from MSU News Service

Irrigated alfalfa is one of Montana's premier cash crops. Though irrigated alfalfa hay production averages 3.3 tons per acre statewide, many producers routinely harvest 5 to 7 tons per acre with high management.

A major consideration for optimum alfalfa tonnage and quality is the timeliness and amount of irrigation applied. Insufficient or poorly-timed irrigation reduces productivity. On the other hand, over-irrigation is inefficient, and can cause loss of nutrients, soil and the stand itself.

Due to a late fall in 1998 and dry spring conditions, many alfalfa fields began the 1999 season with a subsoil moisture deficit, and irrigation during July 1999 was intensive. Hot winds occurred during first harvest, and many producers have had a difficult time "catching up" with their irrigation. As a result, there is extreme variability in alfalfa regrowth in fields that previously appeared uniform. It is easy to spot the variability due to soil texture, slope or gravel bars in many irrigated hay fields.

Irrigation water management of an alfalfa hay field is significantly different from managing small grain, sugarbeets, corn, potatoes or other annual crops. In order to schedule hay harvests, adequate moisture must be present in the soil profile to keep the crop growing through the dry period of haying until the field is re-irrigated.

The major factors that affect irrigation scheduling and rates for an established alfalfa stand are the amount of currently stored soil moisture, the soil's water holding capacity and the rate of plant evapotranspiration.

Available water holding capacity varies from field to field and within a field, due to differences in soil texture and depth across a field. Available water holding capacity of a soil varies from about 0.5 inches of water per foot of coarse sandy soil, up to about 2.2 inches per foot on finer-textured loams or silt loam soils. In most years, a deep-rooted crop like alfalfa can get enough moisture from winter precipitation and early summer irrigations.

Peak water use by alfalfa in Montana range from 0.2 to 0.3 inches per day. Across Montana, water use during July was nearly 2 inches per week. Hot, windy conditions during much of July provided for good haying conditions, but after first cutting was completed, many producers have not been able to keep pace with high evapotranspiration rates.

Producers with access to the Internet can monitor water use rates at the Bureau of Reclamation's AGRIMET site: http://www.gp.usbr.gov/agrimet_station_list.htm. (Many newspapers print weekly summaries). You can get an overview of irrigation information at the link called "irrigation guide," but if you want to know how much to irrigate, you need to select the link to "crop evapotranspiration."

What steps should be taken for the remainder of the 1999 season?

First, replenish the root zone of the alfalfa crop by early August, and prepare for proper fall management to maintain a healthy stand through the winter. Your last summer cutting should occur 30 to 45 days prior to the average first frost date in your area. Fall harvest or grazing should then occur after a "killing" frost - typically in October.

During the "hardening-off" period from mid-August to early October, it is usually best to NOT irrigate, since this encourages rapid regrowth that could predispose the alfalfa crop to severe winterkill. If irrigation water is available in early October, then bring the soil profile back to field capacity.

Second, if you are considering irrigation system improvements, this is an excellent time for an efficiency audit and planning. In many cases, there are federal monies available for cost-sharing programs to upgrade to more efficient irrigation systems. Field irregularities are very easy to detect this time of year, and you are likely operating at high capacity.

Contact your local NRCS or conservation district office and irrigation suppliers to get assistance in designing an optimum system.