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Variable rate fertilizer techniques not ready for manure application

Manure applicator technology needs more refinement before variable rate
(VR) fertilizer application techniques can be adapted to manure
application, according to a recent Canadian study.
Manure applicator technology needs more refinement before variable rate (VR) fertilizer application techniques can be adapted to manure application, according to a recent Canadian study.

The three-year project – conducted by Agra-Gold Consulting and Farmer’s Edge Precision Consulting, both of Manitoba – did show that VR fertilizer rates could be adapted to manure applications but “overall … did not achieve its fullest potential,” reported Scott Dick, project leader.

The $54,750 study – funded by the Manitoba Rural Adaptation Council (MRAC), the Manitoba Pork Council and producer participants – involved 13 producers, each of who provided about 150 acres of land, a sampling base that represented a cross-section of Manitoba cropping conditions. Manure was applied using the drag hose application method. Tractor operators varied application rates according to GPS field maps indicating differing nutrient requirements in different parts of the fields. The three different rate zones were determined using satellite imagery. The zones were then individually soil tested to record the variability across zones and to determine the optimal nutrient application rate.

A total of 4632 acres was zoned and mapped for VR application with 3466 acres receiving the VR applied manure.

“Although we received generally positive feedback from producers on the methodology and agronomic benefits of the management style, producers had difficulty in justifying the cost of this increased management,” said Dick.

According to a survey presented to the participating producers, it was estimated the cost of applying manure during the study using VR techniques was about $8 per acre.

Even so, Dick says adapting VR techniques to manure application appears to have good potential but still requires refinement in the technology on the applicator.

“The technology used by commercial drag-line applicators today cannot efficiently and practically carry out this type of application,” he reported. “(The equipment) just couldn’t accurately and efficiently vary application rates to match specific nutrient needs.”

Equipment operators could only adjust application rates by speeding up or slowing down the tractor speed. “On a half-mile run,” he said, “that could mean between one and three speed adjustments – not a major problem – but enough to affect accuracy.”

Another limitation was a lack of “on-the-go” nutrient sensing to determine soil nutrient levels. Instead, planners had to rely on past soil analyses to determine application rates.

“The only way to accurately measure nutrient levels was to send a sample to the lab after application,” Dick stated.

Finally, unlike commercial fertilizer applications where nitrogen and phosphorus can be applied independently to correct soil deficiencies, the nitrogen and phosphorus mix in manure is fixed. Operators can’t adjust individual N and P application rates as they go.

“(By) varying the rate based on nitrogen requirements … we unnaturally induced more phosphorous variability into the field,” the report stated.

“Though increases in technology and management can help to overcome these hindrances, it is our opinion that variable rate manure applications are not commercially feasible today.”

The study does outline ways producers can use precision farming techniques to increase yields and reduce environmental risks. The researchers, which also included Cliff Loewen and Wade Barnes, suggested producers apply a base rate of manure using conventional techniques then follow up with a variable rate application of commercial starter fertilizer at seeding time.

“This would allow the agronomist time to receive the manure analysis back from the lab and give a more accurate as-applied application report before the commercial fertilizer prescription map is developed,” the report stated.

A complete report of the project – Applying Manure to Defined Management Zones Using Precision Farming Techniques – is available at the Manitoba Livestock Manure Management Initiative’s website – www.manure.mb.ca – filed under Completed Projects.