Key Takeaways 

• Check soil moisture first — Fields may have more stored water than expected. 

• Pre-irrigate only when needed — Use pre-irrigation only if the drill cannot place seed correctly. 

• Irrigate after planting in dry soils — Post-planting irrigation can help ensure strong emergence. 

• Expect rapid water use — Higher seeding rates and faster canopy closure will increase water demand quickly. 

• Use full-canopy scheduling principles — After full canopy, most crops use similar amounts of water each day. 

• Prioritize vegetative growth — Fully irrigate during vegetative stages to maximize forage yield.

This year, hay and grass in pastures are in short supply across much of Nebraska, and many farmers are turning to annual forage crops under their center pivot to fill the gap. The big question on everyone’s mind is straightforward: We know how to irrigate corn and soybean, but how do we irrigate annual forages in dry years using the least amount of water while still getting a good stand and strong yields?

The good news is that with the right strategy, annual forages can be established successfully and produce excellent tonnage — even when irrigation water is limited.

Fields May Have More Water Than You Think

One of the first steps is to evaluate how much water is already in the soil profile. Fields that were irrigated last year — especially those watered late into the season — often start the new year with more stored soil moisture than expected. This stored water can significantly reduce the amount needed from irrigation.

Soil type plays a big part in the amount of water that may be stored in the root zone. Silt loam soil may have over 6 inches, while sandy soil may have only about half that much. Sandy soil surfaces also dry out quicker, which can cause germination problems with the small seeded forages that are planted at an inch or less deep in the summer months. Corn is usually planted about 2 inches deep in the spring when the days are cooler and less drying than in June, when many summer annuals are planted.

A soil probe or shovel is the best tool for checking moisture before making any irrigation decisions. Crops use soil water from the surface layer first and then move deeper into the soil profile. When rain or center pivot irrigation rewets the soil, that water starts at the surface and moves downward. The goal is to identify where the soil profile is dry and where it is still moist.

In fields that were reasonably well watered the previous year, the crop may have only used water down to 18–24 inches, leaving moist soil below that depth. Off-season precipitation may also have rewetted 6-12 inches of the upper part of the profile, even in a dry year.

In those situations, a producer might find a few inches of dry soil at the surface, then 5 to 10 inches of moist soil, follow by few more inches of dry before getting to the deeper moist soil. Under those conditions, it may only take about 1 inch of water to refill a sandy soil or about 2 inches to refill a silt loam.

Irrigation for Stand Establishment

Getting a uniform stand is a critical step for annual forage success.

Pre‑Irrigation: Use Only When Necessary

Pre‑irrigation should only be used when the soil is too hard or too dry for the drill to place seed at the correct depth. If the drill can penetrate and place seed properly, pre‑irrigation is usually unnecessary.

Irrigating After Planting

If the drill can plant seed at the correct depth in dry soil, applying water with a center pivot right after seed is in the ground is often a better option than pre-irrigating before seeding from a water efficiency perspective and helps achieve good seed-to-soil contact.

If irrigation is applied to help establish good stands, it is important to:

• Apply enough water to move moisture down to the depth of moist soil or at least 12 inches.
  • This prevents the new seedlings from germinating in 2 or 3 inches of good soil water and then dying from moisture stress. Plant roots cannot grow through dry soil to get to moist soil below.
• Avoid light, shallow applications that only wet the soil surface an inch or two, encouraging weed germination and poor crop rooting.
  • Exception: Sandy soil may require more frequent, lighter irrigation during the seedling establishment stage because the surface dries rapidly. This helps ensure the roots of new seedlings do not dry out. Once the crop is established, return to larger irrigation applications to improve water-use efficiency and encourage good root development. Aim for 0.8- to 1 inch of water per application, or as much as possible while still avoiding runoff.

A single, well‑timed irrigation can dramatically improve emergence and stand uniformity.

Annual Forages Reach Full Canopy Fast

Annual forage crops are planted at much higher seeding rates than corn.Irrigated corn is usually planted at about 30,000-34,000 seeds per acre, whereas many forages are planted at 1-2 million seeds per acre, which means:

• Annual forages increase water use faster in the early growth stages than corn because there are so many more plants.
• Annual forages can reach full canopy in just a few weeks.

Full canopy is defined when the crop has enough leaves to capture essentially all the sunlight (about 90%) at noon. To determine when the crop has reached full canopy, scout the field at noon to see how much of the soil is receiving sunlight. If it is less than 10%, the crop has reached full canopy. Some densely planted annual forages can achieve full canopy when only a few inches tall compared to corn, which needs to be over 4 feet tall.

This rapid canopy development is one reason annual forages can produce so much biomass in a short time — but it also means crop water use increases much faster than for corn.

Full Canopy Water Use: What to Expect

Once annual forages reach full canopy, their daily water use is similar to other full-canopy crops.

On any given day:

• Corn, millet, soybean, sorghum–sudangrass, etc. will all have about the same crop water use.
• Oats and foxtail millet typically run about 10% lower than corn.

This means that once the crop has developed a full canopy, irrigation scheduling can follow the same principles used for corn or soybean. The water needed can range from 0.75-inch to 2.5 inches per week, making it important to choose an irrigation scheduling system.

Nebraska irrigation scheduling uses alfalfa reference crop ET. After annual forages reach full canopy, that reference crop ET can be used directly to estimate crop water use for each day or week. 

Reference crop ET data can be obtained from an ET gage or the High Plains Regional Climate Center (HPRCC). 

To find reference crop ET data on HPRCC, go to the Automated Weather Data Network page and then select: 

• Products: Penman-Monteith Evapotranspiration
• Networks: NEMesonet (Nebraska) 
• Stations: The location closest to your field 

The alfalfa reference ET values will be displayed in the preview area as inch/day. If you want the data in an Excel spreadsheet, click the Clippy SVG Icon to the right of the “Full Web Service Call” box, then paste the link into a browser search box.

For resources to help understand how to schedule irrigation using soil water data, check out Nebraska Extension NebGuide EC3036, “Irrigation Scheduling Strategies When Using Soil Water Data.” If you prefer videos, watch the CropWatch series “How to Schedule Irrigations with Soil Water Data.”

Watering for High Forage Yield: Focus on the Vegetative Stage

To maximize forage yield, annual forages should be fully watered during the vegetative growth stages. This is when the plant is growing stems and leaves — the foundation of total tonnage. When growing corn for silage, producers should keep the crop adequately irrigated during the vegetative stages to maximize vegetative growth and biomass production. However, when growing corn for grain, producers can save some water during the vegetative stage, resulting in a slightly smaller plant while still maintaining maximum grain yield.

Stress from lack of soil water during vegetative growth reduces:

• Plant height
• Leaf area
• Tillering (in grasses)
• Total biomass

Once the crop enters reproductive stages (i.e. begins to put out seedheads), additional irrigation has less impact on total forage yield but will increase the grain yield.

With careful irrigation management, annual forages can be a reliable and productive option for filling forage shortages — especially in a dry year.