9 August 2002

Seed rate the way to lift marketable yield

High value crops have the

most to gain from precision

farming techniques, says a

leading Scottish grower who

is exploiting GPS technology

to boost the marketable

yields of potatoes.

Andrew Swallow reports

VARIABLE seed rate is the key to boosting marketable yield of potatoes on variable soil types, says a Scottish grower who is pioneering such precision farming techniques on his farm.

"Variable rate fertiliser isnt the big one, it is varying the seed rate," says Jim Wilson, of Hilton of Fern, Brechin, Angus, who has been yield mapping his potato crops since 1997.

That has revealed that even after all limiting factors within a growers control are removed soil type and topography still cause considerable yield variation across a potato field.

For example, north facing slopes have been found to consistently yield 10t/ha less than flat or south facing areas, while yields from thin soil on the top of humps can tumble to 25% of the maximum. That variation changes the size grade of the crop across the field.

"From uniform plantings, a 20t/ha crop is going to have much smaller tubers than an 80t/ha crop," he reasons. Hence, since 1999 Mr Wilson has been splitting fields into areas of low, medium and high yield potential based on previous experience, soil and/or yield maps.

"On rented fields I look at the EMI soil scan results, then walk the field and test dig and sample the different depths and types of soil that highlights. Ill also ask the farmer what different areas are like."

SAC or MAPP recommendations for seed rates according to yield potential are then programmed into the planter.

In low yielding areas, less seed is planted to give each plant more space, water and nutrient with which to grow. In high yielding areas, more seed is planted to limit tuber size by competition.

"The whole point of changing the seed rate is to have the whole crop reaching the same size grade at the same time."

Results from a field of Estima grown for seed in 2000 at Hilton of Fern show the potential of the system (see table). Seed rate was reduced by 10% in the low yield potential area and increased by 10% in the high rate area.

"We didnt lose much marketable yield in the low area but have increased marketable yield substantially in the high seed rate area, without changing the overall seed rate for the field," he says.

"Now, we are changing seed rates by plus or minus 15% because I dont think 10% was enough. There were still too many tops in the high population area."

Lime and fertiliser applications are also varied across fields according to sample results. Sampling for pH is done on a 50m grid to give 12 sub-samples for each hectare sampled.

"One sample a hectare systems of sampling are a pet hate of mine. They never produce a repeatable result."

Variable lime applications avoid unnecessary application on already high pH areas which could exacerbate common scab, he notes.

Fertiliser is adjusted by volume of liquid injected as the planter travels across the field. For example, an 8.13.11 NPK solution might be used on seed crops at 1000-1500 litres/ha, hence varying the application from a low of 80kg/ha of N, 130kg/ha of P and 110kg/ha of K to 120, 195 and 165kg/ha, respectively.

For ware crops a 9.9.12 mix is variably applied on the planter with variable rate straight N applied as a top up if required.

On the rare occasions that irrigation is needed on Mr Wilsons seed crops, that too is variably applied according to soil type.

EMI scans give a measure of clay content which is a good guide to water retention, he explains. Where clay content is high less water is applied to avoid ponding, waterlogging and increased powdery scab risk. Conversely, where soil is sandy more water is needed to prevent common scab.

"Most modern reels can vary the speed on the run."

Siting of tensiometers or theta moisture-testing probes could also be targeted by soil type with the EMI maps, he suggests.

lJim Wilson is one of three directors of precision farming services company Soil Essentials (www.soilessentials.com). &#42

Variable rate results

Yield Seed 35-55mm % potential rate/ tubers/ change acre plant

Low 23,100 24 -4%

Med 25,600 25 0

High 28,200 29 +16%

Results of variable planted Estima for seed at Hilton of Fern, autumn 2000.

Field monitoring

Specially fitted load cells on Jim Wilsons Reekie Cleanflow weigh the potato crop as it is lifted. Data is logged by the same LH Agro GPS monitor as is used in the combine for cereal yields and to vary seed and fertiliser rates. "As I already had the yield monitor it has only cost me about £5000 to equip the potato harvester," he says.

Coming next week: Full report from the Potatoes in Practice event at SCRI Dundee.

PRECISIONPOTATOES

&#8226 Yield mapping since 1997.

&#8226 20-80t/ha yield range.

&#8226 Topography and soil causes.

&#8226 Seed rate varied to maximise target grade.

Angus-based grower Jim Wilson is using GPS-adjusted seed rates to boost marketable yields of his potato crops. More tubers/ha are planted on high yielding areas, less on low (inset).