Extended Abstract
Introduction: Cultivation of two or more crop species concurrently, leading to their coexistence for a significant part of their growth cycle, is known as intercropping. The intercropped species may neither seeded at the same time nor harvested simultaneously, but they grow together in the field for a major portion of the growth periods. Increase in final yield, greater efficiency in using resources, conversation of resources, improved soil health, and in the meantime decrease in pests population and application of plant protection chemicals and hence improved sustainability of agricultural production are the most significant benefits of intercropping. Intercropping research has been mainly focused on cereals and legumes intercropping in tropical and subtropical regions; less attention has been given to research on intercropping of vegetable crops. Publications around intercropping of onion and lettuce in Iran are scarce. Thus, this research was conducted to determine the best planting pattern and plant density in intercropping of onion and lettuce towards achieving the highest yield and monetary benefits.
Materials and Methods:This research was conducted from 2018 to 2020 in the Research Farm of Behbahan Agricultural Research Station, Behbahan, southwest of Iran. The experiment was set up in randomized complete block design with 14 treatments and three replications. The treatments included, sole crop of onion and lettuce, three planting patterns of intercropping: 75% onion and 25% lettuce, 50% onion and 50% lettuce as well as 25% onion and 75% lettuce, in four planting densities (distance between onion plants were 5 and 7 cm and between lettuce plants were 20 and 30 cm). Before planting, soil in experimental plots was plowed and nitrogen, phosphorus, and potassium fertilizers were applied based on results of soil analysis and recommendation of Soil and Water Institute of Iran. All plots received the equivalent of 46 kg P ha-1 as triple super phosphate and 100 kg K ha-1 in form of potassium sulfate during soil preparation. Nitrogen was applied at a rate of 120 and 130 kg N ha-1 as urea for onion and lettuce, respectively.  Seeds of onion and lettuce were sown in nursery simultaneously in mid-October. Seedlings of onion and lettuce were transplanted into the plots in late November and late December, respectively. Lettuce plants were harvested in early March and onions were harvested in mid-May. Data gathered for different traits in 2 years were combined and subjected to a combined analysis of variances using SPSS software. Significant differences among treatments means were determined at 0.05 probability level using Duncan's test. 
Results and Discussion: The onion and lettuce yield/ha in sole cropping were higher in comparison to intercropping. The effect of lettuce plant density in different planting patterns was not significant on onion bulb weight. By increasing lettuce planting density, the mean lettuce plant weight decreased in all planting patterns. By increasing the proportion and planting density of onion and lettuce in intercropping the mentioned differences in yield of these crops in intercropping and sole cropping decreased. Yield of intercropping using 75% onion and 25% lettuce and 50% onion and 50% lettuce with the planting distances of 5-30, 5-20, and 7-20 cm and 25% onion and 75% lettuce with the planting distances of 5-20 and 7-20 cm were significantly higher than yield of onion and lettuce in monocultures. Given the notable differences between morphological attributes and growth stages of onion and lettuce, the increased yield of the intercropping stemmed from an enhanced resource use, compared to the monoculture of these vegetable species. The highest final yield was achieved from 50% onion and 50% lettuce intercropping with the planting distances of 5-20 cm. In all intercropping treatments, both land equivalent ratio and total relative value were greater than unity and monetary advantage index was positive, confirming the feasibility of intercropping in terms of resource use and economic advantages. 
Conclusions:Our results suggested that the intercropping of 50% onion and 50% lettuce with the planting distances of 5-20 cm leads to the highest  yield, land equivalent ratio, total relative value, and monetary advantage indices, and hence it may be practiced by the farmers in the Behbahan region.