Deficit Irrigation, or Irrigating with Limited Water
Dwindling surface and groundwater supplies lead to concern about the amount of water that is used to irrigate crops. Thus, crop producers who irrigate are expected to find ways to reduce the amount of water they use to irrigate crops. Never mind that this will necessarily lead to a lower yield from these irrigated crops, or that they may not be planted at all. So far, reduced food production has not been a part of this conversation.
A concept that is taking root among producers faced with this dilemma is deficit irrigation, or using an amount of irrigation water that is less than that needed to maximize yield from the crop being irrigated. Irrigating soybeans with a limited water supply is explained in an article titled “Seasonal water needs and opportunities for limited irrigation for Colorado crops” from Colorado State Univ. It likely is what the future will be if irrigation and crop management practices used for water conservation are not widely adopted or are not successful on a wide scale in the Delta and other U.S. regions where crop irrigation is widely used.
Below are some points for soybean producers to consider if irrigation water becomes limited.
• In cases where only a limited amount of irrigation water is available, and it is not enough for full reproductive phase irrigation of soybeans, it can be allocated for use during early reproductive development to establish maximum number of seeds, or to the latter stages of reproductive development to maximize weight of seeds.
• Neither of the above options will produce the maximum yield that may be required to maximize net returns unless adequate rain is received during periods of no irrigation.
• The use of limited irrigation in early soybean reproductive phases [R1 to R3+] can ensure maximum number of pods, and is advantageous if rains are received during the latter stages of reproductive development.
• The use of limited irrigation only during the seedfill period [R5-R6.5] can be advantageous for ensuring maximum seed weight, but this approach assumes that adequate rain was received during early reproductive phases to achieve adequate podset and number of seeds.
• The probability of receiving rain during early reproductive development is greater than that during the seedfill period [mid-July through August in ESPS plantings]. Thus, in cases with limited irrigation water, irrigation during later soybean reproductive phases appears to provide the greatest probability for maximizing yield using this approach. This appears less risky than irrigating earlier and depending on infrequent late-season rain to enlarge the high number of seed that were set as a result of irrigation during early soybean reproductive phases. This approach assumes that a reasonably high number of pod sites were formed in the absence of irrigation during early reproductive development.
• Limited irrigation will be a most effective water conservation measure if the water used for it is not withdrawn from underground aquifers, which are becoming depleted. The subject of using water that is captured on-farm is covered in a Southeast FarmPress article titled “Using holding ponds for deficit irrigation can work”.
Midsouth soybean producers should be prepared for the time when water for irrigation is limited by law or supply, and this time is likely in the not-too-distant future. Thus, they are encouraged to devote a small acreage of irrigated soybean [preferrably a single small field] to using both of the scenarios outlined above. They can do this by using each method on one-half of the selected field for a period of time that is deemed long enough [multiple years?] to determine the likely best approach to use to optimize yield and profit. They will then be prepared to use a limited amount of irrigation water to achieve the best outcome from deficit or limited irrigation of soybeans. Also, research to investigate this approach to irrigation should be conducted in U.S. regions where irrigated agriculture is practiced.
Composed by Larry G. Heatherly, Sep. 2024, larryh91746@gmail.com