Managing alfalfa in high temperatures

By Jeremiah Vardiman 

High temperatures, grasshopper reports and drought conditions have made this year interesting.  

These events could be affecting certain producers greatly, or only impacting the fringes of operations depending on where producers are located and if they have access to irrigation. These conditions have stimulated various questions, and one question that has been asked multiple times is how best to manage alfalfa during temperatures over 100°F. This question is primarily for irrigated alfalfa fields.  

Alfalfa is native to the Middle East, such as Turkey and Iran, making it well-adapted for hot and dry conditions. Although alfalfa can tolerate hot conditions, extended periods of high temperatures can have an impact on alfalfa plants and limit the ability to produce high-quality hay.  

A decrease in stem diameter, accelerated crop maturity, an increase in lignin and a decrease in plant height, leaf-to-stem ratio and digestibility is commonly seen when temperatures are consistently over 100°F.  

Overcoming heat 

Maintaining adequate soil moisture is the best management during hot periods. Alfalfa commonly experiences moisture stress during this period of heat stress, even through soils may be moist.  

Alfalfa stands experiencing moisture and heat stress will respond by shutting down, which slows the growth of the plant and encourages maturing of the crop. Irrigation helps alfalfa respond to this stress, however, yields can still be reduced.  

Slow plant growth is triggered by the increase of respiration, or the breakdown of carbohydrates into energy, especially when respiration exceeds photosynthesis.  

Unlike photosynthesis, which depends on light, respiration occurs 24 hours per day. Respiration rates increase with temperature and can exceed photosynthesis rates when high temperatures are maintained, especially at night.  

Fortunately for Wyoming, there are typically good fluctuations in daytime and nighttime temperatures. For example, on July 27, Powell’s daytime high was 102°F with a nighttime low of 73°F. Lower temperatures at night gives a plant time to recover from any heat or water stress during the day and replenish on water. 

If nighttime temperatures do not cool down, alfalfa plants expend more nutrients, which results in producing finer stems high in fiber, and even though they can contain high protein, this results in low relative feed values. 

Managing maturity 

The early maturity of alfalfa crops is another concern during high temperature summers. Alfalfa fields may begin to bloom early when temperatures exceed 90°F. Typically, bloom is an indication for harvest timing. This early blooming in high temperatures can be misleading because the alfalfa plants have not had enough time to rebuild nutrient reserves in their roots since the last harvest.  

Watch the calendar and the plants to schedule the next cutting. Plants should be allowed to grow for 30 days or more prior to cutting again to maintain the health of a plant. There are instances when the field could bloom within 20 days or less because of hot and dry conditions. 

When to cut is the last consideration during high temperatures. Cut alfalfa that maintains 50 percent moisture or more will continue to respire, decreasing the quality of hay and yield. Research has proven hay cut in the afternoon produces higher quality hay than if cut in the morning.  

Everyone is striving for the best hay. It is important to keep in mind cutting hay when the best drying conditions are available to get the hay below 50 percent as soon as possible also needs to be considered. Cutting hay in the morning during good drying conditions may still be wise. 

The best way to manage alfalfa in high temperatures is to maintain adequate soil moisture, track crop maturity on a calendar as well as in the field and select the best drying conditions for cutting hay, even if this means cutting in the morning.  

Wyoming is known for difficult growing conditions, but its natural fluctuations between daytime and nighttime temperatures allows for quality hay to be produced during hot conditions. 

Jeremiah Vardiman is a University of Wyoming Agriculture and Horticulture Extension educator. He can be reached at jvardima@uwyo.edu.