Variable-rate irrigation has proved effective and profitable to manage crop moisture on varying soil types. However, variation under normal irrigation can do just the opposite: Reducing uniformity of water application (UWA) could dramatically exceed the remedy’s cost.
“A well-maintained center pivot is designed to operate with a UWA of over 90%,” explains Bruno Lena, assistant University of Nebraska Extension educator for water and cropping systems. However, he notes, Factors such as the sprinkler package’s natural wear, and installing or replacing nozzles with improper sizes can reduce UWA, ultimately reducing yield. “To make it worse,” he says, “the negative impact of low UWA is generally larger during dry growing seasons, when total irrigation water requirements increase.”
Testing UWA
After talking with a central Nebraska grower who experienced lower-than-expected yields, Lena and his colleagues set up an Extension project in 2022 to assess three center pivots’ UWA before and after replacing the nozzles, and low UWA’s potential negative impact on crop yield.
After analyzing the grower’s yield map, the team identified an area with substantial yield reduction. “These areas resembled rings sometimes one-fourth of a span length,” Lena described. Consequently, they conducted two “catch-can tests” at three center pivots in eastern Nebraska. This involved setting up cups attached to electric fence posts every 9 to 10 feet and then running the pivot over the cups. The team tested each center pivot before and after equipping them with new nozzle packages. With the old nozzles, which had received minimal maintenance since installation, pivots 1, 2, and 3 exhibited UWA values of 78%, 59%, and 23%, respectively. Following nozzle replacements, Lena says UWA increased to 92%, 90%, and 74%, respectively. He noted pivot 3 still fell short of the recommended values due to reduced pumping capacity.
“In 2022, a substantial portion of the area yielded below the grower’s specified threshold, exemplifying the impact of an inadequately maintained center pivot,” Lena continues. Approximately 36.3 acres fell below the threshold, averaging a yield of 52 bushels per acre (bpa) for soybeans. Similar trends were observed on the corn acreage, with 31.5 acres below the threshold and an average yield of 172 bpa. This discrepancy from the farmer’s anticipated yield values resulted in a significant potential loss of about $9,000 for soybeans and $14,500 for corn.
The total cost to replace the nozzle package, including labor, was $6,500 in April 2023, Lena recalls. “If the grower had opted to replace the nozzles before the dry 2022 growing season, and assuming the field would have averaged above 65 bpa in soybeans and 230 bpa in corn, the investment could have been recouped,” he asserts, “yielding a profit of approximately $17,000 … highlighting the economic benefits of proactive maintenance and timely equipment upgrades.”
A few growers have asked Lena’s researchers how often they should replace the nozzles; some dealers say ideally after 10 years, while others claim new materials permit longer life. Lena recommends conducting a simplified catch-can test, with fewer cups, to assess a particular pivot’s UWA before deciding whether a nozzle needs replacing. Another way is to check the yield map data, which can provide a good insight of issues regarding low UWA.
“Either way, upgrading and maintaining the pivot system, particularly in the face of varying environmental conditions, is essential to ensure optimal crop performance and minimize potential losses,” he sums up