How Delayed Spraying Cuts Wheat Yield

Canadian research demonstrates that sprayer wheel tracks can damage yield

Research in wheat shows that crop trampling while spraying can result in up to a 4% yield loss if spraying is delayed. While not huge, that’s over 1½ bushels of wheat on a 40-bushel crop, and that 4% yield loss doesn’t take into account the loss (or reduced herbicide effectiveness) due to delayed weed control.

“We were doing a lot of sprayer testing in the 1990s, and with the wide range of tires, tank sizes, and wheel configurations, we were getting a lot of questions on wheel track damage,” says Brian Storozynsky, Project Technologist with the Agricultural Technology Centre (ATC) at Lethbridge, Alberta. As a result, the ATC conducted a three-part research trial to see how various combinations of tractor, sprayer tires, and sprayer tank sizes, at different crop stages, affected yield.

The first study looked at the effect of single versus tandem sprayer tires, sprayed at the 2 to 3-leaf stage, 5 to 6-leaf stage, and tillering stage. There were 400 and 800 gallon water loads compared. Six combinations of wheel assemblies were compared, including single turf sprayer tires, tandem walking beam sprayer tires, tractor tires pulling a sprayer tank with single turf tires, and tractor tires pulling a sprayer tank with a tandem walking beam transport system.

The results showed a clear trend of reduced yield at larger crop stages, due to trampling by the wheels. It also highlights the need to run the sprayer tires in the wheel tracks caused by the tractor tires.

“We definitely were seeing some yield effects of wheel tracks as the crop was larger,” says Storozynsky.  “The earlier you get in the crop the better.”

The second part of the study compared single versus dual wheels on the tractor. Storozynsky says that many farmers run dualies in the hopes that the wheel pressure will be less. The results showed that there was not any difference in crop damage between single and dual tires, except with dual tires at the tillering stage, which showed significantly lower yield than the single tractor tires. Again, this would have been because of crop trampling. And, once again, under all tractor tire loads, whether single or dual, the yield loss was higher when the crop was sprayed later.

The final study compared single, tandem and lugged sprayer tires at two growth stages; 2 to 3-leaf stage, and tillering.  In this trial, no trends were apparent between tire types, indicating all would be suitable for crop spraying. 

Effect of Wheel Configuration and Crop Stage on Yield

Wild oats are 1-leaf stage ahead of the wheat	Crop Stage (% yield reduction)
Wheel Configuration	2 to 3-leaf stage	5 to 6-leaf stage	Tillering
Single sprayer tires in tractor tracks	1.170	1.428	2.177
Tandem sprayer tires in tractor tracks	0.577	1.089	2.294
Single sprayer tires plus tractor tracks*	1.906	1.644	3.857
Tandem sprayer tires plus tractor tracks*	0.651	2.687	4.216
*Sprayer tires do not follow tractor wheel tracks.   Source: Ag Tech Centre.

The overall results indicated that sprayer tires used today are suitable for crop spraying. Both single and dual tractor tires are suitable, reducing the need to drop the dual tires when spraying. However, with all tires and combinations of configurations, the later the spraying operation took place, the greater the amount of crop damage. The other recommendation coming out of the study is to ensure that the sprayer tires follow the tractor tires.

“The damage can really show up if spraying later, like what happened in 2005.  Farmers couldn’t get on the land to spray because of the wet weather in June.  When the crop gets up to seven inches tall, some of the narrow tires on high clearance sprayers leave tire tracks that are still visible at harvest, so there is likely some loss there,” explains Storozynsky.

Early weed removal also important
While crop trampling losses at later spraying stages can be significant, they can be overshadowed by crop losses due to weed competition.   In wheat, generally, the earlier the weed is removed from the crop, the greater the chances the crop yield will be unaffected.

Studies from Agriculture and Agri-Food Canada and the Alberta Research Council have found that delaying wild oats control will result in higher yield losses, especially if wild oats are left to compete past the 5 to 6-leaf stage, or past 20 days. Additionally, if wild oats emerge before the crop, yield losses are higher as well.

Patrick Haikal, a researcher with Arysta LifeScience agrees that many independent studies show that early weed removal often results in the highest yields.  Arysta is also conducting time of weed removal trials looking at common grassy and broadleaf weed herbicides.  He says the trends in their research support other findings that early weed removal is best.

Impact of Wild Oat Emergence and Density on Wheat Yield

Wild Oat Time of Emergence	Wild Oat Density — Number per Square Metre
	1	2	4	6	8	10	12	14	20	30
Wild oats are 1-leaf stage ahead of the wheat	1	2	4	6	8	10	12	14	19	26
Wild oats are at the same stage as the wheat	1	1	2	4	5	6	7	8	11	16
Wild oats are 1-leaf stage behind of the wheat	0	1	1	2	3	3	4	5	7	10
Source: O’Donovan; Alberta Environmental Centre (Vegreville, Alberta)

“Where we are seeing some differences is with a product like Everest, which has both leaf contact and soil activity. It can be applied early to control existing weeds, and then the soil activity will control later flushes,” Haikal says. “That is where we are seeing the difference, with Everest applied early versus other post-emergent products. Other products can suffer from either being applied too early, resulting in later flushes impacting yield, or applied later in an effort to target more weeds, resulting in yield losses due to early weed competition.”

Obviously, weather conditions in any given year can impact weed removal timing and crop trampling damage. But it’s evident that controlling weeds as early as possible is the best way to set wheat crops up for their full yield potential.