A model developed by Cambridge University Farm that predicts the nitrogen requirements and final yields in potato crops could help growers better plan harvesting schedules.

The model will tell growers if and when a potato crop will achieve their desired grade, helping plan lifting timetables, the model’s architect CUF’s Marc Allison says.

For example, a simplified version of the model tested in the USA last year pinpointed a grower who was in danger of going over size with his crop.

“The prediction enabled him to burn off earlier and to find an alternative market for the potatoes that did go over size.”

The model will also be very useful as a planning tool to show growers where they may have lost potential yield in previous crops, he says.

Potentially it could also be used as an indicator on whether a top dressing will be required on certain crops during the season.

“The jury is still out on this approach, but it may be the window of opportunity is larger in more indeterminate varieties such as Russet Burbank,” says Dr Allison.

The key to the model is the relationship between dry matter yield, light and nitrogen.

To increase yield, the crop needs to absorb more radiation [light], he explains.

“We can use nitrogen to modify the persistence of the potato canopy, so growers can attain the maximum marketable yield according to size specification.”

But it not quite as simple as just increasing nitrogen dose and destroying the haulm when appropriate, he says.

In fact, increasing nitrogen can decrease the tuber yield as a percentage of the total yield of the haulm and tubers.

“That means growers may be worse off.”

The critical factor is understanding how the crop is using the nitrogen within the haulm and tubers in relation to the amount and intensity of light being absorbed.

“By understanding that relationship we can estimate peak haulm nitrogen uptake and how quickly the nitrogen pool is being depleted, and use that to predict yield.”

To make an accurate prediction of tuber yield and size distribution, the model needs data such as crop emergence, the date of tuber initiation, ground cover development, and the amount of incoming radiation from a local weather station, together with a determination of haulm and tuber nitrogen uptake and the number of tubers from a crop sample taken 45 days after emergence.

“Within the limits of error it is a good prediction for most varieties,” says Dr Allison.

However, factors that limit the life of the canopy, such as disease, pests, water and heat stress, and compaction will decrease accuracy, he notes.

“It should be used in combination with test digs, at least until more validation has been carried out.”

Depending upon progress in 2006, Dr Allison anticipates the model will fit into a potato agronomy package available to growers hopefully in the near future.

fwarable@rbi.co.uk