Turning down the irrigation spigot

Partial irrigation can be defined as any irrigation strategy that does not meet the full evapotranspiration demand of a crop. Evapotranspiration is simply the total water use of the crop through evaporation and transpiration. Partial irrigation is dictated in some cases due to water shortages, water right restrictions, irrigation system failures, or other related reasons. It might also be a strategic choice in some cases for income through water leasing (or “water banks”), or where conservation payments or program incentives support cutting back on irrigation.

Whether intentional or not, partial irrigation will often cause yield, quality, and profit loss. Partial irrigation strategies with varying effectiveness can be employed in many cases to maximize production and profit. These strategies can include modifying irrigation schedules, concentrating irrigation to critical crop growth stages, reducing rates, terminating irrigation early, or improving irrigation efficiency in a variety of ways. The crop of interest will also impact best partial irrigation practices. General considerations for partial irrigation of alfalfa, small grain forage, corn silage, and teff grass hay will be discussed here and followed by recent research results.

Alfalfa uses water most efficiently during spring growth, so irrigation should be targeted earlier rather than later in the season, if possible. The first cutting will use about 5 acre-inches of evapotranspiration per ton of alfalfa, compared to about 7 acre-inches for later cuttings. Plus, the first cutting is usually the highest yielding crop. Initially, target alfalfa partial irrigation to the most productive alfalfa land — areas with the best soils, best irrigation efficiency, best alfalfa stem density, least weeds, youngest alfalfa, and/or the best varieties.

Small grains are early users
Small grains such as wheat, barley, and oats are good annual forage crops during a drought because their water use is early in the season when greater precipitation and spring runoff generally occur. Forage harvest generally occurs in June or July and requires no late water, except for late planting or double cropping scenarios.

Small grain forages require slightly less total irrigation than small grains harvested for grain, but they also have less opportunity for partial irrigation than grain. Vegetative growth of small grains requires adequate irrigation and is crucial for producing high forage yields.

Target corn growth stages
Expected water supply and timing of availability should be considered before planting corn to ensure the plants can reach maturity. It is a large investment to plant corn, and partial irrigation strategies will be of little value if no grain is produced to enhance corn silage yield and quality. If water availability is uncertain or water may not be available through the whole growing season, consider other cropping options.

When planting corn, selecting appropriate hybrids is important. For example, shorter day corn that matures quicker can be planted in cases where water may not be available toward the end of the growing season. Target partial irrigation to critical growth stages where near full irrigation levels are crucial to yield determination. These include tasseling, silk, and yield formation. The best time to water stress corn, if needed, is generally during the vegetative and ripening stages. Since grain is such a vital part of good quality corn silage, strategies for partial irrigation of corn silage are usually similar to those for partially irrigating corn grain.

Few partial irrigation studies have been published for teff, but recent research in the Pacific Northwest showed teff required about half the amount of irrigation that alfalfa requires, and that excessive irrigation decreased forage quality. Given the lower irrigation requirement of teff compared to other forages, it may not require partial irrigation. If partial irrigation is desired, the best strategies may include planting in mid-July and only harvesting once and/or increasing irrigation frequency and reducing the rate such that less total water is used than for other forages.

And the research says . . .
A team of researchers at Utah State University started testing partial irrigation strategies for major forage crops in 2019. We evaluated irrigating corn silage at a site near Logan, Utah, with full irrigation (as close to potential evapotranspiration as possible) compared to a 25%, 50%, and a targeted 50% reduction in total irrigation rates for the growing season. This was accomplished by adjusting sprinkler nozzle sizes on a linear irrigation system.
The 50% targeted rate applied half the total irrigation for the season, but this irrigation was targeted toward critical growth stages such as during corn tasseling or mid- to late-vegetative growth for other forages. This work was repeated and expanded in 2020 to include sites in Logan and Vernal, Utah. Logan included a new alfalfa stand with two cuttings and corn silage, while Vernal included spring three-way forage (wheat, oat, and barley), teff, and corn silage (see figure).

Alfalfa: Two cuttings of alfalfa were harvested in 2020 in Logan. A total of 13.5 inches of irrigation was applied for both cuttings for the full irrigation rate. When irrigation was reduced by 25%, alfalfa yield was reduced by an average of about 0.75 tons per acre (20%) total for the two cuts. A 50% reduction in irrigation produced an average of 30% less yield (1.15 tons per acre). Results were mixed for the 50% targeted irrigation. It was much worse than the straight 50% reduction for the first cutting but much better in the second cutting.

Small grain forage: The full irrigation rate for the three-way small grain forage in Vernal was 15 inches. This irrigation rate produced a yield of 4.2 tons per acre. All three partial irrigation strategies lowered yield by an average of 1.5 tons per acre (35%). These results show that the two 50% irrigation reductions were equal to the 25% reduction, which signified that the larger reduction of 50% can be made without losing extra forage yield and that targeting the 50% reduction provided no benefit.

Corn silage: Corn silage was evaluated in three growing seasons at two sites. The 25% reduction in irrigation rate always produced the same yield as the full irrigation rate in all three conditions. This means that a small reduction in irrigation all season might be ideal for partial irrigation. The 50% irrigation reduction was not as consistent. In Logan, only the targeted 50% irrigation reduction lowered yield in 2019 by an average of 2 tons per acre (9% loss). The next year, results were similar for Logan and Vernal, where both 50% reductions in irrigation reduced silage yields by 3 to 9 tons per acre (7% to 23% loss). The targeted 50% rate was never superior to the flat 50% reduction.

Teff: Three teff varieties were evaluated for their response to partial irrigation in Vernal in 2020. The full irrigation rate was 12 inches. The 25% reduction in irrigation rate decreased the teff yield of only one variety (Dessie) by 1 ton per acre (27% loss). The 50% reductions always lowered yield by 17% to 42% for all three varieties. These results demonstrated that some varieties can handle partial irrigation better than others.

Though preliminary, our results to date have shown that 25% reductions in irrigation of forages result in zero to 35% loss in crop yield, depending on the crop. These 25% irrigation reductions were usually but not always superior to 50% reductions, and corn handled this reduction better than other crops. We found no advantages of targeting 50% reductions in irrigation rates compared to a flat 50% reduction all season. Both 50% reductions decreased yield by 7% to 35% across crops. As more data is collected, these recommendations will be refined to further guide best practices for partial irrigation of forages.