Effects of different plant growth retardants and herbicides on the yield and interspecific competition of the alfalfa/silage corn relay intercropping system

To explore the feasibility of reducing the competition between alfalfa (Medicago sativa) and silage corn (Zea mays) seedlings in an alfalfa-silage corn relay intercropping system, three plant growth retardants (prohexadione-calcium, paclobutrazol, and mepiquat chloride), and two herbicides for corn fields (a compound herbicide of butachlor, propisochlor, and atrazine; and metolachlor) were applied after corn interseeding in 2018–2019, and their effects on suppressing alfalfa regeneration, improving corn growth, and system productivity were analyzed. No interactive effects were found between the plant growth retardants and herbicides. The plant growth retardants had no significant inhibitory effects on alfalfa yield, and there was no influence on alfalfa, corn, or system yields, except that the paclobutrazol improved the alfalfa yield during the coexisting period in 2019. The compound herbicide of butachlor, propisochlor, and atrazine significantly suppressed alfalfa regrowth (P < 0.05), and lowered the alfalfa plant height, SPAD value, and the yield during the coexisting period. It improved the corn yield but also significantly reduced the alfalfa yield of the next spring as well as the system total yield and total food equivalent unit in 2019 (P < 0.05). Metolachlor had no effect on the yield of both corn and alfalfa (P > 0.05) but showed a trend of improving alfalfa yield in the spring, system yield, and the total food equivalent unit. None of the plant growth retardants or herbicides used in the study prevented the alfalfa growth without negative effects on system productivity, and none reduced the competition between alfalfa and the intercropped corn.