Center pivot crop clearance
Hills, valleys, crop height, span length and tower leg length all work together to create the crop clearance your center pivot can clear.
The excellent growing conditions of 2014 produced a very tall crop in many fields. In a few unfortunate situations, the tall crop and lack of sufficient span clearance led to pivot that flipped or rolled. According to Michigan State University Extension the most common form of damage leaves the towers standing but the span rolls and twists the pipe structures at each end. Less common is if both spans on each side of the tower have insuffient clearance and start to drag the bellies of the span the tower in the center, causing both spans to roll. In either situation, the wrecked span on the ground will drag until it creates an angle that should safety off the system.
End gun booms or overhangs can also become victims to tall crop and a lack of ground clearance but have the advantage of allowing the producer to drop the damaged end boom and place the modified end gun on the last tower to finish out the season. The longer the end boom is the more likely a hill or tall crop combination can cause a problem and the less tolerant the structure is to dragging in the crop.
With the drastic reduction in seed corn production this year there will be many pivots used to grow commercial corn that have never had a potential crop height issue until now. To avoid problems, identify potential areas that may need to be addressed. Hill tops that are between towers are potential problem areas. Towers that ride the ridge should have good clearance. Most pivot designers take this into consideration and shorten the length of some spans to keep the tower running on the ridge or hill top rather than the valley. If you have any questions have the pivot supplier map the hill tops and compare to the designs wheel tracks. A preventive upgrade to taller towers is about one fifth the price of replacing the damaged span and a lot more cost effective than the upgrade to taller towers after a wreck.
The majority of irrigated fields in Michigan and Indiana have corn in the rotation setting the minimum clearance requirements. Some shorter feed corn varieties and most popcorn and inbred used in hybrid seed corn production can be raised with as little as seven feet of clearance. Crops like Sudan grass and silage corn may need all of the nine foot traditional design clearance or more. Ideally only the tassel of the plant would brush the belly of the span structure. Problem areas will push stalk ahead of the lead lower brace of the structure then rock forward over the accumulation creating the rocking motion that twists the pipe at the tower attachment.
Shorter span lengths offer greater crop clearance since there is shorter length has less distance for the slope to be expressed over. As an example a small hill with 3 percent slopes sets in a field. If the adjacent tower striatal the hill a typical 187 foot span would use up almost 34 inches of the spans potential clearance. If the tower wheels are centered on the peak of the hill the span will have an additional 34 inches of clearance. This has become a more common issue as pivot span lengths have increased. Twenty-five years ago a typical 1300 foot pivot was made up of eight spans and an end boom. Most of today supplier would build the system from seven longer spans and a longer end boom if hills were not an issue. Longer spans are less expensive for the same coverage area unless the lack of clearance results in damages.
Standard tower legs are designed to provide 9 to 10 feet of clearance at the midpoint of the spans belly. Most center pivot suppliers have alternative products to accommodate taller crop or hills. For a couple thousand dollars or less per tower the taller legs and longer base beam between the tires provide more ground clearance and a wider wheel space on the tower to reduce potential flipping.
Wheel and tire size contribute to the height of the crop your pivot span can go over without drag. On flat ground a center pivots crop clearance will be seven inches higher with 11.2 x 38 potato tires than 11.2 x 24 that resemble a semi-truck tire in size.