Study On Effects Of Topdressing After Cutting On Alfalfa In The Longdong Loess Plateau Of China

Fertilization is one of the important management measures in alfalfa production,but the effect and management plan of topdressing after cutting is not clear.A two-year field trial was conducted with alfalfa（Medicago sativa L.cv.Longdong）in Longdong Loess Plateau of China.Nitrogen fertilizer at 0（N0）,25（N25）,50 kg N/ha（N50）and/or phosphorus fertilizer at 0（P0）,30（P30）,60 kg P₂O₅/ha（P60）were applied on the day of cutting（T0）and 7 d after cutting（T1）.The aim of this study was to investigate the effects of topdressing time and combination of topdressing after cutting on the growth,yield,quality,nutrient utilization of alfalfa and soil nutrient content.The main findings were as follows.1)The time of topdressing after cutting significantly affected the growth of alfalfa in the establishment year,and the effect of immediate topdressing（T0）was more obvious.However,the effect on stem diameter and leaf traits was weakened in the second year,and there was no significant effect on plant height,coverage and photosynthetic characteristics.The combination of N and P fertilizer could promote the growth more than single application.In the establishment year,the plant height under topdressing immediately after cutting was generally higher than that 7 d after cutting（T1）.Topdressing time had no effect on the plant height in the second year,but N and P fertilizer had a significant effect on the plant height,and the corresponding treatments of different cuts were T0N50P60,T1N25P30 and T1N50P30,respectively.Topdressing time and N fertilizer after cutting had significant effects on the stem diameter of the first cut,which was significantly increased by 18.21%under T0N25P30 compared with the control.In the second year,the specific leaf weight and photosynthetic performance of alfalfa were not significantly affected by topdressing after cutting.The leaf thickness of the first and second cuts of alfalfa under T0N25P30 treatment was significantly increased by 12.57%and 22.08%compared with the control.However,the effect of topdressing time on the leaf thickness and leaf area of alfalfa was weakened with the increase of cutting times.After cutting,topdressing of N and P fertilizer significantly affected the growth of alfalfa roots in the year of establishment,and root surface area and volume were significantly increased under N25P30 treatment.But had no significant effect in the second year.2)High nitrogen and phosphorus（N50P60）could improve hay yield more than low N and P,and T0 was better than T1.Topdressing P significantly affected the producti performance of alfalfa at each cut,and the effect of topdressing high P（P60）was better,while the effects of topdressing time and nitrogen fertilizer gradually weakened.In the year of establishment,the yield of hay and crude protein was the highest under T0N50P60 immediately after cutting,which was 3.58 t/ha and 0.94 t/ha,and the effect of T0 was better than T1.In the second year,the effects of topdressing P fertilizer on the yield of fresh biomass,hay and crude protein were significant,while the effects of topdressing time and N fertilizer were gradually weakened.The annual fresh biomass yield of T1N25P60 treatment was the highest,reaching 73.97 t/ha,but the annual crude protein yield of T0N25P60 treatment was the highest.Topdressing time had a significant effect on the total nitrogen content of the first cut of alfalfa in the second year,T0 was better than T1,and then the effect weakened.However,topdressing N and P could significantly affect the total phosphorus content of the first and second cut of alfalfa.T0N25P30 treatment significantly increased the total phosphorus content of the first and second cut of alfalfa by 15.09%and 10.95%.The supplementation of high P（P60）was more beneficial to the absorption and utilization of P at the third cut and the whole year.However,the washing fiber content of the first cut of alfalfa in the second year was significantly decreased.The effects of topdressing time and nitrogen fertilizer were gradually weakened.The comprehensive efficiency of alfalfa was improved obviously by TOPSIS and dry matter yield analysis.The ideal topdressing method was T0N25P30 in the establishment year,but the effect of topdressing time was not significant in the second year.High P（P60）could significantly improve the quality comprehensive Score of alfalfa at each cut,and the effect of combined application of N fertilizer was better.3)Topdressing immediately after cutting was more beneficial to maintaining soil nutrient supply,and fertilizer utilization rate was significantly increased under low amount topdressing level.In the second year,the nitrogen content in 50 cm soil layer continued to decrease,and adding high nitrogen immediately after cutting was more beneficial to maintain soil nitrogen level.Topdressing after cutting increased the contents of nitrate nitrogen and available phosphorus rapidly,especially at T0,but the contents of soil nutrients below50 cm had little change.In the second year,the partial productivity of alfalfa fertilizer was significantly higher at low topdressing level（P30,N25）than at high topdressing level（P60,N50）.However,there was no significant difference in the agricultural efficiency of P fertilizer under high P and low P conditions when combined with N fertilizer.In conclusion,topdressing after cutting could effectively promote the regeneration,increase yield and improve feeding quality.It would be more efficient when the topdressing was applied immediately after cutting and when N and P fertilizers were applied together.In alfalfa production on Longdong Loess Plateau,it is recommended that 25 kg N/ha and 30 kg P₂O₅/ha should be applied immediately after cutting in the establishment year,and in the second year,25 kg N/ha and 60 kg P₂O₅/ha should be applied immediately after cutting.