The purpose of irrigation management is to ensure an adequate amount of water is available for the crop at all times. Moisture stress, at any time during the growing season, will reduce the yield potential of an irrigated pasture. The seasonal consumptive use of a pasture is approximately 590 mm, depending on climatic and agronomic conditions. The peak consumptive use, when averaged over 30 days, can be as high as 7 mm/day with a 10 day peak as high as 7.2 mm/day. Typically, these peaks occur in mid July. Soil moisture conditions in the 80 cm root zone must be monitored and analyzed to determine the amount of water available to the plant. The allowable depletion level is 50% of the available moisture for the entire root zone or 80% in the top 50 cm; as long as, the depletion in the whole root zone does not exceed 50%. It is important to know the water holding capacity of the soil; as well as, the amount of water applied by irrigation, so water and nitrogen fertilizer will not pass through the root zone.
Generally, irrigated pasture in Alberta is an intensively managed business. It requires management skills similar to many of the specialty crops. Irrigation planning must include both irrigation and non-irrigation time. A pasture should not be grazed during or shortly after irrigating, as this will cause soil compaction. Similarly, a portion of the crop may be cut for hay to keep the crop in the vegetative stage and prevent it from heading out. During these two instances, irrigation should be stopped. Irrigation should begin early in the spring and continue until a killing frost has occurred. Fall irrigation is an important consideration for pasture as growth may begin in the spring before it can be irrigated. Fall irrigate to 75% available moisture.
Pivot irrigation systems
Cross fencing may have some implications to irrigation management. The pivot may need to travel through a cross fence which necessitates gates at each tower. The pivot may reverse at a fence or run dry through a pasture to get to the field to be irrigated.
Because grass seed is small and is seeded shallow, germination moisture may need to be supplied after seeding. Center pivots can do an excellent job of watering a crop and the pivot should be kept running until the crop has emerged. Pivots generally cannot supply all of the crop's water requirements during peak use periods. The difference should be made up from soil moisture reserves. Therefore, it is important to build up soil moisture reserves in the fall or spring and maintain them during the peak use period.
Fertigation of liquid nitrogen is an option with a pivot and should be considered on sandy soils to prevent leaching. It is also an alternative way to apply fertilizer when fields are to wet for conventional fertilizer spreading equipment.
Wheel move irrigation systems
The water holding capacity of the soil must be known in order to determine the time between irrigations. This will also affect the length of irrigation sets. The nozzles should be sized ensuring the root zone is filled to field capacity at the determined length of the irrigation set. For sandy soils, the time between irrigations will need to be short, so the next irrigation will occur before the crop is stressed.
Because wheel move systems generally apply 75 -100 mm of water, ponding is a potential problem. This may lead to leaching water and nitrogen, drowning the crop and causing soil compaction. To apply moisture for germination during the establishment year, a light, uniform application using 2-4 hour sets is required. This is very labour intensive and is not recommended unless absolutely necessary.
Surface irrigation systems
Border dyke irrigation is the most common form of surface irrigation for pasture. Corrugations and furrow irrigation do not lend themselves very well to pasture irrigation. Level basin and border ditch irrigation are suited to pasture irrigation, although not widely used.
Cattle trample down the sides of ditches requiring the ditches to be re-pulled every year. Also, animals must be kept away from turnout and control structures.
Irrigation for germination moisture with border dykes is not recommended. Light applications may not be possible, soil erosion may occur and seed may be relocated. The field intended for irrigated pasture should be properly leveled. It must not contain any low areas where water could pond. Ponding will drown the crop and foster growth of weeds. Any contaminated water that leaves the field as runoff or in the irrigation supply ditch must be avoided.
Research and experience has shown
This information was prepared by the Irrigation Branch.
- Optimum production is affected by:
- the growth stage of the crop
- cattle stocking rates
- the length of the grazing period
- the varieties of grass and or legume
- Soil compaction will occur and mediation practices are recommended
- Haying may be required to keep the forage in the vegetative stage
- Calves sleep in pivot wheel tracks and can be run over