Monday, November 20, 2017

Drought Trials--- what kind of drought are we trying to test?

We do our share of drought work.  Yuma Arizona is drier than most of the Sahara Desert—we receive only 2.8 inches (75mm) of precipitation a year!  When combined with our two growing seasons for most crops, this region lends itself to this kind of work.  But all not drought programs are created equal.  Giving some thought to what the type of drought situation you want to simulate is paramount to determining the right approach.

Drought means that the plant is in moisture stress for a period of time.  That period can be anything for a couple of hours on a hot afternoon to being severely water deficient for most of the plants life. 

The cause or reason for the drought is also pertinent. 
  • ·         A situation where there is rationing of water, and now a grower is forced to grow his crops on 10, 20 or 50% less water than “normal”. 
  • ·         Perhaps the situation is that the water is only available on a schedule, and the grower gets it every 7, 14 or? Days regardless of the plants need.
  • ·         It could be a dry land situation, and the rains do not come for 3 weeks when the plants need it most.

How to administer the drought becomes critical.  Typically, we like to use drip irrigation as we can assure uniformity, and we can regulate and confine it accurately.  Surface water can be utilized in some situations.  Sprinklers can also be used, but again, only in some situations.

The generic program we see requests a percentage reduction in water applied.  Reductions of 25 and 50% are common requests.  So, getting to the first discussion above… What is the “normal 100%” amount of available moisture?  We have worked with some very high-tech software to predict moisture needs using soils, crop growth stages and forecast temperatures.  To date we have watched them all FAIL miserably.  Corn in a full “pineapple roll” at 9 am when it should be fine for several more days.  And we have followed conventional wisdom on irrigation and cut back 20% only to have no effect on yield at all.  All this means is that drought work is as much art as it is science.

When do we want drought stress? Do we want to get the crop out of the ground and then reduce it?  Most places with wet or snowy winters and then spring plantings come out of the winter wet.  This means that the crop comes up in moisture, and the suffering is later.  Many in the corn belt feel it is V-8 before they can dry things down significantly.  This is the most common scenario, and based on our experience, much drought stress at germination is next to impossible to overcome and have a normal yield.

Do we want to grow the crop with just enough water and then at fruit set, or tasseling, etc, we want to stress for a month as if the rains did not come or the reservoir went dry?  To do this effectively we need to keep the lower soil horizons water quantity as dry as possible so when we induce the stress, the roots don't make up the deficit by getting the water from the lower depths. (The same concept holds true for nutrient deficiency trials.)  In our experience, a full flood irrigation will fill those lower profiles and deep-rooted crops like corn or tomatoes will take 30-60 days to show much stress.  But this strategy can backfire.  In summer of 2017 here in Yuma, we were keeping our tomatoes and watermelons on the light side of wet, with not much at all for available moisture in the second foot of soil.  June 20, the first day of summer we had 120 Fahrenheit. —the 4th hottest day on record, and abnormally early.  We were caught flat footed and the crops went down in a hurry without having a reserve of deep water to pull from.

Whatever scenario is targeted, a decision has to be made about frequency of the water.  Are we putting on a little bit every day or three with “normal” being an acre inch and the reduced regimes ¾”?  Or are we watering every other day with the normal amount one day and a reduced quantity every other irrigation?  Another possibility is to water Several acre inches every 10-14 days like we would with a surface irrigation, and then withhold water Longer on the dry section.  Part of this decision is if we are mimicking a situation of water rationing, or if we are looking at a dryland scenario of no rain for a few weeks.

When in a water rationing situation, most growers will water it out of the ground well, and then irrigate for a good fruit set and try to cut back some during the bulking stage.  But so much depends on the crop.  Shorting tomatoes or peppers during the fruit bulking growth stage will typically cause blossom end rot, even if there is enough water to make most of the tonnage.

This discussion can be endless!  Determining the situation you want to mimic is the starting point.  The more we understand what the product might do, combined with lots of discussion about what might and might not work is the foundation of a good trial.

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