Irrigating Corn in Extreme West-Central Kansas

Declines in groundwater levels in this area have sharpened interest in irrigation efficiency. During the last three years we evaluated irrigation amounts and timing influence on corn yields Res_ults that timing Cit arngat1on wtll maantaan corn ytelds and use less water. Our 3-year test was with furrow irrigation on Ulysses silt lo·am. All plots received a preplant irrigation. Each in-season irrigation was approximately 7.5 inches. Application efficiency may range from 50 to 75% for furrow irrigation. Phosphorus was applied uniformly each year. We compared irrigation schedules. usi ng preplant only ·and at three soil-moisture tensions with two nitrogen fertilizer levels, two plant population lev:els, and three commercial corn hybrids. Whe n to irrigate was determined by monitoring soi_l mo.isture tension. Tension is the re lative .difficulty of extracting moisture from the soil. Watering when the soil water tension was 0.8

Declines in groundwater levels in this area have sharpened interest in irrigation efficiency. During the last three years we evaluated irrigation amounts and timing influence on corn yields Res_ults that timing Cit arngat1on wtll maantaan co rn ytelds and use less water.
Our 3-year test was with furrow irrigation on Ulysses silt lo· am. All plots received a preplant irrigation. Each in-season irrigation was approximatel y 7.5 inches. Application efficiency may range from 50 to 75% for furrow irrigation. Phosphorus was applied uniformly each year. We compared irrigation schedules. usi ng preplant only · and at three soil-moisture tensions with two nitrogen fertilizer levels, two plant population lev:els, and three commercial corn hybrids.
Whe n to irrigate was determined by monitoring soi_ l mo. isture t ension. Tension is the re lative .difficulty of extracting moisture from the soil. Watering when the soil water tensio n was 0.8 bar a! 2 feet deep ":'atering an 2.7 t1mes a se·ason: tw1ce m 1973 and f\..... _.'e times in 1972 and 197 4 (Table 1). Watering when soil water tension was 0.8 bar produced the highest yield, 134 bushe[s an acre (Figure l ). When pre-plant irrigation filled the soil profile to six feet, then either two or three in-season irrigations (depending on the year) produced maximum yields. Irrigating up to eight times a yea r produced no higher yields during three years, 1972-197 4. lrrig'atin'g preplant only, produced 85% as much grain as the top yield with 43% as much applied water.
The 0.8 bar treatment required that the first in-season irrigation be applied July 7; later irrigations w ere 22 days apart (3-year average). Irrigating when soil moisture tension was lower required earlier ·and more freque nt irrigations. The 0.6 bar tension required irrigation by June 30 with later irrigations ave raging 16 days apart; the 0.4 bar tension, by June 19 a nd 10 days apart. The intervals b etween irrigations varied with p lant-use and rainfall. An entire fi e ld cannot be watered so precise ly as our plots were; however, our results should be used as a manageme nt guide to timing or scheduling irrigation.
Regardless of other treatments , 160 lbs/ A nitrogen was required a nd resulted in 139 bushels an acre versus 113-bushel average with 80 lbs/ A of nitrogen ( Figure 1 ). So, it wou ld be a mistake to sharply reduce nitrogen applied, anticipating a water shortage. Nitrogen rates might be reduced slightly but our 50% nitrogen reduction cut yields too much.
The effect of plant population d epended on other treatments (Figure 2). The higher population was fa vored where nitrogen w as adequate . However, when nitrogen was short (80 lbs/ A), the high population reduced y ield. That was more apparent when both water and nitrogen were in short supply. Most surprising was that plant population did not affect yields for the pre-plant only treatment. We expected the lower population to be favored under pre-plant irrig·ation only.
The three commercia l hybrids averaged about the same over the three years. They responded differently but their responses va ried from year to year giving no clear-cut conclusions . It is im-  portant to recognize that hybrids react individually to various environmental conditions.
Results of our tests are summarized in more detail in Table 1. Highest yields were from 2 or 3 in-season irrigations with 24,000 plants ar acre and 160 pounds per acre of applied nitrogen. Limited in-season irrigation is most practical when a moderate to large amount of available water is stored before planting.
Soil moisture tension can be monitored by tensiometers or electrica I resistance blocks. Tensiometers, limited to tensions below 0.85 bar, cost about $20 each. Tensiometers used in this test were obtained from Soil Moisture Equipment Corporation, Santa Barbara, California.
Information in this report is for farmers, producers, colleagues, industry cooperators, and other interested persons. It is intended to help in irrigation management. It is not a recommendation but represents three years' research at one location.
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