High Volume, Low Frequency Irrigations vs. Low Volume, High Frequency Irrigations
"I want to grow alfalfa to put into a feedlot operation. The area has a summer irrigation requirement of 18-24" of water and maximum summer air temperatures from 82°F to 100°F. Is it a better use of water, in terms of dry matter produced, to heavily irrigate for a short period of time going into summer or to irrigate more often right through the summer?"
There is a fine balance between high volume, low frequency irrigation and high frequency, low volume irrigation. The former provides efficient use of equipment and reduces losses due to evaporation, while promoting downward movement of salts. The latter, on the contrary, minimizes the degree of stress to the plant, but promotes high evaporation losses, salinization of surface soil, leaf injury, and rootborne plant diseases. One of the basic rules of alfalfa is that it uses a lot of water but does not like wet feet. In other words, it will not tolerate a waterlogged root zone for extended periods of time.
In an 18-24 inch ET (evaportranspiration) zone where the goal is to maximize alfalfa production, combining irrigation management with soils knowledge gets the best results. On the deep soils, irrigate heavily early in the season, since 40-50% of the production of a 3-cut system comes in the first harvest. Irrigate heavily immediately after the first harvest, then once in the middle of the second crop, after the second harvest, and in the middle of the third crop. On shallow soils with low water holding capacity, irrigate on a 8-12 day frequency, but apply smaller volumes to minimize leaching. Natural precipitation will provide the leaching necessary on shallow, low water holding capacity soils.
The best management practice with regard to irrigation (assuming you have a soil suited to irrigation that can store some water) is to fill the profile and then let it deplete to not more than 50% available water consumed. Alfalfa is a "cumulative consumer" of water, meaning as long as the crop can consume water without severe stress, it will continue producing biomass if seed production is not initiated. Once seed production begins, water use diminishes significantly, and plant energy goes into seed production rather than biomass production. Thus, it is best to target your harvesting at the point where biomass production peaks, during early bloom.
Growing Season Limitations
Generally speaking, a well-inoculated, productive stand of alfalfa will produce approximately 1/5 to 1/4 ton of hay for each inch of water consumed. After that, prevailing climatic conditions dictate yield. If the summer water requirement is 18-24", you can figure on a yield of approximately 3.5-5 tons per acre. Here in Montana we can produce a maximum of 5-6 tons/acre per season, since the alfalfa growing season has a span of only 140-150 days. In contrast, Texas, southern California, and New Mexico can produce yields of 12 tons/acre per year because of their longer growing seasons.