Precision Irrigation Pays Off in Challenging Conditions

Dr. David Sloane

The demonstration plots at the Water Conservation Center (WCC) are looking good, despite the incredibly hot weather we have been experiencing lately. They were well served by the excellent soil moisture they were planted into, thanks to the permanent soil moisture probes that guided pre-irrigation to fully wet the soil profile prior to planting. The early planted corn tasseled in mid-July and is now into grain fill. While the later planted corn is shaping up to tassel in the first week of August. The sub-surface irrigation (SDI) fields are faring better than the pivot irrigated fields and this is probably related to the frequency of irrigation rather than the amount of water used. The hot, dry conditions should be a great test for the drought tolerant hybrids planted on the SDI fields and the differences due to plant population and irrigation regime are now very prominent. Figure 1 shows a recent satellite NDVI image of the SDI fields and the plant vigor is closely matched to the amount of irrigation each zone gets every 2^nd day. The low-water zones are existing on approximately 2.7 gpm/ac of water!

The two pivot fields were split-planted to an early corn silage crop and a later planted corn crop that will be harvested for grain. By split-planting, we have been able to favor irrigating the early planted corn to get it through pollination and have now switched to favoring the late planted corn to give it a boost as it approaches tassel.

Apart from sharing water between crops on the pivots, this season we are also demonstrating how precision mobile drip irrigation (PMDI) compares to regular nozzles run in bubble mode. The mobile drip uses the same hose used for sub-surface drip but, rather than being buried, it is attached to each irrigation drop and dragged around the surface of the field behind the pivot. We simply include as many emitters on the drip hose as are necessary to match the size of the nozzle it replaces, meaning the hoses are longer on the outside of the pivot and get progressively shorter towards the center. These hoses replaced the nozzles on spans 5 and 7 of each pivot (Figure 2) and ever since they were installed on June 18^th, they have irrigated both early and late planted corn. So far, the results have been quite remarkable but were more obvious in the young corn due to the more open canopy. Since the late planted corn has grown up and offered more protection, the differences in infiltration depth between the PMDI and LEPA (i.e. bubble mode) have been less obvious.

Figure 3 shows the drip hoses fitted to the irrigation drops in the PMDI compared to the regular nozzles running LEPA (bubble mode) and the resulting wetting patterns that were observed. The PMDI only wet half the soil surface compared to the LEPA and the water infiltrated approximately twice as deep. The dry soil between the wet strips produces no evaporation and the deeper wetting pattern also gets it down and away from surface evaporation from the wet strip. Another benefit of the drip hose is that it gives a much longer soak time due to the time it takes for the full length of hose to pass each plant. This can reduce run-off in heavy soils and get more of the water to the root zone where it is needed. The narrow and deep wetting pattern should be more efficient than using spray nozzles and it was evident in the NDVI map (Figure 4) showing greater plant vigor under the PMDI spans. This difference in crop vigor was not detectable in the early planted corn where the rows were already closed by the time the PMDI was fitted. It will be interesting to see how this translates into yield and if it is more evident were the crop had a longer period of vegetative growth under PMDI. It will also be interesting to see the differences next year during pre-irrigation, where there is no crop canopy and the conditions are generally windier. We have soil moisture probes installed under the PMDI and LEPA on both halves of both pivots, so we will have plenty of data to make some comparisons and really examine the differences between the two systems. We will also be able to make some value calculations and explore the return on investment of PMDI.

All the work done at the WCC is to promote technology and farming practices that enhance water use efficiency in a way that can be replicated on producer farms. We encourage you to come and see for yourself what might work on your farm or to reach out and ask questions. Please come along to the field day on September 1^st and see first-hand at how the various drought tolerant hybrids are handling the heat in such a dry season. We are really putting them to the test and they will be on show, along with the comparison between PMDI and LEPA.