31 May 1996

WHY IRRIGATION PAYS WITH BEET

Sugar beet responds well to irrigation in dry years. By following a few simple rules, yields can be maximised at little cost. Robert Harris finds out how it is done

IN an average year, about 1m tonnes of sugar beet are lost through lack of water. Yet very little of the national crop is irrigated despite the fact that 70% is grown on sandy soils.

Just over 2000 growers can water all or part of their sugar beet, some 35% of the crop area. In practice, less than half of that is irrigated in any one season, says Simon Fisher, agricultural development officer at British Sugars Bury St Edmunds factory.

"Other higher value crops like potatoes, onions and carrots take priority. But if water is used carefully, even hard-pressed growers can irrigate beet and achieve considerable returns."

On average, sugar beet crops grown on sandy soils, irrigated to maintain soil moisture levels at the minimum deficit (see table), yield an extra 2.5t/ha (1t/acre) of adjusted weight roots for every 25mm (1in) of water applied, he says.

That is worth £100/ha (£40/ acre) at A and B quota prices.

Over the past 24 years, during which the average water deficit has been 55mm (2.1in) a season, irrigation would have boosted returns by £200/ha (£80/acre), he explains.

Applying 25mm costs £35-50/ha (£14-20/acre) so growers stand to make money, Mr Fisher maintains. It is fixed costs that make or break the equation. These vary from farm to farm.

"The actual cost depends on the type and age of equipment used, the water source, and the irrigation area. For example, fixed costs for a new hosereel/ borehole system might amount to £70/ha."

At that level, irrigating beet alone may not pay, or at best break even. "On the other hand, if a grower is using irrigation primarily for other crops, fixed costs will be a fraction of that."

Another way to reduce costs is to apply less water. Irrigating crops at half the rate needed to maintain limiting deficits means crops use a higher proportion of the water.

Run off is minimised. And if rain follows irrigation, the soil can absorb more before returning to field capacity. That prevents drainage, saving water and nutrients.

While overall yield increase may be less, response per unit of water is often much greater, says Mr Fisher. Yield increases of 3-4t/ha (1.2- 1.6t/acre) per 25mm of water are achievable, worth £140-160/ha (£56 – 65/acre) at A and B quota prices.

Twice the area can be irrigated in the same time, good news for growers who have other crops to irrigate and have to limit sugar beet applications. "It is much better to irrigate all of the crop a little, than to concentrate on one particular area."

Besides higher yields, irrigating sugar beet has other advantages too, he notes. Yields are more consistent, so the area of crop needed to meet contract tonnage can be estimated more accurately. There is also less risk of drought-prone root rots like fusarium, reducing rejection risk.

Harvest losses are lower, since roots are less likely to break in moist, irrigated soils. Losses can be high in dry seasons especially during early liftings. It is also easier to create good seed-beds for following crops, he maintains.

FOR the best results, water needs to be applied at the right time and in the right amounts, says Mr Fisher. "There are three key points to remember. Start early, operate to a schedule and apply water sparingly."

The crop typically uses 460mm (18in) of water from May to October. Crop demand is lower in the early months due to limited crop cover, and there is usually enough moisture in the soil to meet it.

June to August is the peak time, when long sunshine hours coincide with rapid crop growth, and crops cannot yet tolerate high moisture deficits. "An early drought which delays full leaf cover can reduce yield more than a late one. Irrigating to ensure full leaf cover as early as possible is particularly valuable."

It is particularly important to make an early start on highly fertile fields or those treated with organic manures, he stresses. "A late start can be detrimental to quality. The sudden uptake of nitrogen will cause high amino-N levels which will still be present in early-harvested crops."

Irrigation need should be monitored using a schedule, like that operated by BS and Levington Agriculture, he stresses. "That will ensure the crop only gets what it needs and when it needs it."

The scheme costs £93 for each site, which may be a field or a particular soil type. Growers send in a soil sample at the start of the season, and each week report crop cover and rainfall or irrigation applied. They are then notified of the soil moisture deficit, the amount the crop can stand, and the irrigation needed to correct the imbalance.n


Limiting soil moisture deficits for sugar beet (mm)

Coarse sandLoamy sandSandy loam

Mid-June253035

Mid-July354050

Mid-August506075

Mid-September6575125

(Source: British Sugar)

Irrigating at half the recommended rate reduces waste and cost but still boosts yields in dry years.