Evaluation of forage production potential in the sorghum (Sorghum bicolor L.) and red clover (Trifolium pratense L.) intercropping systems under drought stress

According to studies, the agricultural sector is the largest consumer of water and in this sector, paying attention to optimal water consumption is very important. On the other hand, climate change and the spread of environmental stresses in recent years have reduced crop yields; therefore, the need to identify appropriate solutions to deal with such situations is fully felt. Restoring diversity to agricultural ecosystems and its effective management is recognized as one of the important strategies in sustainable agriculture. Mixed cropping as a sample of sustainable systems in agriculture pursues goals such as creating ecological balance, greater utilization of resources, and increasing crop yields. The most important benefit of intercropping systems compared to monoculture systems is the increase in production per unit of area, which of course has not always been achievable and this advantage is achieved only when the plants that make up the mixture are completely different in terms of how and how much to use growth factors (water, light, and nutrients). Therefore, when plants with different morphological characteristics are grown in a mixture next to each other, they can make optimal use of environmental factors and as a result, their total yield increases per unit of area. The present study aimed to evaluate the forage production potential in the replacement and additive intercropping systems of sorghum and red clover under different irrigation regimes.

This experiment was conducted as split plots based on a randomized complete block design with three repetitions during the 2017 and 2018 cropping seasons at the Research Farm of Damavand Natural Resources Department. The main factor was the irrigation regime at three levels including irrigation with 100% field capacity (full irrigation as control), irrigation with 75% field capacity (moderate stress), and irrigation with 50% field capacity (severe stress). Different cropping systems were considered as sub-factors, including 75% sorghum + 25% clover, 50% sorghum + 50% clover, and 25% sorghum + 75% clover as replacement intercropping systems; 100% sorghum + 50% clover, 50% sorghum + 100% clover, and 100% sorghum + 100% clover as additive intercropping systems, and the monocultures of sorghum and red clover as control. Each experimental plot consisted of six planting rows with a length of six meters with a distance between the lines of 60 cm. In order to prevent water leakage to adjacent plots, the distance between the main plots was four planting lines. In this experiment, forage sorghum of the Speedfeed cultivar and red clover of the Nassim cultivar were used. For planting sorghum and clover in monoculture treatments, 15 and 25 kg of seeds per hectare were used respectively. Also, for planting intercropping treatments based on mixing ratios of 25, 50, and 75%, 3.75, 7.50, and 11.25 kg of sorghum seeds and 6.25, 12.50, and 18.75 kg of clover seeds per hectare were consumed respectively. Experimental treatments were irrigated by the furrow method. The irrigation cycle was adjusted based on draining 40% of available water in the root zone under full irrigation conditions. Irrigation water depth in full irrigation treatment was determined based on the soil moisture deficiency (relative to the point of field capacity) at the time of irrigation through sampling. Irrigation water depths in moderate and severe stress treatments were considered based on 75% and 50% of full irrigation water depth, respectively. It should be noted that after the establishment of sorghum plants, thinning operations were performed and its density was adjusted based on 20 plants per square meter. In this experiment, the planting pattern was in rows and the change of mixing ratios was applied based on the change in the number of planting rows of sorghum and clover. In monocultures and replacement intercropping systems, planting operations were performed in the middle of the ridges, while in additive intercropping treatments, a two-row pattern was used and each species was planted on one side of the ridges.

The results of variance analysis showed that the effect of year, irrigation regime, and cropping system and the interaction of irrigation regime × cropping system on fresh and dry forage yield of clover, sorghum, and total yield were significant. The highest fresh and dry forage yields (65.169 and 14.059 ton ha-1, respectively) were obtained in the additive intercropping system of 100% sorghum + 100% clover under the full irrigation regime, whereas the minimum fresh and dry forage yields (4.191 and 0.920 ton ha-1, respectively) were recorded in clover monoculture under severe drought stress. Under moderate and severe drought stress, the maximum fresh and dry forage yields were obtained in sorghum monoculture and then in the additive intercropping system of 100% sorghum + 100% clover. Furthermore, the effect of the cropping system and the interaction of irrigation regime × cropping system on the land equivalent ratio (LER) for fresh and dry forage production were significant. The highest land equivalent ratio for fresh and dry forage production (1.719 and 1.723, respectively) was obtained in the additive intercropping system of 100% sorghum + 100% clover under full irrigation, whereas the lowest land equivalent ratio for fresh and dry forage production (1.024 and 1.022, respectively) was recorded in replacement intercropping system of 25% clover + 75% sorghum under full irrigation.

According to the results of this study, the additive intercropping system of 100% sorghum + 100% clover can be recommended as the superior treatment in all irrigation regimes, whereas sorghum monoculture was suitable only in moderate and severe drought stress regimes.