Safety of Oilseed Rape Straw Mulch of Di ﬀ erent Lengths to Rice and Its Suppressive E ﬀ ects on Weeds

: Rice is widely grown after harvesting of oilseed rape ( Brassica napus L.) in many countries. Experiments were carried out under greenhouse and ﬁeld conditions to assess the impact of oilseed rape straw mulch on rice and paddy weeds. Oilseed rape mulch (900 g m − 2 ) from straw 1-to-7 cm long was found to be safe for rice, when applied four days after sowing (DAS) in direct-seeded rice or four days after transplanting (DAT). In the meantime, the biomass of Echinochloa crus-galli (L.) P. Beauv. was reduced 65.74%, 80.18%, 81.15%, 70.99%, 55.65%, and 27.22%, respectively, when mulched with powder, and 1, 3, 5, 7, and 9-cm long oilseed rape straw, respectively, and the biomass reductions in Leptochloa chinensis (L.) Nees., Ludwigia prostrata Roxb., Ammannia auriculata Willd., and Cyperus di ﬀ ormis L. were all above 97% when mulched with 1 cm-length straw. The results of a ﬁeld trial conﬁrmed that oilseed rape straw mulch (900 g m − 2 ) of 1 cm length was safe for rice. Moreover, weed suppression was as e ﬀ ective as the standard herbicide (oxadiargyl + butachlor 525 g ai ha − 1 ) treatment. These ﬁndings demonstrate the potential to manage paddy rice weeds in an e ﬀ ective and environmentally sound manner by mulching with the straw of a preceding crop, oilseed rape.

• C to 30 • C, humidity ranged from 50% to 90%, and 14 h of natural light was provided. The soil used was a silty loam, with a pH of 6.6, 1.43% organic matter content, and a total nitrogen of 0.14 %. The soil was oven-dried at 110 • C for 3 h to kill any resident weed seeds, and then a 3-cm thick layer of soil was placed in a 42-cm-diameter plastic pot. Each pot received 2.5 g (180 kg ha −1 ) compound fertilizer (N + P 2 O 5 + K 2 O ≥ 45% (15-15-15), Jiangsu Zhongdong fertilizer Co., Ltd., Jiangsu, China) and mixed with water. Rice seeds were germinated in an incubator (28 • C), and 40 seeds with a visible hypocotyl (~0.5 cm long) were sown on the soil surface of each pot. At the same time, seeds of Echinochloa crus-galli (L.) P. Beauv., Leptochloa chinensis (L.) Nees., Ammannia auriculata Willd., Ludwigia prostrata Roxb. and Cyperus difformis L. which were collected from Jiaxing, Zhejiang Province in 2017 and stored in a refrigerator (4 • C) until needed, were sown at a density of 70, 130, 110, 100, and 120 seeds per pot, respectively. The germination levels of these weeds were approximately 90%, 50%, 90%, 75%, and 50%, respectively, and the densities were based on these for each of the species to provide typical field densities found in paddy fields in Zhejiang province. Water was added to keep the soil moist. At 4 days after sowing (DAS), pots were subjected to various mulch treatments: pots were mulched with oilseed rape straw (900 g m -2 ) of 1 cm (T1), 3 cm (T3), 5 cm (T5), 7 cm (T7), and 9 cm (T9) length. A rice-only control without mulch treatment (CK) and a rice + the five-weed species control without straw mulch treatment (WCK) were also included. Treatments were set up in a completely randomized design (CRD) with four replicates. After mulching, 500 ml of water was evenly applied to each pot to maintain the straw moisture. The soil was kept moist until 6 DAS and then kept flooded to a depth of 2 cm. Urea (N≥46.4%) (Shandong Hualu-hengsheng Chemical Co., Ltd., Shandong, China) was applied at 108 kg ha −1 after 20 days of straw mulching (DAM) and re-applied (144 kg ha −1 ) at 30 DAM. The number of weeds was recorded at 10, 20, 30, and 40 DAM. At 40 DAM, aboveground weed tissues were harvested in each pot, dried at 110 • C for 1 h, followed by drying at 80 • C to constant weight before weighing. The number of rice plants and the height of 10 randomly selected plants per pot was also recorded at 10 DAM. At 20,30, and 40 DAM, ten rice plants were randomly selected to determine plant height and tiller number. Aboveground tissues of 20 rice plants per pot were harvested, dried, and weighed at 40 DAM, as described above for weeds.

Impact of Oilseed Rape Straw Mulch of Different Lengths on Simulated Transplanted Rice and its Effect on Major Paddy Weeds
Twelve tufts (3 plants/tuft) of rice seedlings were transplanted into 42-cm-diameter plastic pots when rice was at the four-leaf stage. At the same time, the seeds of E. crus-galli, L. chinensis, A. auriculata, L. prostrata, and C. difformis were sown at densities of 70, 130, 110, 100, and 120 seeds per pot, respectively. Water was added to keep the soil moist. At 4 days after rice transplanting (DAT), pots were subjected to various mulch treatments: pots were mulched with oilseed rape straw (900 g m −2 ) of powder (T0), 1 cm (T1), 3 cm (T3), 5 cm (T5), 7 cm (T7), and 9 cm (T9) length. A rice-only control (CK) and rice + five-weed species control (WCK) treatment not receiving the straw mulch were also included. Treatments were set up in a completely randomized design (CRD) with four replicates. After mulching, 500 ml of water was evenly applied in each pot to maintain the straw moisture. The water level was adjusted to a 2 cm depth at 6 DAT. The experimental conditions, procedures, and data collection were similar to those described above for the direct-seeded trial.

Field Study on Impact of Oilseed Rape Straw Mulch on Rice and Its Effect on Paddy Weeds
Experiments were conducted in direct-seeded and transplanted rice fields in 2017 in Linhai City, Zhejiang Province (28.76 • N, 121.13 • E). The rice cultivar used was Yeyou 12. The oilseed rape straw was derived from cultivar Zheda 619. The straw material was pulverized into a powder of 1-cm length with a mower. The field soil was comprised of 3.3% organic matter content, 2.1 g kg −1 total nitrogen, 231.8 mg kg −1 alkali-hydrolyzable nitrogen, 44.0 mg kg −1 available phosphorus, and 171.0 mg kg −1 available potassium. The main weeds in the field were E. crus-galli, L. chinensis, A. auriculata, L. prostrata, C. difformis, Monochoria vaginalis (Burm. f.) C. Presl ex Kunth (heart shape false pickerelweed), and Rotala indica (Willd.) Koehne (Indian toothcup).
At 4 days after sowing (DAS), direct-seeded rice fields were subjected to three treatments: 700 g m −2 oilseed straw mulch (A), 900 g m −2 oilseed straw mulch (B), and a no-straw mulch control (CK). At 7 days after transplanting (DAT), transplanted rice fields were subjected to five treatments: 700 g m −2 oilseed straw mulch (A), 900 g m −2 oilseed straw mulch (B), 35% oxadiargyl + butachlor EW 1.5 L ha −1 (Bayer) (C), hand weeding (D) which was carried out at 10 and 30 DAT, and a no-straw mulch and non-herbicide control (CK). Compound fertilizer (N + P 2 O 5 + K 2 O ≥ 45 % (15-15-15) was applied at 525 kg ha −1 before rice sowing or transplanting, urea (N > 46.4%) was applied at 75 kg ha −1 after 15 and 30 DAS in direct-seeded rice. Urea was applied at 75 kg ha −1 7 DAT and re-applied at 150 kg ha −1 17 DAT. All other agricultural operations, such as water and pest control followed local management protocols. The temperature range during the experiment was 20~32°C. There were two rainfall events within 15 days after treatment, one of 1.0 mm 3 DAM and one of 2.6 mm 7 DAM.
The field studies were performed as a randomized complete block design with three replicates. Measurements were taken within a 20-m 2 area in each plot. Seven days after mulch was added (DAM), the number of rice seedlings in the direct-seeded rice trial was recorded from three (50 by 50 cm) subplots that were located randomly along a diagonal line of the plot. At 35 DAM, the number of rice tillers in the transplanted rice trial was recorded by randomly selecting 10 plants in each plot. The rice yield was measured at 136 DAS and 125 DAT, respectively. In both the direct-seeded and transplanting trials, the density of weeds was assessed at 20 and 35 DAM from three (50 by 50 cm) subplots that were located randomly along a diagonal line of the plot, and the aboveground fresh weight of weeds determined at 35 DAM.

Statistical Analysis
Data were analyzed using Fisher's Analysis of Variance (ANOVA) technique. Straw length and days after mulching (DAM) were considered fixed factors in the greenhouse study, and treatments were considered fixed factors in the field study. Treatment means were compared using the least significant difference (LSD) test at p ≤ 0.05. Statistical analyses were carried out using the SPSS statistical analytical package (version 21). Rice seedling numbers, plant height, tiller numbers, and dry weight data were analyzed by comparing them with data from the control treatments (CK and WCK). Weed number and dry weight data were calculated as a percentage by comparing the data with control treatments (WCK or CK) before analysis.

Impact of Oilseed Rape Straw Mulching in Simulated Direct-Seeded Rice
After 10 days of oilseed rape straw mulching (DAM), the 1~7 cm straw length treatments had no effect on rice emergence. However, the number of rice seedlings in the 9-cm length straw mulch treatment decreased by 8.8% compared with the control (CK) treatment ( Table 1). The height, tiller number, and dry weight of rice plants at 40 DAM for the 1~9 cm straw length mulch treatments did not differ from the rice control (CK) treatment. In contrast, the height and tiller number of rice plants in pots without straw mulching (WCK) decreased significantly. Oilseed rape straw mulch was generally effective at suppressing the emergence and growth of the weed species tested, although effects varied by species. Suppression of E. crus-galli was best achieved with the straw mulch of 1-3 cm length, which resulted in at least a 64.6% decrease in dry weight at 40 DAM ( Table 2). The inhibitory effect of the mulch was reduced when the straw mulch of 5~9 cm length was used. For example, the 9-cm straw mulch only reduced E. crus-galli aboveground dry weight by 22.9% ( Table 2). The 1-3 cm length mulch treatment was most effective at suppressing L. chinensis and L. prostrata. Applying 1-cm long straw mulch decreased dry weight of these two troublesome weeds by as much as 99.7% and 97.4%, respectively. In contrast, the suppressive effects of the mulch on weed dry weight decreased when straw mulch greater than 3 cm in length was used (Table 2). However, straw mulch lengths of 1~5 cm were most effective for suppressing A. auriculata and C. difformis dry weight, with reductions above 98.7% and 99.1%, respectively. Substantial reductions in dry weight of these two weed species were also achieved using the other straw mulch lengths ( Table 2). For all five weed species combined, oilseed rape straw mulch lengths of 1~3 cm were most effective in reducing both density and aboveground dry biomass of weeds. For instance, these straw mulch length treatments reduced overall weed densities and aboveground dry biomass by more than 83.5% and 68.3%, respectively ( Table 2). The efficacy of the straw mulch to suppress the growth of the five weed species decreased with sampling time, especially for the longer length straw mulch treatments. Oilseed rape straw mulch length had a significant impact on weed suppression when applied 4 DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli aboveground dry biomass decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground dry biomass decreased by only 27.2 % when subjected to the 9-cm long straw mulch treatment (Table 4). Reductions in aboveground dry biomass for L. chinensis and C. difformis were above 96.5% when subjected to the 1~5 cm straw treatments. The suppressive ability of straw mulch for the 7 cm and 9 cm treatments was also acceptable even if lower than the 1-5 cm straw treatments. Aboveground dry biomass reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm straw mulch treatments. Although decreases in the biomass of these two weeds were lower for the other straw mulch lengths, reductions were still above 91.4% (Table 4). Overall, mulch treatments of 1~3 cm straw length were most effective in reducing weed densities (77.9%) and aboveground dry biomass (86.7%). The optimal suppressive ability of the straw mulch appeared to occur for intermediate (1~5 cm) oilseed rape straw mulch lengths (Table 4). Oilseed rape straw mulch length had a significant impact on weed suppression when applied 4 DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli aboveground dry biomass decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground dry biomass decreased by only 27.2 % when subjected to the 9-cm long straw mulch treatment (Table 4). Reductions in aboveground dry biomass for L. chinensis and C. difformis were above 96.5% when subjected to the 1~5 cm straw treatments. The suppressive ability of straw mulch for the 7 cm and 9 cm treatments was also acceptable even if lower than the 1-5 cm straw treatments. Aboveground dry biomass reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm straw mulch treatments. Although decreases in the biomass of these two weeds were lower for the other straw mulch lengths, reductions were still above 91.4% (Table 4). Overall, mulch treatments of 1~3 cm straw length were most effective in reducing weed densities (77.9%) and aboveground dry biomass (86.7%). The optimal suppressive ability of the straw mulch appeared to occur for intermediate (1~5 cm) oilseed rape straw mulch lengths ( Oilseed rape straw mulch length had a significant impact on weed suppression when applied 4 DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli aboveground dry biomass decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground dry biomass decreased by only 27.2 % when subjected to the 9-cm long straw mulch treatment (Table 4). Reductions in aboveground dry biomass for L. chinensis and C. difformis were above 96.5% when subjected to the 1~5 cm straw treatments. The suppressive ability of straw mulch for the 7 cm and 9 cm treatments was also acceptable even if lower than the 1-5 cm straw treatments. Aboveground dry biomass reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm straw mulch treatments. Although decreases in the biomass of these two weeds were lower for the other straw mulch lengths, reductions were still above 91.4% (Table 4). Overall, mulch treatments of 1~3 cm straw length were most effective in reducing weed densities (77.9%) and aboveground dry biomass (86.7%). The optimal suppressive ability of the straw mulch appeared to occur for intermediate (1~5 cm) oilseed rape straw mulch lengths (Table 4).

Leptochloa chinensis
Agronomy 2019, 9,  Oilseed rape straw mulch length had a significant impact on weed suppression when applied 4 DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli aboveground dry biomass decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground dry biomass decreased by only 27.2 % when subjected to the 9-cm long straw mulch treatment (Table 4). Reductions in aboveground dry biomass for L. chinensis and C. difformis were above 96.5% when subjected to the 1~5 cm straw treatments. The suppressive ability of straw mulch for the 7 cm and 9 cm treatments was also acceptable even if lower than the 1-5 cm straw treatments. Aboveground dry biomass reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm straw mulch treatments. Although decreases in the biomass of these two weeds were lower for the other straw mulch lengths, reductions were still above 91.4% (Table 4). Overall, mulch treatments of 1~3 cm straw length were most effective in reducing weed densities (77.9%) and aboveground dry biomass (86.7%). The optimal suppressive ability of the straw mulch appeared to occur for intermediate (1~5 cm) oilseed rape straw mulch lengths ( T refers to oilseed rape straw mulch treatment (900 g m −2 ) initiated 4 days after sowing; T1, straw length = 1 cm；T3, straw length = 3 cm；T5, straw length = 5 cm；T7, straw length = 7 cm；and T9, straw length = 9 cm；DAM, days after mulch application; In each column, different lowercase letters indicate significant differences at p ≤ 0.05 while different capital letters indicate highly significant differences at p ≤ 0.01.

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Simulated Transplanted Rice
Oilseed rape straw mulch length had a significant impact on weed suppression when applied 4 DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli aboveground dry biomass decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground dry biomass decreased by only 27.2 % when subjected to the 9-cm long straw mulch treatment (Table 4). Reductions in aboveground dry biomass for L. chinensis and C. difformis were above 96.5% when subjected to the 1~5 cm straw treatments. The suppressive ability of straw mulch for the 7 cm and 9 cm treatments was also acceptable even if lower than the 1-5 cm straw treatments. Aboveground dry biomass reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm straw mulch treatments. Although decreases in the biomass of these two weeds were lower for the other straw mulch lengths, T refers to oilseed rape straw mulch treatment (900 g m −2 ) initiated 4 days after sowing; T1, straw length = 1 cm；T3, straw length = 3 cm；T5, straw length = 5 cm；T7, straw length = 7 cm；and T9, straw length = 9 cm；DAM, days after mulch application; In each column, different lowercase letters indicate significant differences at p ≤ 0.05 while different capital letters indicate highly significant differences at p ≤ 0.01.

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Simulated Transplanted Rice
Oilseed rape straw mulch length had a significant impact on weed suppression when applied 4 DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli aboveground dry biomass decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground dry biomass decreased by only 27.2 % when subjected to the 9-cm long straw mulch treatment (Table 4). Reductions in aboveground dry biomass for L. chinensis and C. difformis were above 96.5% when subjected to the 1~5 cm straw treatments. The suppressive ability of straw mulch for the 7 cm and 9 cm treatments was also acceptable even if lower than the 1-5 cm straw treatments. Aboveground dry biomass reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm straw mulch treatments. Although decreases in the biomass of these two weeds were lower for the other straw mulch lengths, There was no difference in the number of rice plants, height, tiller number, and dry weight between the rice-only control (CK) treatment and the straw mulching treatments of 7-cm length or shorter at 10 DAM (Table 3). However, the height of rice plants subjected to the 9-cm straw mulch treatment was lower than for rice plants in the control treatment (CK) at 10~30 DAM. No differences in rice height were observed at 40 DAM (Table 3).

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Simulated Transplanted Rice
Oilseed rape straw mulch length had a significant impact on weed suppression when applied 4 DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli aboveground dry biomass decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground dry biomass decreased by only 27.2 % when subjected to the 9-cm long straw mulch treatment (Table 4). Reductions in aboveground dry biomass for L. chinensis and C. difformis were above 96.5% when subjected to the 1~5 cm straw treatments. The suppressive ability of straw mulch for the 7 cm and 9 cm treatments was also acceptable even if lower than the 1-5 cm straw treatments. Aboveground dry biomass reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm straw mulch treatments. Although decreases in the biomass of these two weeds were lower for the other straw mulch lengths, reductions were still above 91.4% (Table 4). Overall, mulch treatments of 1~3 cm straw length were most effective in reducing weed densities (77.9%) and aboveground dry biomass (86.7%). The optimal suppressive ability of the straw mulch appeared to occur for intermediate (1~5 cm) oilseed rape straw mulch lengths ( Table 4).
The suppressive ability of straw mulch decreased slightly with sampling time, especially for L. chinensis, A. auriculata, L. prostrata, and C. difformis. These differences in the suppressive ability of the straw mulch over time were not very pronounced.

Impact of Oilseed Rape Straw Mulch on Rice and Its Effect on Paddy Weeds in the Field
There was no difference in the emergence of rice plants between the rape straw mulch treatments and control (CK) treatment in direct-seeded rice. However, rice yield was significantly higher than for the control (CK) treatment. In the transplanted-rice field trial, the number of tillers and yield of rice for the 900 g m −2 mulch treatment were significantly higher than for the oxadiargyl + butachlor herbicide treatment and the hand-weeded control treatment (Table 5). Rice was transplanted at the four-leaf stage; CK, rice control; WCK, rice and weed control; T refers to the oilseed rape straw mulch treatment (900 g m −2 ) applied 4 days after transplanting; T0, straw powder; T1, straw length = 1 cm; T3, straw length = 3 cm; T5, straw length = 5 cm; T7, straw length = 7 cm; T9, straw length = 9 cm; DAM, days after mulch application; In each column, different lowercase letters indicate significant differences at p ≤ 0.05 while different capital letters indicate highly significant differences at p ≤ 0.01.

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Simulated Transplanted Rice
Oilseed rape straw mulch length had a significant impact on weed suppression when applied 4 DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli aboveground dry biomass decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground dry biomass decreased by only 27.2 % when subjected to the 9-cm long straw mulch treatment (Table 4). Reductions in aboveground dry biomass for L. chinensis and C. difformis were above 96.5% when subjected to the 1~5 cm straw treatments. The suppressive ability of straw mulch for the 7 cm and 9 cm treatments was also acceptable even if lower than the 1-5 cm straw treatments. Aboveground dry biomass reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm straw mulch treatments. Although decreases in the biomass of these two weeds were lower for the other straw mulch lengths, reductions were still above 91.4% (Table 4). Overall, mulch treatments of 1~3 cm straw length were most effective in reducing weed densities (77.9%) and aboveground dry biomass (86.7%). The optimal suppressive ability of the straw mulch appeared to occur for intermediate (1~5 cm) oilseed rape straw mulch lengths (

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Simu
Oilseed rape straw mulch length had a significant impact on weed s DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-gall decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground only 27.2 % when subjected to the 9-cm long straw mulch treatmen aboveground dry biomass for L. chinensis and C. difformis were above 9 1~5 cm straw treatments. The suppressive ability of straw mulch for the was also acceptable even if lower than the 1-5 cm straw treatments. reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 Although decreases in the biomass of these two weeds were lower for the reductions were still above 91.4% (Table 4). Overall, mulch treatments o most effective in reducing weed densities (77.9%) and aboveground optimal suppressive ability of the straw mulch appeared to occur for in rape straw mulch lengths (Table 4).  T refers to oilseed rape straw mulch treatment (900 g m −2 ) initiated 4 days after sowing; T1, straw length = 1 cm；T3, straw length = 3 cm；T5, straw length = 5 cm；T7, straw length = 7 cm；and T9, straw length = 9 cm；DAM, days after mulch application; In each column, different lowercase letters indicate significant differences at p ≤ 0.05 while different capital letters indicate highly significant differences at p ≤ 0.01.

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Simulated Transplanted Rice
Oilseed rape straw mulch length had a significant impact on weed suppression when applied 4 DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli aboveground dry biomass decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground dry biomass decreased by only 27.2 % when subjected to the 9-cm long straw mulch treatment (Table 4). Reductions in aboveground dry biomass for L. chinensis and C. difformis were above 96.5% when subjected to the 1~5 cm straw treatments. The suppressive ability of straw mulch for the 7 cm and 9 cm treatments was also acceptable even if lower than the 1-5 cm straw treatments. Aboveground dry biomass reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm straw mulch treatments. Although decreases in the biomass of these two weeds were lower for the other straw mulch lengths, reductions were still above 91.4% (Table 4). Overall, mulch treatments of 1~3 cm straw length were most effective in reducing weed densities (77.9%) and aboveground dry biomass (86.7%). The optimal suppressive ability of the straw mulch appeared to occur for intermediate (1~5 cm) oilseed rape straw mulch lengths (

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Simu
Oilseed rape straw mulch length had a significant impact on weed s DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground only 27.2 % when subjected to the 9-cm long straw mulch treatmen aboveground dry biomass for L. chinensis and C. difformis were above 96 1~5 cm straw treatments. The suppressive ability of straw mulch for the was also acceptable even if lower than the 1-5 cm straw treatments. A reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 c Although decreases in the biomass of these two weeds were lower for the reductions were still above 91.4% (Table 4). Overall, mulch treatments o most effective in reducing weed densities (77.9%) and aboveground optimal suppressive ability of the straw mulch appeared to occur for int rape straw mulch lengths (

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Simulated Transplanted Rice
Oilseed rape straw mulch length had a significant impact on weed suppression when applied 4 DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli aboveground dry biomass decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground dry biomass decreased by only 27.2 % when subjected to the 9-cm long straw mulch treatment (Table 4). Reductions in aboveground dry biomass for L. chinensis and C. difformis were above 96.5% when subjected to the 1~5 cm straw treatments. The suppressive ability of straw mulch for the 7 cm and 9 cm treatments was also acceptable even if lower than the 1-5 cm straw treatments. Aboveground dry biomass reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm straw mulch treatments. Although decreases in the biomass of these two weeds were lower for the other straw mulch lengths, reductions were still above 91.4% (Table 4). Overall, mulch treatments of 1~3 cm straw length were most effective in reducing weed densities (77.9%) and aboveground dry biomass (86.7%). The optimal suppressive ability of the straw mulch appeared to occur for intermediate (1~5 cm) oilseed rape straw mulch lengths (Table 4).

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Simul
Oilseed rape straw mulch length had a significant impact on weed su DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground only 27.2 % when subjected to the 9-cm long straw mulch treatment aboveground dry biomass for L. chinensis and C. difformis were above 96 1~5 cm straw treatments. The suppressive ability of straw mulch for the was also acceptable even if lower than the 1-5 cm straw treatments. A reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm Although decreases in the biomass of these two weeds were lower for the reductions were still above 91.4% (Table 4). Overall, mulch treatments of most effective in reducing weed densities (77.9%) and aboveground optimal suppressive ability of the straw mulch appeared to occur for inte rape straw mulch lengths (

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Simulated Transplanted Rice
Oilseed rape straw mulch length had a significant impact on weed suppression when applied 4 DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli aboveground dry biomass decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground dry biomass decreased by only 27.2 % when subjected to the 9-cm long straw mulch treatment (Table 4). Reductions in aboveground dry biomass for L. chinensis and C. difformis were above 96.5% when subjected to the 1~5 cm straw treatments. The suppressive ability of straw mulch for the 7 cm and 9 cm treatments was also acceptable even if lower than the 1-5 cm straw treatments. Aboveground dry biomass reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm straw mulch treatments.

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Simu
Oilseed rape straw mulch length had a significant impact on weed s DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-gall decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground only 27.2 % when subjected to the 9-cm long straw mulch treatmen aboveground dry biomass for L. chinensis and C. difformis were above 9 1~5 cm straw treatments. The suppressive ability of straw mulch for the was also acceptable even if lower than the 1-5 cm straw treatments. reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 Rice was transplanted at the four-leaf stage; CK, rice control; WCK, rice and weed control; T refers to oilseed straw mulch treatment (900 g m −2 ) applied 4 days after transplanting; T0, straw powder; T1, straw length =1 cm; T3, straw length = 3 cm; T5, straw length = 5 cm; T7, straw length = 7 cm; T9, straw length = 9 cm; DAM, days after mulch application; In each column, different lowercase letters indicate significant differences at p ≤ 0.05 while different capital letters indicate highly significant differences at p ≤ 0.01. Table 5. Effects of various oilseed rape straw mulch treatments and herbicide application on direct-seeded and transplanted rice in field trials.

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Simulated Transplanted Rice
Oilseed rape straw mulch length had a significant impact on weed suppression when applied 4 DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli aboveground dry biomass decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground dry biomass decreased by only 27.2 % when subjected to the 9-cm long straw mulch treatment (Table 4). Reductions in aboveground dry biomass for L. chinensis and C. difformis were above 96.5% when subjected to the 1~5 cm straw treatments. The suppressive ability of straw mulch for the 7 cm and 9 cm treatments was also acceptable even if lower than the 1-5 cm straw treatments. Aboveground dry biomass reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm straw mulch treatments. Although decreases in the biomass of these two weeds were lower for the other straw mulch lengths, reductions were still above 91.4% (Table 4). Overall, mulch treatments of 1~3 cm straw length were most effective in reducing weed densities (77.9%) and aboveground dry biomass (86.7%). The optimal suppressive ability of the straw mulch appeared to occur for intermediate (1~5 cm) oilseed rape straw mulch lengths (Table 4).

Suppression by Oilseed Rape Straw Mulch of Major Weeds in S
Oilseed rape straw mulch length had a significant impact on wee DAT. When subjected to the 1-5 cm straw mulch treatments, E. crusdecreased more than 71% at 40 DAM (Table 4). In contrast, abovegro only 27.2 % when subjected to the 9-cm long straw mulch treatm aboveground dry biomass for L. chinensis and C. difformis were abov 1~5 cm straw treatments. The suppressive ability of straw mulch for was also acceptable even if lower than the 1-5 cm straw treatmen reductions for L. prostrata and A. auriculata were above 97.9% in the 1 Although decreases in the biomass of these two weeds were lower for reductions were still above 91.4% (Table 4). Overall, mulch treatmen most effective in reducing weed densities (77.9%) and abovegrou optimal suppressive ability of the straw mulch appeared to occur for rape straw mulch lengths ( ) initiated 4 days after sowing; T1, straw length = 1 cm；T3, straw length = 3 cm；T5, straw length = 5 cm；T7, straw length = 7 cm；and T9, straw length = 9 cm；DAM, days after mulch application; In each column, different lowercase letters indicate significant differences at p ≤ 0.05 while different capital letters indicate highly significant differences at p ≤ 0.01.

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Simulated Transplanted Rice
Oilseed rape straw mulch length had a significant impact on weed suppression when applied 4 DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-galli aboveground dry biomass decreased more than 71% at 40 DAM (Table 4). In contrast, aboveground dry biomass decreased by only 27.2 % when subjected to the 9-cm long straw mulch treatment (Table 4). Reductions in aboveground dry biomass for L. chinensis and C. difformis were above 96.5% when subjected to the 1~5 cm straw treatments. The suppressive ability of straw mulch for the 7 cm and 9 cm treatments was also acceptable even if lower than the 1-5 cm straw treatments. Aboveground dry biomass reductions for L. prostrata and A. auriculata were above 97.9% in the 1~3 cm straw mulch treatments. Although decreases in the biomass of these two weeds were lower for the other straw mulch lengths, reductions were still above 91.4% (Table 4). Overall, mulch treatments of 1~3 cm straw length were most effective in reducing weed densities (77.9%) and aboveground dry biomass (86.7%). The optimal suppressive ability of the straw mulch appeared to occur for intermediate (1~5 cm) oilseed rape straw mulch lengths (Table 4).

Suppression by Oilseed Rape Straw Mulch of Major Weeds in Si
Oilseed rape straw mulch length had a significant impact on wee DAT. When subjected to the 1-5 cm straw mulch treatments, E. crus-g decreased more than 71% at 40 DAM (Table 4). In contrast, abovegrou only 27.2 % when subjected to the 9-cm long straw mulch treatm aboveground dry biomass for L. chinensis and C. difformis were abov 1~5 cm straw treatments. The suppressive ability of straw mulch for was also acceptable even if lower than the 1-5 cm straw treatment reductions for L. prostrata and A. auriculata were above 97.9% in the 1~ Although decreases in the biomass of these two weeds were lower for reductions were still above 91.4% (Table 4). Overall, mulch treatment most effective in reducing weed densities (77.9%) and abovegrou optimal suppressive ability of the straw mulch appeared to occur for rape straw mulch lengths (Table 4). Oilseed rape straw mulch length had a significant impact on weed suppression in the field trials. In direct-seeded rice, the 700 g m −2 and 900 g m −2 straw mulch treatments reduced weed density by more than 98.0% at 20 DAM, and weed fresh weight by more than 96.9% at 35 DAM. For transplanted rice, there was no difference in weed suppression between the 900 g m −2 straw mulch treatment and the oxadiargyl + butachlor herbicide treatment, but both treatments were more effective than the hand-weeded treatment. The 700 g m −2 straw mulch treatment suppressed weeds as effectively as the hand-weeded treatment ( Table 6). Table 6. Effects of various oilseed rape straw mulch treatments and herbicide application on the density and fresh biomass of weeds in direct-seeded and transplanted rice field trials.

Discussion
Crop residues including straw are an important resource in modern agriculture. The use of crop residues plays a critical role in stabilizing agricultural systems by providing ecosystem services, increasing farmer income, and alleviating energy and environmental pressures. Our findings demonstrate that oilseed rape straw (900 g m −2 ) may be safe to use in direct-seeded rice cropping systems when mulching with 1~7 cm length straw at four DAS. A possible reason for this finding is that rice seedlings were largely at the two-leaf growth stage and the leaf incline angle was relatively small when mulch was applied. Rice stems are typically thick and at the seedling stage still protected by the coleoptile [26,27], such that rice seedlings at four DAS are strong enough to overcome the downward pressure caused by the straw mulch. In contrast, the use of straw mulch of 9-cm length decreased the number of rice seedlings emerging from the mulch. It is possible that the longer straw mulch (i.e., 9 cm), may simply be too heavy to allow rice seedlings to push through it and emerge. Similarly, in transplanted rice, the straw powder and 1~7-cm straw length treatments had no significant effect on any of the rice parameters measured. However, when 9-cm long mulch straw was applied, the height of rice plants was significantly reduced. This negative effect of the mulch on rice plant height may be due to leaf damage caused by the longer straw mulch, the effect of which gradually declines with the emergence of new leaves over time. Importantly, our field trial results confirmed that oilseed rape straw mulch (700 or 900 g m −2 ) of 1-cm length was safe to use in direct-seeded and transplanted rice.
The effect of straw length on weed suppression showed that 1~3 cm straw is suitable for the management of E. crus-galli and L. prostrata, while 1~5 cm straw is suitable for the management of L. chinensis, A. auriculata, and C. difformis. Interestingly, weed suppression was reduced when the straw mulch length is too short (i.e., powder) or too long (e.g., 9 cm). This suggests that a straw layer of intermediate length probably allows greater straw overlap and coverage of the soil surface, thus more effectively shading weeds from sunlight and also physically suppressing their emergence. Weed seedlings need to overcome greater mulch resistance under such conditions, which may also be the reason why the weed suppressive ability of straw powder was relatively lower than intermediate straw lengths. Application of mulch comprised of longer straw (e.g., 9 cm) may allow more soil gaps, which are suitable for weed growth.
The efficacy of the straw mulch to suppress the monocot, E. crus-galli, was not as good as for the other weeds tested in our trials. This finding may be due to the early growth characteristics of the different weeds used. We found that the germination of E. crus-galli was relatively fast, with germination beginning within two days of sowing. Moreover, the young seedlings have needle-like narrow leaves with a small incline angle [28]. Thus, seedlings of this grass weed can germinate and grow for a few days prior to the application of the mulch and, therefore, likely "escape" to some degree the straw mulch or are strong enough to overcome the suppressive effect of the straw layer. In contrast, seedlings of L. chinensis are very short and its first two leaves are relatively flat and held horizontally [29], thus making it difficult for seedlings of this weed to push through the straw mulch. Although seedlings of the sedge, C. difformis, are also needle-like, its stem is relatively thin. Hence, seedlings of this weed species would have more difficulty penetrating through a mulch layer than E. crus-galli. Our field trial results confirmed that oilseed rape straw mulch (700 or 900 g m −2 ) of 1cm length can effectively suppress troublesome paddy weeds. Certainly, factors such as the precise timing of mulch application, the amount of straw applied, and water level, all affect rice growth, productivity, and weed suppression. Further research is needed to better understand how these factors alone and in combination affect crop growth and weed suppression. This additional research would aid in optimizing management strategies in this cropping system.
There are several possible mechanisms responsible for the strong weed suppression reported in our study. First, we observed that the amount of water absorbed by the oilseed rape 1-cm long dry straw might result in a weight several times that of its original weight, such that the heavy straw layer may form an effective physical barrier against the emergence of weed seedlings. Allelopathic substances released from crop straw may be another important mechanism by which straw mulching may effectively suppress weeds [30]. Numerous secondary metabolites have been reported in oilseed rape straw including isothiocyanate, glucosinolates [31,32], and erucic acid [33] that have allelopathic effects on germination and growth of various weed species [34,35]. Asaduzzaman et al. [36] found oilseed rape cultivars Pak85388-502, JC134, Roy47-99P1, and Av-opal to have strong allelopathic effects on Lolium rigidum Gaudin (rigid ryegrass). Lolium rigidum root biomass was reduced by up to 70% when grown with oilseed rape at a density of 30 plants pot −1 . Similarly, oilseed rape straw extract from cultivar Huiyou 50 (1 g straw/20 ml water) resulted in complete suppression of germination in L. chinensis. The straw of this cultivar includes primarily organic acids, phenols, alkaloids, and neutral substances with these compounds decreasing seed germination levels in L. chinensis by 100%, 10.5%, 5.6% and 12.9%, respectively [33]. Although not directly assessed in our study, differences in weed suppression of the oilseed rape straw mulch might have been due to differences in the type and concentration of allelochemicals present in the straw. Future research should focus on identifying the specific allelopathic compounds and their concentrations present in oilseed rape straw as well as their biological activity.
In a rapidly changing agricultural landscape where sustainable, economical, and environmentally acceptable management practices are sought, the increased use of oilseed rape straw in rice cropping systems provides significant benefits in all these areas. Currently, there is a large overlap in the area planted to oilseed rape and the area planted to rice in China, especially since the oilseed rape-rice rotation is a traditional cropping system in the country. To our knowledge, this is the first report on the use of oilseed rape straw from a previous crop to manage weeds in a subsequent paddy rice crop. The appropriate use of rape straw mulch in rice cropping systems is likely to have the added benefit of effectively controlling herbicide-resistant weeds and of markedly decreasing herbicide use. Moreover, decomposition of the straw in rice systems leads to several soil health benefits including reductions in soil bulk density, increases in soil organic matter content, and decreases in fertilizer use. The use of oilseed rape straw as a mulch holds great promise as both an effective weed management tactic and contributor to soil health, thus allowing more sustainable and profitable rice production.

Conclusions
Oilseed rape mulch (900 g m −2 ) from straw 1-to-7 cm long was found to be safe for rice, when applied four days after sowing (DAS) in direct-seeded rice or four-to-seven days after transplanting (DAT). Straw of 1~3 cm length is suitable for the management of Echinochloa crus-galli and Ludwigia prostrata, while 1~5 cm long straw is suitable for the management of Leptochloa chinensis, Ammannia auriculata, and Cyperus difformis. Weed suppression was as effective as the standard herbicide (oxadiargyl + butachlor) treatment.

Conflicts of Interest:
The authors declare no conflict of interest.