Pasture and Forage Minute: Alfalfa Irrigation and Last Cutting, Winter Annual Brome Control

Pasture and Forage Minute: Alfalfa Irrigation and Last Cutting, Winter Annual Brome Control
Sep 09, 2022

By Todd Whitney and Jerry Volesky

Late Fall Alfalfa Irrigation

Applause to our Nebraska irrigators. This year’s drought weather patterns and alfalfa growing season have been tracking similar to 2012, so it has been an extremely long irrigation season with very little rainfall relief. However, avoid the temptation to shut off your alfalfa irrigation applications too early.

Even though alfalfa is a relatively drought-tolerant forage, surface moisture is still needed to prevent alfalfa roots from drying and dying over winter. Soil moisture will also help moderate soil temperatures and keep alfalfa plants alive.

Unlike corn and soybeans, alfalfa is a perennial crop, so developing roots can reach soil depths of eight feet or more. Thus, possible over-irrigation in the fall is less of a problem, since alfalfa plants will likely benefit from extra stored soil moisture next growing season without high water leaching losses. Irrigating now through October and into possibly November until soils freeze will potentially protect plants and improve yields.

Irrigation scheduling efficiency can be improved with using ET gages and/or soil moisture monitoring technology. Accurate weather forecasts can also allow growers to take full advantage of possible rainfall and reduce irrigation applications.

Daily water use drops into the fall. However, each cutting typically requires six to seven inches of irrigation. Peak water usage is about one-third-inch per day in July and August, but may lower to less than one-fourth-inch per day in late fall due to cooler days.

More alfalfa irrigation details are on our UNL Water website. Our NebGuide, G1778, “Irrigation Management and Crop Characteristics of Alfalfa” is a free, useful resource.

Control of Winter Annual Bromes

Was cheatgrass — sometimes called downy brome — or wild oats abundant in your pastures this spring? Although difficult, they can be controlled and your pasture revitalized.

Winter annual bromes often invade thin or overgrazed pastures in fall and early spring. Livestock dislike grazing them after they become mature, and over time they can take over and make large patches in a pasture.

Research by the University of Nebraska evaluated herbicides for controlling cheatgrass. Products containing rimsulfuron and imazapic (Plateau®) can provide good control of cheatgrass from a single application, but control can vary widely from year to year depending on when the application is made, maturity of cheatgrass plants, and the weather patterns. A one-time management operation utilizing grazing, mowing or a non-residual herbicide, like rimsulfuron or imazapic, usually does not have a lasting impact on the cheatgrass in the seedbank. To reduce the seedbank, control needs to be close to 100% and repeated over several years.

Rejuvra™ is a new rangeland herbicide product from Bayer that works differently from existing cheatgrass herbicides. Rejuvra™ has limited activity on emerged plants and only controls seedlings as they germinate, so it is best to apply Rejuvra™ in early fall before seeds germinate. This herbicide can provide control up to two years post-application.

In warm-season grass pastures, there is another option. You can use glyphosate herbicides after top growth of these grasses has died due to a hard freeze or two. This can kill emerged annual brome seedlings without harming the desirable grasses. However, do not use glyphosate in cool-season pastures because it will injure or kill the pasture grasses as well. With any herbicide, always read and follow label directions.

With these herbicide options and proper grazing management, your pastures can develop thicker stands of the more desirable grasses.  

Alfalfa Last Cutting

Has the decision been made for when the last cutting of alfalfa will happen? This year it might not be an easy decision.

September is here and the dry conditions continue in much of Nebraska. When alfalfa is cut for the last time in the fall, it affects winter survival as well as the spring regrowth. As long as it is cut at the right time, the effects won’t be bad. Alfalfa needs 500 growing degree days or approximately six weeks of uninterrupted growth in the fall to fully prepare for winter by building up nutrients in the roots. This typically means that the beginning of the six weeks of growth will be about three weeks before the first frost. 

The last cutting can either be before the winterization process or after. If cut during, it adds more stress to the alfalfa. During stressful years for alfalfa such as drought, insect and disease pressure, or more than four cuttings, the risk of poorer spring regrowth increases. Newer stands, winter hardy varieties, and more disease resistant varieties can typically handle more plant stress. With multiple years of drought and water use, cutting to allow plenty of growth to catch snow may be a good idea. 

Another factor to consider is how badly alfalfa hay is needed. If drought has forced the hand to cut alfalfa in less-than-ideal times, the risk of cutting during the winterization process may outweigh the cost of buying expensive hay. Weather can always throw a wrench in our plans, so waiting until after the winterization process to cut again would be less risky.

Any cutting of alfalfa is a stress event for the plant. Minimizing additional stress by avoiding the winterization window will help with winter survival and vigorous spring growth.

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