March 2008
Measuring for consistency can help manage tuber profile

Seeding rate is often measured by the number of trucks used; if X number of trucks have been used, Y number of acres should be seeded, and vice versa.

But the size of the seed pieces and the spacing of the planting could affect the total yield and profile of the crop. Researchers at the University of Idaho sought to find out if seeding a field at 20 cwt. would give the same yield as seeding the field at a 13-inch spacing.

The seeding rate study used Russett Norkotah, Ranger Russett and Altura varieties hand-cut to 1.5, 2.25 and 3 ounces. The three size treatments were then planted at a spacing of 8, 12 and 16 inches. A seeding rate of 20.4 cwt. was the same for 1.5-ounce seed planted at 8 inches, 2.25-ounce seed at 12 inches and 3-ounce seed at 16 inches.

Yield was affected by the seed size and spacing, however. The largest seed planted at the shortest spacing produced the highest yield. As the size of the seed goes up, the yield goes up, and as the spacing gets wider, the yield goes down, said Bill Bohl, Extension educator with the University of Idaho.

But generating a higher yield may make the most economic sense. Planting 3-ounce seed pieces at 8 inches resulted in almost 41 cwt. more per acre, which amounts to a return of 9 cwt. for every 1 cwt. of seed. The most efficient return is around 20 cwt. of seed per acre, Bohl said. For fresh market Norkotahs, returns were around $2,900 at 20 cwt. of seed planted per acre.

The profile of the harvest can be affected by the seed size and spacing, which may be more important to contract growers, which is why the Ranger and Altura varieties were included in the trial. But at 20 cwt. of seed per acre, there was no difference in the yields of U.S. No. 1 potatoes.

As you’re planting bigger seed at wider spacing, you tend to get bigger tubers,” Bohl said. “We can affect the tuber profile by the size of seed.”

The problem with fitting a study like this to real-world conditions is there is no such consistency in seed piece size.

“Uniform spacing is hard to achieve when you don’t have uniform seed piece size,” Bohl said.

One grower that he works with in Idaho had a planting efficiency of around 60 percent, and couldn’t figure out why his spacing was off and thought it was a problem with the planter. Bohl suggested he switch with the employee overseeing the seed cutting operation, and the grower’s efficiency increased to 80 percent because he was watching the cutters and making sure the seed sizes were uniform.

He recommends growers carefully develop criteria for judging what tuber profile and yield they are looking for to determine the seed piece spacing. Once planting begins, he said they should check 15 feet to 20 feet of every row three or four times a day to make sure the planter is getting the desired spacing and depth.

Seed Depth and Hilling

Some varieties have a tendency to grow up or out, which can cause greening to exposed tubers. It would make sense to plant seed deeper, Bohl said, but that would significantly reduce yields.

“So if anyone has a problem with too many tubers in storage or too many coming out the field, just plant your seed deeper. You’ll be done earlier in the season, too,” he joked.

To reduce the chance of greening, Bohl studied the hilling and planting depth. Seed pieces were planted at 6 inches, at 3 inches and hilled to 6 inches and 9 inches at emergence, planted at 6 inches and hilled to 9 inches at emergence, and some were hilled postemergence.

Hilling postemergence significantly reduced total yield and U.S. No. 1 yields, while the emergence treatments resulted in no significant differences. The treatment that was planted at 3 inches and hilled to 9 inches had significantly less greening, but also had the lowest total yield and U.S. No. 1 yield. There’s also the question of labor, with few growers likely to manually hill 6 inches of soil over their acreage.

“Who wants to hill 6 inches over a 1,000 acres?” Bohl said. “You’re never going to get 6 (inches).”

It’s possible to get about 3 inches, but there was no significant difference in reducing greening, he said.

The shape of the hill could also be a factor, so Bohl looked at four hilling treatments to see if shape could affect greening. This study, using the Summit and Defender varieties, found no significant differences in the shape of the hill on yield. U.S. No. 1 yield was lower for the plant and drag method in Defender, but didn’t affect Summit.

The results indicated that normal hills were better for preventing greening, with the shape or height of hills having negative or few effects.

“The best thing to do is watch the planting depth. Measure 6 inches from the top of the hill to the top of the seed piece. That’s about all you can do to reduce greening,” Bohl said.

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