Achieving the correct criteria for the market is an essential part of crop management for any potato grower, but for Perthshire farmers, Bob Strachan and his son, Rob, critical tuber size is all important for his 40ha crop of Cultra, which is destined for the pre-pack market.

Whilst skin finish and shape are largely determined by the varietal characteristics of the ‘breed’, the size can be easily manipulated by the grower using different management tools.

Spring barley, broccoli and vining peas make up the balance of the rotation at his 240ha Blairgowrie farm (part of the acreage is rented) but it’s the potato crop that is the predominant feature at Lochlands Farm and reasonable yields can be achieved on its irrigated, silty loams.

Maris Piper traditionally made up 30% of the crop, but difficulty in controlling common scab meant a switch to Cultra for the entire acreage, which has proved an easier variety to manage.

Achieving an ideal size for their buyers (40% baking grade over a 65mm riddle), meant that careful attention to detail, with test-digging regularly and monitoring of the tubers is an important part of the process prior to desiccation.

Flailing of the canopy has been the preferred method of haulm destruction and timing is crucial, usually at first signs of early senescence. Ideally, this should be carried out leaving stalks of 20-30 cm, with the haulm being deposited in the bottom of the ridge away from the exposed stems.

“We usually start around September 1,” said Mr Strachan. “Flailing for us, followed by an application of Spotlight Plus (60g carfentrazone-ethyl) definitely produced the best results. We have used both acid and diquat in the past, but with the withdrawal of acid, this combination suits our system.”

Spotlight Plus, a contact desiccant, is applied at 1.0l/ha in 300l/ha water, preferably within two days after flailing and in sunny, dry conditions. Desiccation then occurs rapidly, being complete usually within 10-14 days, according to Mr Strachan.

This product is said to be rainfast in one hour and has approval for all soil types, with only a seven-day harvest interval and no buffer zone.

Said to be excellent at preventing regrowth – Mr Strachan only usually carries out one application – occasionally crops might benefit from two doses to complete desiccation under exceptional and prolonged growing conditions.

In this instance, 0.6l/ha should be used about a week after the first application. Longevity of the crop can be an issue, but he also used other management tools to reduce this risk.

Limiting nitrogen application on the crop is another useful means of regulating the amount of luxury canopy growth, which in turn allows better efficacy from Spotlight Plus. A total N application for the crop generally sits at about 190-200kg/ha and irrigation is applied when necessary ( or this year, as allowed) with anything up to six, one-inch applications.

Mr Strachan said that good management practice and close attention to detail helped get the best from the product and maximised return at the same time.

“We hope to achieve an adequate skin-set about 21 days after application,” concluded Mr Strachan. “For us, flailing of the crop, followed by Spotlight Plus, ticks all the boxes without having to resort to diquat.

“Working with a variety like Cultra, using this system, seemed to deliver us the best returns. Spotlight Plus is an integral part of that operation for us now – we’re very happy and see no reason to change.”