Most older-generation herbicides used at planting time have to be water activated.

Longtime University of Georgia Extension Peanut Specialist John Baldwin probably said it best when talking about the use of moisture-activated herbicides: “If it don’t rain, it don’t matter.”  Without proper moisture, it won’t matter whether the herbicides are there or not.

Irrigation is often a poor substitute for Mother Nature’s water, but in some cases, it can be a better way to water-in herbicides. The labels of several herbicides state that as long it is incorporated within three weeks, the herbicide will be effective. The best way to get full activation is to have one rain event that provides all the water needed to activate the herbicide.

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Most water-activated herbicide labels also say these materials need water within 21 days. Many times, growers think they need a cumulative amount of rainfall within that three-week period to activate the herbicide. Research indicates that may be a recipe for failure.

Peanuts are predominantly grown on sandy soils. If these soils are allowed to dry after a rain, or after a low rate of irrigation, and are followed between similar showers or low levels of irrigation, the herbicide will bind tightly to the top layer of the soil without adequately moving through the soil.

By applying adequate irrigation water at planting, growers can insure these type herbicides will keep their peanuts clean for the first few weeks of growth.

Irrigation is an expense, and it can be significant in some parts of the country. To produce peanuts profitably, growers must manage irrigation just as they manage other crop inputs. Balancing the irrigation cost checkbook is equally as important as justifying the use of fertilizer, fungicides and other peanut inputs.

Just like money in the bank, the water bank account can fall quickly when things don’t go as planned. To manage the water bank, the grower has to measure the amount of moisture in the soil.

Growers irrigate crops to maximize yield. To maximize yield, the grower has to maximize evapotranspiration. To accomplish those goals, the farmer has to measure water accurately.

Evapotranspiration (ET) is the transport of water into the atmosphere from surfaces, including soil (soil evaporation), and from vegetation. Soil and vegetation are the most important contributors to evapotranspiration.

Other contributors to evapotranspiration may include evaporation from wet canopy surface and evaporation from vegetation-covered water surface in wetlands.

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The evaporation component of ET is comprised of the return of water back to the atmosphere through direct evaporative loss from the soil surface, standing water (depression storage), and water on surfaces (intercepted water) such as leaves and/or roots.

Transpired water is that which is used by vegetation and subsequently lost to the atmosphere as vapor.

There are a number of monitors that can be used to determine exactly how much irrigation water peanut plants need. These irrigation water guides are available from Extension specialists in any peanut-producing state and from any irrigation equipment dealer.