Standard equipment for a center pivot includes an end gun for many farmers, but others would never consider installing one. Why do we have such a difference in opinions?

End guns provide a low-cost method of irrigating an extra 8 to 10 acres on a full quarter-section machine. The downside is that they do not apply water very uniformly compared to the rest of the sprinklers on the machine for a variety of reasons.

First, the application pattern of the gun is not that good, even in low-wind environments. Moderate-to-high wind increases the problem because the water is sprayed high into the air over a long distance, resulting in the wind distorting the 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, and if not planted far enough, you will waste water and may stimulate the growth of large weeds.

Insect, weed and plant disease pressure can build up in these areas and can move into the rest of the field.

Another challenge is keeping the water off the public road. Occasionally the shut-off device will fail, and water is applied to the road, which must be fixed as soon as possible. But the bigger challenge is deciding how close to the road to turn off the end gun.

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The sprinkler must be turned off a distance 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, shorter-throw end gun that can be turned on to fill in part of the corner after the long throw would hit the road. The shorter-throw guns often have less drift potential, helping keep water off the road.

End gun considerations

The extra water the end gun needs must be considered when sizing the pump. The larger pump will result in the system using more energy even when the gun is off.

There are several considerations to keep in mind. The decision of using an end gun should be made based on the availability of water, the cost of pumping the water, and most importantly, whether the corner will be planted to the same crop as under 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 is usually designed correctly when it was included as part of the original sprinkler package design.

The sprinkler chart describes how the end gun is to be set up, 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 give you a new one.

Problem-solving

The biggest problem with many end guns is that the operating pressure is not correct. Low pressure is the most common problem resulting in the sprinkler being very 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 level shown in the sprinkler chart when the end gun is at the highest point in the field is the most important factor to optimizing the water application efficiency, for both the end gun and the sprinklers on the pivot lateral.

Keep in mind that sprinkler packages, including pressure regulators, should be replaced every 10 years or 10,000 hours of operation. However, end guns often last much longer than the other sprinklers. Trees that have grown up around the end gun path and hail damage are two of the main causes requiring repairs.

A good method to visually inspect an end gun is to look at the end gun at a right angle from the pivot lateral while it is spraying water. Look to see that the water pattern and the movement of the gun is correct. Then determine if the arc is about right based on what is listed in the sprinkler chart.

A typical design could be 100 degrees to the reverse (starting with the gun aimed straight out from the pivot lateral) and 70 degrees 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, but does often result in a small, overwatered ring around the field.

Also, make sure end guns turn off when they are supposed to, and that the valve does not leak when it is off. Often, end guns are allowed to irrigate all the way around the field except over the road. This not only wastes water, but also dramatically overwaters the areas that are already irrigated by another pivot.

Doubling the amount of water applied will lower crop yields. So, do the neighboring field a favor and turn off the end gun if the field is already irrigated.

Manage correctly

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 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 irrigated cropping systems Extension educator.