Each drop counts for green pastures

Doug Warnock

Capital Press

March 7, 2008

It looks like there will be ample irrigation water in the Northwest this year due to the high snow pack building up in the mountains. That's welcome news to producers with irrigated pastures, but we shouldn't allow this to make us careless in managing irrigation water.

Poor water management can lead to less-than-optimum production and even to weakening of desirable pasture plants. Good pasture management includes proper irrigation management to sustain ecosystem health. A healthy ecosystem will not only give us a better return, but will also help sequester carbon. A vigorous plant community is the best means to fix carbon dioxide in the air, reduce soil erosion and build soil resilience.

The proper amount of irrigation water to apply to a particular pasture depends on the type of soil prevalent. Clay soils retain water longer, while sandy soils dry out much faster. Soil type, water-holding capacity, soil depth and other characteristics are listed on soil surveys that are available through the Natural Resources Conservation Service and the local extension office. This information helps determine the proper amount of water to apply to fill the soil profile.

While there are several types of moisture-measuring devices available, most grazers use less expensive and less sophisticated methods to schedule irrigations. A very simple and effective method is to dig up a small sample of soil from the root zone and squeeze it with your hand. If the ball of soil falls apart, the soil is low in usable moisture. If the ball sticks together, there is still moisture available.

The plants will tell us much about what is happening, also. When plants reach the wilting point from lack of water, they become darker in color, the leaf tips of grasses begin to turn yellow or brown and the shallow-rooted forbs on higher ground begin to wilt.

Plants with adequate moisture retain a brighter green color. It is best if plants are irrigated before they experience any wilting and stress from low moisture.

Applying the correct amount of water is important. Either too much or too little water can be detrimental in achieving optimum forage production. A coffee can set under a sprinkler is an easy measure of water depth, indicating the amount of water that's being applied during an irrigation. As a general rule, two to four inches of water per application is sufficient to recharge the root zone of most soils. However, there is no substitute for knowing the water-holding capacity and characteristics of your pasture soil.

The type of irrigation system being used makes a difference because of water application efficiency. Sprinkler systems generally apply water more uniformly and are more efficient than surface application systems. Because of their efficiency, sprinkler systems are recommended for areas that have limited water availability. Sprinklers are also better suited for rolling terrain and steeper slopes, where surface systems allow water to drain off rapidly and leave plants dry.

In a year with ample water available, it is easier to be less judicious in applying water. Over application of water can be detrimental. Applying too much water can cause leaching of valuable plant nutrients and erosion of topsoil, which can create problems for those producers who are downstream from you.

Over watering also suppresses the vigor of grass and legume species that thrive best on well-drained soils and cause them to be replaced over time by plants that are less palatable and less productive. Prolonged over watering encourages plants that like wet soils, such as meadow foxtail, reed canary grass, sedges and rushes. Prolonged under watering weakens the good producing forage plants and shifts the plant community toward forbs with deep taproots, such as Canada thistle and knapweed. Less experienced irrigators can get helpful information at the local extension office, the local conservation district, or NRCS.

For pastures to remain healthy and productive, the appropriate amount of water needs to be applied on a timely basis and coordinated with the movement of livestock.

It is best not to irrigate a paddock that is occupied by grazing animals. This can lead to soil compaction, which is harmful in the long run. Compacted soil won't allow water to infiltrate properly, leading to water runoff and soil erosion. Soil aeration and soil microorganism activity are reduced in compacted soil.

Ideally, a paddock should be irrigated just after it was grazed. This promotes plant regrowth and recovery as soon as possible. Waiting to irrigate a grazed area will delay plant recovery. The number of irrigations needed to foster good plant recovery depends on the length of the grazing schedule, the water-holding capacity of the soil, the moisture needs of the plants and the weather.

Retired Washington State University extension agent Doug Warnock consults and writes on ranch and farm management. 

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