End guns have been on center pivots from almost the beginning, and the merits of their use have been debated about as long. Some farmers would not have a pivot without an end gun, and others would not have a pivot with one.
Center pivots are designed to have the lateral pipe just long enough to irrigate as large of a circle or part of a circle in the field as possible, and still not go past the property boundaries. So as you can imagine, the corners of a square field cannot be irrigated.
End guns are a large sprinkler that can deliver water up to 120 feet on the outer end of the machine. It is turned on when the pivot goes past the corners to provide a low-cost method of irrigating an extra 8 to 10 acres on a 160-acre field. The downside is that it is the most inefficient part of the pivot because it does not apply the water uniformly.
Challenges with end guns
The poor uniformity is caused by multiple factors. First, the application pattern of the sprinkler is not uniform even in low-wind environments when operating as an end gun. Moderate to high winds increases the problem because the water is sprayed a long distance and high into the air, resulting in the wind distorting the application pattern and causing high drift and evaporation losses.
In addition, the water application depth drops off dramatically toward the outside edge, making it difficult to decide how far to plant the crop past the end of the pivot. Plant too far, and the crop will suffer drought stress.
If not planted far enough, the end gun will waste water and may stimulate the growth of large weeds. Weed, insect and plant disease pressure can build up in this area, and can move into the rest of the field.
Another challenge is keeping the water off public roads. Occasionally, the shutoff device will fail, and water is applied to the road, which must be fixed as soon as possible. However, the bigger challenge is deciding at what angle to turn off the end gun.
The sprinkler must be turned off well before it gets to the road, not only to prevent spraying the road, but also to prevent water from drifting onto the road on windy days. The result is leaving part of the corner dry. The problem can be somewhat alleviated by adding a second smaller end gun with a shorter throw and usually less wind drift potential.
The extra water the end gun needs must be taken into account when sizing the pump. However, when the end gun is turned off, the extra capacity of the pump will generate higher pressure in the system than needed. The extra pressure will not cause any problems with the system, but it will cause the pump to use more energy than is necessary.
There are several reasons not to use end guns. The decision of using an end gun should be made based on the availability of water, the cost of pumping water, and most importantly, whether the corner be planted to the same crop as the pivot. If the same crop will be planted into the corners, then the nonuniformity around the outside edge will not result in wasted water.
End guns, like every other mechanical device, need to be well designed, maintained, adjusted correctly, and operated at the correct pressure. The end gun usually is designed correctly when it is included as part of the overall sprinkler package design.
The system's sprinkler chart describes how the end gun should be set up. It includes the needed pressure, the designed part circle arc angles, and the suggested throw radius. If you do not have the sprinkler chart for your system, your dealer can provide a new one.
The biggest problem with end guns in operation today, and a big part of the reason they get such a bad rap, is when the operating pressure is not correct. Low pressure is the most common problem, often resulting in the sprinkler being aggressive and having extremely poor uniformity.
When the pressure is too high, the result is a lot of mist and drift. So, making sure the pressure is at the design rate shown in the sprinkler chart when the end gun is at the highest point in the field is the most important maintenance issue for maintaining the water application efficiency for both the end gun as well as all the other sprinklers.
A good method to visually inspect an end gun is to look at it at a right angle from the pivot lateral while it is spraying water. Then determine if the arc is about right based on the sprinkler chart. In addition, does the water pattern and the movement of the gun look correct?
A typical design could be 100 degrees to the reverse and 70 to the forward for a total of a 170-degree arc. The 100 degrees reverse helps ensure good overlap with the sprinklers on the pivot lateral under windy conditions.
End guns can provide a low-cost method of irrigating more acres with a center pivot. The device should be well designed, maintained, adjusted correctly, and operated at the correct pressure to optimize efficiency.
However, they are less efficient than the sprinklers on the lateral pipe because they do not apply the water very uniformly, and thus should not be used in every situation — such as when the corners are planted to a different crop, or when water supplies are short or expensive.
Melvin is a Nebraska Extension irrigated cropping systems educator.