Pasture and Forage Minute: Early Season Alfalfa Irrigation, Trigger Dates and Weed Control
Spring Grazing to Control Weeds
Given the drought conditions that Nebraska experienced in 2022, it is likely that many pastures will have an abundance of spring and summer weeds this year.
Drought last year also has led to the general recommendation of delaying turn out to pasture, but early flash grazing can be an option to capitalize on growth of some of those weeds. Flash grazing is the process of quickly rotating through pastures early, before they are scheduled for their main summer grazing period.
When flash grazing mixed cool- and warm-season grass pastures, we do want to be a little more cautious as to not overgraze any desirable cool-season grasses. In areas where cheatgrass or downy brome is a problem, grazing at strategic windows — such as during the cheatgrass elongation phase right before seed set — appears to be the best time to apply grazing. Grazing at this time matches diet preference by grazing animals with the cheatgrass growth period and limits over use on perennial cool-season grasses growing at the same time. Targeted grazing is a long-term management option that can utilize cheatgrass as a forage resource and limit the potential seed proliferation within a system.
In warm-season grass pastures, an abundance of early weeds will remove moisture that could be used for grass growth later on and they remove valuable nutrients from the soil. Early weeds also can develop so much growth that they can shade, smother and reduce early growth of your summer pasture grasses.
While early flash grazing of some pastures will not eliminate all the weeds, it can actually make for some pretty timely and valuable pasture.
As we head into the growing season following last year’s dry conditions, assessing pasture conditions at the correct time is critical to successful planning. How can trigger dates complement your drought planning this year?
One of the main factors driving annual forage production in Nebraska’s grazinglands is available moisture. Both cool- and warm-season grasses in the state rely heavily on spring and early summer precipitation at a time when the plant is rapidly growing. This period of rapid growth varies by species and is driven by air temperature, day length and soil moisture. Speed of spring growth and recovery after grazing depend on this. Once optimal conditions have passed, getting significant growth even if it does rain is difficult.
With this in mind, we can set up some trigger dates to assess moisture levels and pasture conditions, informing the implementation of a drought management plan.
- April 15 to May 10: Smooth bromegrass pastures with below-average precipitation, annual production may be reduced 25-50%.
- May 20 to June 10: Assess earlier precipitation levels. If March-May precipitation was 50-75% of the long-term average, reduce stocking rates 30-40% or more depending upon grass species and plant health;
- June 15 to June 30: Approximately 75% to 90% of grass growth on cool-season dominated range sites and 50% of grass growth on warm-season dominated range sites will have happened. Rainfall after late June results in limited benefit to cool-season grass production.
- July 15: Precipitation after this date will have limited benefit to warm-season tallgrass production but can still result in some forage growth from shortgrass warm-season species such as buffalograss and blue grama.
Knowing when to pull the trigger on drought plans is not an easy decision, but it can mean the difference between managing with conditions or scrambling to catch up. This year, use trigger dates for your operation to successfully implement drought mitigation strategies.
Early Season Alfalfa Irrigation
Early season irrigation may be delayed if there are limits for yearly irrigation amounts. However, if soil profiles are currently very dry, spring irrigation should be considered, especially since first cutting alfalfa may be twice as productive as any subsequent cuttings. For maximum season production, the first alfalfa cutting typically requires six to seven inches of water. Since established alfalfa is a deep-rooted perennial, the risk of excess water running off fields is low. Even if rainfall comes after early spring irrigation, the water will likely be stored in the soil profile for use later in the growing season when summer heat increases water demand.
During the peak of summer heat and plant growth, alfalfa may use over one half inch of water per day. This compares to cooler spring days water usage of less than ¼ inch of water per day.
Still irrigation timing is important, and adequate subsurface moisture can help control weeds. If irrigation occurs before the alfalfa plants have begun to regrow after cutting though, weed growth will be promoted instead. When summer heat arrives, alfalfa plants will likely draw moisture from their eight feet rooting depths to maintain full forage production. Conversely, lower spring irrigation amounts may result in shallower rooting decreasing summer irrigation efficiency. For early season irrigation, the target is to refill the top six feet of subsoil profile for the late spring and summer.
More information is available on our UNL Extension website and our free NebGuide G1778, “Irrigation Management and Crop Characteristics of Alfalfa”.