Spray coverage in potato

This past June, 30 growers attended a spray coverage demonstration in a potato field in Alliston. Our goal was to explore three questions:

  1. What is the effect of droplet size on coverage?
  2. What is the effect of volume on coverage?
  3. What is the effect of spray angle on coverage?

In order to see spray coverage, we placed water sensitive paper in the potato canopy (see Figure 1). Water sensitive paper turns from yellow to blue when it is contacted by spray (in this case, water). Normally, we use a digital scanner to quantify spray coverage. However, it was a very humid day, and this made it difficult for the scanner to discern spray from background. We decided to assign a qualitative value to the cards based on coverage. Low (or no) coverage got a score of zero. Moderate coverage (enough to offer good control) received a score of one. Cards with excessive coverage (anything more than moderate) received a score of two.

Figure 1 - The location of water-sensitive papers in the potato plant canopy. Two plants were papered for each nozzle.

Figure 1 - The location of water-sensitive papers in the potato plant canopy. Two plants were papered for each nozzle. (text version)

Droplet Size

To answer the first question, we compared coverage from two hollow cone nozzles. The TeeJet TXR80028, which creates a fine/medium droplet size, and the TeeJet AITX8002VK, which is air-induced and creates a coarse/very coarse droplet size. In both cases the boom was approximately 50 cm (20 in) above the top of the crop, traveling at 10 km/h (6.2 mph) and spraying about 110 L/ha (~11.5 gpa).

Generally, coarse droplets tend to move in a straight line, and are not easily deflected by moderate wind or travel speed. Fine droplets don't move very far, and move erratically depending on the forces acting on them.

Figure 2 shows the coverage results in each position. We see that finer droplets were better able to penetrate the canopy than the coarser droplets. We also see that under-leaf coverage was difficult to achieve. This figure may appear to favour small droplets, but be aware that fine droplets are prone to drift, and should not be used without making every effort to prevent off-target movement. Shrouds, low wind, and slower ground speed help.

Figure 2 - Droplet size comparison. Cumulative spray coverage achieved in four positions, on two plants per nozzle. Low-to-no coverage = 0. Moderate coverage = 1. High-to-excessive coverage = 2.

Figure 2 - Droplet size comparison. Cumulative spray coverage achieved in four positions, on two plants per nozzle. Low-to-no coverage = 0. Moderate coverage = 1. High-to-excessive coverage = 2. (text version)

The best drift-mitigating option would be air-assist on the boom, which would also improve under-leaf coverage. It's disappointing that there aren't more self-propelled sprayers that offer this feature.

Volume

To answer this question, we compared coverage from Syngenta's potato nozzles. The VP04 (gold) was operated at 1.5 bar (22 psi) and sprayed 135 L/ha (~14.4 gpa). The VP05 (Orange) sprayed 180 L/ha (~19.2 gpa) at the same pressure. The boom travelled at 10 km/h (6.2 mph) at approximately 50 cm (20 in) above the top of the crop.

Generally, raising the volume-per-hectare translates to improves coverage, but at some point there is a diminishing return. Imagine comparing coverage between 1 L/ha and 100 L/ha - there would be a big difference. Now imagine comparing 500 L/ha to 1,000 L/ha - probably not much difference, because drenched is drenched.

According to Figure 3, the higher volume did not improve coverage. In fact, the lower volume appears to have superior coverage. It's possible that at this stage of growth, our 150 L/ha was close to the threshold of diminishing return. Note the absence of under-leaf coverage.

Figure 3 - Spray volume comparison. Cumulative spray coverage achieved in four positions, on two plants per nozzle. Low-to-no coverage = 0. Moderate coverage = 1. High-to-excessive coverage = 2.

Figure 3 - Spray volume comparison. Cumulative spray coverage achieved in four positions, on two plants per nozzle. Low-to-no coverage = 0. Moderate coverage = 1. High-to-excessive coverage = 2. (text version)

Spray Angle

To answer this question, we used HyPro's Guardian Air nozzle (GA11003), which is a 110° wedge-shaped flat fan that we alternated between 15° forward and 15° backward on the boom. We compared it to Greenleaf's TADF nozzle (a blue and yellow 02), which is an asymmetrical, 110° twin-fan tip, where one fan is at 50° and has a higher flow compared to the second fan at 10°. We also alternated this tip forward, and backwards. Both tips sprayed 100 L/ha (10.9 g/ac) from a boom travelling 10 km/h (6.2 mph) and about 35 cm (~14 in) from the top of the canopy.

Personal experience in soybean, field tomato and ginseng has shown that the spray angle does not have much bearing on crop penetration in a broadleaf canopy. This isn't a surprise, because the tips were originally intended for vertical targets, such as wheat heads. Figure 4 seems to bear this out.

Figure 4 - Spray angle comparison. Cumulative spray coverage achieved in four positions on two plants per nozzle. Low-to-no coverage = 0. Moderate coverage = 1. High-to-excessive coverage = 2.

Figure 4 - Spray angle comparison. Cumulative spray coverage achieved in four positions on two plants per nozzle. Low-to-no coverage = 0. Moderate coverage = 1. High-to-excessive coverage = 2. (text version)

Take Home

It is important to note that this comparison of nozzles and spray variables isn't definitive. It is only a subjective demonstration. Further, coverage doesn't necessarily imply efficacy: Just because you have more coverage doesn't mean you didn't already have enough to do the job. However, there are a few points to be made:

  • Smaller droplets penetrate dense canopies well, but are prone to off-target drift.
  • Under-leaf coverage is difficult to achieve without air-assist, or some way to position a nozzle beneath the canopy.
  • Higher volumes result in improved coverage, but only to a certain point. Volume should reflect the stage of growth.
  • Spray angles do not seem to have any bearing on broadleaf coverage.

This picture shows the attendees in a potato field seeing spray coverage in situ.

Figure 5 - Discussing spray coverage in Alliston

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