Irrigated pastures a dynamic system, tough to model

Cody – “Anything you can do to increase the depth of rooting will give you a more robust, healthy plant more able to get through hot dry periods,” said Howard Neibling of the University of Idaho at a recent meeting in Cody.
    Neibling was present at the early November joint meeting of the Society for Range Management Wyoming Section, the Soil and Water Conservation Society Wyoming Chapter and The Wildlife Society Wyoming Chapter.
    “A lot of times pastures take short shrift on things – we take care of them after we’ve done all the ‘important’ things,” said Neibling. “They don’t necessarily get the management they deserve.”
    Regarding irrigation, too little or too much water will impact forage yield and is economically unsound, affecting stand density and other factors. He said subsurface water quality is also part of the equation.
    “Excess irrigation can leach plant nutrients like nitrate down into shallow groundwater sources, creating water quality problems,” he said. “Also, inadequate irrigation can increase plant nitrate concentrations if you stress some crops at the wrong time.”
    Neibling said it’s important to match the depth of water applied to the soil’s water holding capacity and the rooting depth of the plants you’re working with. “Irrigation systems are a great tool, but they need to be fit to the plants as best we can,” he added.
    “Irrigation design is very important. It’s not as much of a problem with pasture grasses, but in alfalfa production it’s important the system is designed to provide adequate uniformity and provide the water you really need at the rate you need it,” he continued.
    Irrigating pasture creates a system favorable for compaction. “That’s a tough one to manage around, particularly with some of the sprinkler systems,” said Neibling. “The complicating factor is all the critters out there stomping around and taking away your biomass and changing how much leaf area you’ve got to work with.”
    He said grazing under irrigation also opens things up after pasturing, allowing bare soil to be exposed. “It’s not as easy to figure out how much water we’re really using on a pasture as some other crops because of the changing canopy,” he noted.
    “We know on bare soil evapotranspiration (ET) is highest just after irrigation. As the soil dries out on the surface the ET rate drops off. There’s also ET from the plant leaf area,” he explained. “When animals are turned in they eat off the leaves, which affects the ET rate. It’s a very dynamic system and tough to model.”
    Neibling said grazing pastures simulates the same affect of cutting an alfalfa field. “When you cut it the ET drops off significantly, then it goes up as you start to rebuild the plants,” he said. “Something like that happens with pasture area. You turn the animals in, they reduce leaf area and ET will drop off and rebuild again.”
    He said when the plants are in the rebuilding stage is a good time to catch up on irrigation if it’s been hot and dry. However, if a producer is running a three-day pivot he can only add a certain amount of water at a time.
    “Many times you’re wetting and drying the top foot and a half of soil, and you might as well have rock underneath. The plant doesn’t really care because there’s no water deeper than that,” said Neibling. “With certain system designs you don’t have the ability to wet the deeper rooting zone, so you never get the roots down deeper in the soil.”
    He said if a field has deeper soil a producer needs to be able to take advantage of some of the deeper-rooting characteristics of some of the grasses. “That will give you a healthy plant more able to get through hot, dry periods, and the deeper the system the easier it will be going through the grazing cycle when you’ve got the water off during grazing,” he added.
    To achieve the deeper irrigation, Neibling said it’s a lot easier to keep the soil full or refill it with the surface irrigation of hand lines or wheel lines where you can put on quite a bit of water per set.
    “It’s really tough to fill up a three- or four-foot profile with a center pivot,” he said. “If you put on over an inch you get runoff, or it runs into the low spots, and next thing you know you’re digging the pivot out.”
    In addition to the impacts of livestock grazing on water use, Neibling said climate factors also influence ET, including solar radiation and relative humidity as the main factors. Also, he said the plant will use less water if it’s drought-stressed, and elevation and latitude also affect ET.
    Concerning all of those factors, he cautioned to not fall into the trap of managing based on average. “We have very few average years,” he said. “Tailor your irrigation management to what’s really going on out there.”
    Neibling said the bottom line is, as much as possible, to take advantage of all the soil moisture you have by tailoring your irrigation system and program to suit pasture type and soil profile.
    Christy Hemken is managing editor of the Wyoming Livestock Roundup and can be reached at christy@wylr.net.