Heavy Soil Drying And Skip-row Planting Of Main Crop For Increasing The Grain Yield And Quality Of Ratoon Crop In Mechanized Rice Ratooning System

Ratoon rice,the production of a second crop（ratoon crop）from the stubbles left after harvesting the first crop（main crop）,is a promising cropping system to boost total productivity via increasing harvest frequency.Along with the reduced labor force and increased labor wage resulting from urban expansion,mechanical harvesting(H_MEC)has been developed as an alternative to manual harvesting for the main crop(H_MAN)in ratoon rice production of China.However,there are often decreases in grain yield and milling quality in the ratoon crop caused by crushing damage to stubbles during H_MEC,limiting the further extension of mechanized rice ratooning technology.Since the specialized harvest machines for ratoon rice production has not been commercially available at present,it is urgent to develop crop management strategies that can reduce the crushing damage during the harvest of the main crop using conventional machines.One of the common practices is to drain the paddy field before the harvest of the main crop for soil drying,so that the combines can move easily in the field for the harvest.But limited information is available regarding the effects of soil drying during the grain filling of the main crop on the grain yield and grain quality of the ratoon crop.In addition,skip-row planting（SP）,where the track zone is left unplanted in the main crop to provide traffic paths for harvest machines,might be effective to facilitate H_MEC so that the tracks do not damage plants.However,it is remains unclear if border effects could compensate for the yield loss due to decreased planting density from SP in both the main and ratoon crops,and if SP could improve the milling quality of the ratoon crop.To address these questions,field experiments were conducted on ratoon rice in Qichun County,Huanggang City,Hubei Province,China with an indica hybrid（Liangyou6326）and an indica/japonica hybrid（Yongyou4949）as the experimental materials from 2017 to2019.In 2017 and 2018,two soil drying treatments of the main crop（light soil drying（LD）and heavy soil drying（HD））and two harvest methods of the main crop(H_MAN and H_MEC)were designed in the experiments.The objectives were to determine whether heavy soil drying during the mid-to-late grain filling stage of the main crop can reduce the decreases in grain yield and quality of the ratoon crop from H_MEC,and to identify the underlying agronomical mechanisms of heavy soil drying for alleviating the negative effects of H_MECon grain yield and quality of the ratoon crop.In 2018 and 2019,a SP pattern with 33.3%decrease in planting density was compared to conventional planting（CP）pattern in the field experiments.The objective was to determine the effects of SP on grain yield and quality of the main and ratoon crops in mechanized rice ratooning system.The main results of our study are as follows:（1）Compared with LD,soil hardness of the topsoil（0-20 cm layer）was increased by42.9%-85.3%in HD at the maturity of the main crop.Averaged across the two years,soil hardness at the depths of 5 and 10 cm reached 4.05 and 7.07 kg cm^-2 in HD,respectively.Soil drying treatment did not affect the grain yield of the main crop significantly.Under H_MEC,HD increased the grain yield of the ratoon crop by 9.4%compared with LD.Consequently,the yield loss in the ratoon crop from H_MEC relative to H_MAN was 16.3%in LD,in contrast to only 3.4%in HD.The differences in grain yield and yield loss between the two soil drying treatments were explained by panicles m^-2,spikelets m^-2,and total dry weight,which were associated with the differences in number of regenerated buds,leaf area index,and post-heading dry matter production of the track zone between LD and HD.（2）Compared with CP,the grain yield of the main and ratoon crops in SP was reduced by 4.8%and increased by 4.6%,respectively.The large decrease in planting density from SP did not affect grain yield substantially because of the border effects on grain yield in the rows next to traffic paths in both crops.The border effects on grain yield were 38.9%-89.9%and 43.9%-74.0%in the main and ratoon crops,respectively.Panicle number,spikelet number panicle^-1,and total dry weight were responsible for the border effects on grain yield in the main crop,whereas panicle number and total dry weight were responsible for the border effects on grain yield in the ratoon crop.The border effects in the two crops were associated with number of tillers or regenerated buds,leaf area,post-heading dry matter accumulation and aboveground total nitrogen uptake.（3）Soil drying treatment did not affect the milling quality of the main crop significantly.Under H_MEC,HD increased the head rice rate of the ratoon crop by 2.4percentage point,and increased the head rice yield of the ratoon crop by 12.8%,as compared with LD.Consequently,the decreases in head rice yield in the ratoon crop from H_MEC relative to H_MAN was 20.8%in LD,in contrast to only 6.4%in HD.Compared with LD,better milling quality in HD was results of the improvements in head rice rate and head rice yield of the track zone.（4）Planting pattern did not affect the milling quality of the main crop significantly.Compared with CP,the head rice rate of the ratoon crop in SP was increased by 2.8percentage point,and the head rice yield of the ratoon crop in SP was increased by 9.2%.The poor milling quality in CP was results of the decreases in head rice rate and head rice yield of the track zone compared with SP.In summary,heavy soil drying practice during the mid-to-late grain filling stage of the main crop is effective to reduce the decreases in grain yield and milling quality of the ratoon crop in mechanized rice ratooning system by improving plant development and growth of the track zone.In addition,planting with a skip-row arrangement is effective for improving the milling quality of the ratoon crop in mechanized rice ratooning system by avoiding the crushing damage,while maintaining high grain yield of the two crops due to the border effects.Therefore,these two practices can be considered as important crop management strategies that promoting the sustainable development of mechanized ratoon rice.