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As summer approaches and the soil dries, forage plants become dormant. Some years in drier areas of Oregon dormancy may begin in the late spring. If you have irrigation rights, your pastures can provide supplemental nutrition even during the dry summer months. Although this article target irrigated pastures many of the principles apply to other crops.
There are a number of irrigation methods used in Oregon, including flood, hand line, wheel line, gated pipe, little and big gun, linear, and pivot irrigation systems. The method of choice depends on the system that came with the farm, the size of the farm and the amount of labor, time and money available. Some small farms use solid set systems for pasture. These systems are efficient but require care to protect the pipe from the livestock.
Determining when to irrigate and how much water to apply are specialized tasks. Though many techniques exist, monitoring soil moisture may be the easiest irrigation scheduling technique. This technique can help you determine when to irrigate, whether irrigation periods are sufficiently spaced, and whether the proper amount of water is applied during each irrigation. See the resources for more information that the end of this article for a useful field test for estimating soil moisture. During the growing season, the soil should dry out to about 50% of the soil water hold capacity before it is irrigated back to its capacity. Water holding capacity is a determined by soil texture, organic matter content, and soil depth. The time between irrigations varies depending on the time of year. For instance, during spring in Central Oregon, the frequency of irrigation could be every two to three weeks; in the summer it could be every 5 days, depending upon the water holding capacity of the soil.
Moisture evaporates from the soil and plants are said to transpire, that is, they give off moisture through their leaves. Considered together, these two processes are referred to as evapotranspiration. Evapotranspiration or average daily water loss from the soil plant system varies by season. As you might guess, water losses are greater during the hot, dry, longer days of summer than at any other time of year.
If your goal only is to have a green pasture, irrigate whenever the weather is dry. If you irrigate for production, follow an irrigation management plan based on the infiltration rate, water-holding capacity of the soil, and amount of moisture lost to evapotranspiration. Use weather and soil information to ensure adequate but not excessive irrigation. This information is available for a variety of areas of Oregon through Agrimet (see for more information below). An irrigation specialist at the local USDA Natural Resources Conservation Service office can provide help develop a water management plan.
Do not leave large livestock in the pasture while irrigating; they may damage equipment. To avoid plant damage and soil compaction, wait 3 or 4 days after irrigating before turning large livestock back onto pastures. As always, wait until the pasture is above 6 to 8 inches in height before grazing, and graze no shorter than 3 inches.