For Release July 30, 2002

Water Use By Crops

by Chuck Otte, Geary County Extension Agent

Rains over the weekend, while a nice short term solution, certainly need to be followed up by more rains to get us through to harvest. But for the time being, they were a life saver! Now the question comes up as to how much more rain do we need to make a crop worth harvesting for grain.

Over the last 20 years there has been a lot of research done looking at the very question of how much water does it take to make a crop. Some very interesting results have come out of these studies and we can finally explain many of the things that we have seen in the field for the past 50 years.

Water use is broken down into two stages. The first stage is how much water does it take to grow the plant to the point that it produces itís first unit (pound or bushel) of yield. Interestingly enough, maturity group doesnít impact this part as much as you would think. It doesnít matter if the crop is full season or double crop, it is still the same. The second part of the equation tells us how much more yield we can expect for every inch of rain or irrigation. These formulas assume that, for the most part, you have normal crop growing conditions.

To grow the crop plant and reach that first unit of yield wheat needs 4.2 inches of water, sunflowers 5.5 inches, milo (grain sorghum) 7 inches, soybeans 9.5 inches and corn 10.5 inches. I donít think there are any surprises in that list. It also rationalizes why we have grown the crops, in the various parts of the state, that we have over the years.

Letís try to make a practical application of this information. If you planted soybeans after wheat harvest in June, the plants would need 9.5 inches of water to be able to produce the first bushel of yield. This is normally a combination of available soil moisture and rainfall, or irrigation if you are fortunate to have that option.

If you had a full soil moisture profile at wheat harvest, we could estimate that each foot of standard Kansas soil will have about 1.5 inches of plant available water at capacity. We also know that only the moisture in the top 2 to 3 feet will be used by this crop. But with a full moisture profile at planting, you are already half way to the soybean production minimum. This year, we had no moisture in the rooting profile. The rains over the weekend will finally sprout the seed and start it growing, a full month after planting. The problem now is that we are also running out of time so these double crop beans have a very slim chance of making any yield.

The second part of the equation tells us how much more yield we can expect with each inch of water. Each inch of water will produce 6.5 bushels of wheat, 9 bushels of milo, 5.25 bushels of soybeans, 11 bushels of corn and 150 pounds of sunflower seeds. Again, this doesnít matter if they are full season crops or double cropped. These are averages but are fairly reliable predictors, assuming adequate weather and crop management factors.

What it tells us is that if you have a milo or soybean crop up and trying to flower or head right now, we still have a chance of making a harvestable crop if we can just get some more rain. It also points out the risk of double crop production if you have dry soil and no rain. So, you can probably write off dryland double crop production for 2002, but keep your fingers crossed for continued rain and we can still salvage the fall harvest hopes!


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