experiment was conducted at Varamin, Iran, in 2010 and 2011. The mulch treatments included black plastic mulch, semitransparent biodegradable mulch, an unmulched weeded control, and an unmulched unweeded control. The sowing dates were 5 June, 20 June, and 6 July. Results revealed that delayed sowing (6 July) reduced weed dry weight by 51% and 41% compared with the 5 June sowing date in 2010 and 2011, respectively, without reducing crop yield. The black plastic mulch treatment had the lowest weed biomass. The sowing date 3 mulch interactions on kernel number per ear, length of ear, percentage of unfilled ear tip, and yield of sweet corn were statistically significant (P < 0.01) in both years. The maximum kernel number per ear (535) and the highest fresh ear yield (24,684 kg·ha) in 2010 were obtained on the 6 July sowing date under biodegradable mulch. The plants sown on 6 July produced the highest fresh kernel yield with the black plastic mulch (12,893 kg·ha) and unmulched weeded control (11,777 kg·ha) in 2010 and 2011, respectively. The highest percentage of unfilled ear tips in both years was observed in the unmulched unweeded plots sown on 5 June. According to our findings, to suppress the weeds and avoid the sweet corn yield loss in such a hot summer that we had, using the black plastic mulch and delayed sowing are recommended. Changes in sowing date can strongly affect plants development (Hay, 1986). Sowing dates can be manipulated to avoid the periods of greatest risk from pests, weeds, and diseases and hence increase yield of the crop (Harper, 1999). Many researchers have argued that the correct sowing time for a variety is the date that gets the crop to anthesis at the optimum time. In maize the high air temperature (greater than 38 C) compounded by water stress at anthesis decreases the kernel set under dry land environments (Ramadoss et al., 2004). Herbek (1986) reported that delayed sowing of corn in hot and dry conditions reduced yield and had harmful effects on pollination and grain filling. In contrast, Oktem et al. (2004) obtained the highest fresh ear yields for a 25 July sowing date and the lowest fresh ear yields for an 25 Apr. sowing date in a hot and dry region of Turkey. Determining the optimal sowing date for corn is thus very crucial for maximizing crop yields (Abdel Rahman et al., 2002). On the other hand, sweet corn should be consumed, canned, or frozen immediately after harvest as a result of the rapid conversion of soluble sugars to starch. Accordingly, sweet corn is planted over a 3-month period in the north–central United States to extend the availability of fresh produce for marketing and processing (Williams, 2008). Use of variable sowing dates to control weeds is dependent on time of weed emergence. An awareness of the timing of weed emergence facilitates the planting of crops when weeds are at their lowest density. Differences in the growing environment can also influence the number, structure, germinability, and viability of the seeds produced (Nurse and DiTommaso, 2005). Mulder and Doll (1994) reported that in-row weed density decreased significantly in uncultivated treatments when planting was delayed from 25 Apr. to 5 May in 1991. Delayed planting allows corn to germinate after peak emergence of many weed species (Regnier and Janke, 1990). Weed suppression is one of the largest benefits of mulching. Mulching can inhibit weeds in two ways: 1) by preventing light from reaching the soil, which reduces germination and seedling growth; and 2) by acting as a physical barrier against the growth of weed seedlings. The beneficial effects of mulches on crop yield have been reported in many studies. Mahajan et al. (2007) found that plastic mulch reduced weed dry matter by 63.8% compared with an unmulched control. Zandstra et al. (2007) reported that the marketable yield of sweet corn increased by 25% to 63% with clear plastic mulch compared with bare soil. Plant height, total dry matter, number of grains per cob, 1000grain weight, and grain yield of corn were maximized under mulch treatment (Khurshid et al., 2006). The objective of the present study was to investigate a non-chemical weed control strategy by evaluating opaque plastic and semitransparent biodegradable mulches and different sowing date on sweet corn yield and competitiveness against weeds. Materials and Methods Climate. Climate at Varamin has been classified as arid and semiarid. Summers (June to September) are very hot and dry. Temperature reached 45 C (Fig. 1), and no precipitation fell during June, July, and August. Maximum mean temperature was recorded for July in both years (Fig. 2). Average relative humidity during the summer was 31% to 33%. In addition, a warm wind blows from the south that dries the corn flowers. Site description. Field experiments were conducted in 2010 and 2011 at the research farm of the Faculty of Agriculture on the Varamin-Pishva Branch, Islamic Azad University, Varamin, Iran (lat. 35 17# N, long. 51 40# E). The previous crop was wheat (Triticum aestivum L.). The soil at the beginning of the experiments contained 23% sand, 44% silt, 33% clay, and 0.92% organic carbon and had a pH of 7.6. The experimental setup was a factorial arrangement in a randomized complete block design with four mulch treatments of black plastic mulch film, semitransparent biodegradable mulch film (starch based), an unmulched weeded control, and an unmulched unweeded and three planting dates of 5 June, 20 June, and 6 July with three replicates. We chose these dates to find the best sowing dates producing the highest yield in a short period between two fall crops. Both the plastic and biodegradable mulches were 120 cm wide. The plastic and biodegradable mulches were 30 and 50 mm thick, respectively. We prepared seed beds by a bed shaper and mulches laid manually on the raised beds (70 cm width) 2 d before planting. Raised beds are often preferred with plastic mulches because they warm more quickly than flat beds and Received for publication 31 July 2013. Accepted for publication 10 Jan. 2014. To whom reprint requests should be addressed; e-mail Mirshekari@iaut.ac.ir. HORTSCIENCE VOL. 49(3) MARCH 2014 289 offer superior drainage (Lamont, 1996; Tarara, 2000). Plot size was 6 m · 6 m. Each plot consisted of four beds 75 cm apart with two rows of sweet corn (Zea mays L. var. saccharata Sturt) cultivar KSC 403 (yellow su and 85 d) planted in each bed. Sweet corn was handseeded through the mulches or into bare soil. The crop was sown with a density of 66,500 seeds/ha with 30 to 35 cm between rows in a bed and 20 cm between plants within the rows. To investigate weed–crop competition, we focused on natural weed population, which emerged from the field seed bank. Herbicides were not applied to the control treatments; we controlled weeds by biweekly manual weeding throughout the season. Fields received cow manure at 25 t·ha, zinc at 19 kg·ha (as zinc sulfate), and manganese at 8 kg·ha (as manganese sulfate) in late May and nitrogen at 161 kg·ha (as urea) and iron at 240 g·ha (as Fe-EDDHA) at three stages (fiveto six-leaf stage, tasseling, and kernel filling through a drip irrigation system). We also watered the plants throughout surface drip irrigation system. Diameter of tape was 16 mm, emitter spacing 20 cm, and flow rate 2 L·h. The irrigation frequency varied 3 to 5 d according to plant growth stage, temperature, and the presence or absence of mulch. Data collection. Twelve adjacent plants were hand-harvested from each plot to determine the total biomass and fresh ear yield when the silks were dark brown ( 70% kernel moisture content). We randomly selected five ears from each plot for determining length of ear, unfilled ear tips, kernel number per ear, and fresh kernel yield. At harvest, natural weed population and biomass were assessed by throwing two 50 · 50-cm quadrates two times over the unmulched plots. However, in plastic mulch treatments, weed was assessed from a 3-m length of a row. Harvest index was calculated by the following formula. HI 1⁄4 fresh ear yield biological yield · 100 Growing degree-days (GDD) were determined using minimum (Tmin) and maximum (Tmax) air temperatures from daily reports by the Iran Meteorological Organization. A base temperature (Tb) of 10 C was used as the minimum temperature for sweet corn growth, and 30 C was used as the air temperature associated with optimal growth. The time of crop sowing was used as the reference point for accumulation of GDD. GDD 1⁄4 Tminþ Tmax ð Þ=2 ð Þ Tb 1⁄2 Statistical analyses. Data were subjected to analysis of variance using PROC GLM (Version 9.1.3; SAS, Cary, NC). When F values were significant, the least significant difference test was used to compare differences in means between treatments. Results and Discussion Weed control. The dominant weed species in the field included redroot pigweed (Amaranthus retroflexus L.), jungle rice (Echinochloa colona L.), common purslane (Portulaca oleracea L.), green foxtail (Setaria viridis L.), and Johnson grass (Sorghum halepense L.), all of which have a C4 photosynthetic pathway and high competition ability against summer crops. The total dry weights of weeds were significantly influenced by sowing date and mulch treatment. At harvest, highest weed dry weight was in the plots sown on 5 June, although there was no statistically significant difference between 5 June and 20 June (Table 1). Actually the absence of a significant difference between the first and second planting dates could be related to the peek emergence of weeds occurring in late spring. The seeds of summer annual grasses fall

Sweet Corn Weed Control and Yields in Response to Sowing Date and Cropping Systems