Catch crops for cleaner freshwater

A catch crop is a short-term crop established between two main crops or as part of a pasture renewal programme, primarily to take up nitrogen and reduce nitrate leaching.

Catch crops come in a variety of options to suit different soils, climates and farm systems.

The most common options are cereals because they are generally winter active and can establish under cold conditions. For example, oats, ryecorn, triticale make good catch crops. Italian ryegrass can also be used and is well suited to being sown with a cereal like oats.

While catch crops are not new and have been used in arable systems for centuries, the novel use for them in New Zealand recently is following a winter grazed forage crop to reduce the fallow period. This means they are typically sown during the coldest times of the year (winter), in the key winter forage cropping regions (e.g. Canterbury and Southland).

Catch crops’ primary objective is to catch excess nitrogen in soil that may otherwise be lost through leaching. Catch crops act like sponges. They absorb nitrogen by soaking up water and nitrogen during late winter and early spring.

Intensively grazed winter forage crops are a known hot spot for nitrogen leaching. Catch crops have been found to reduce this leaching by up to 50 percent.

Soil temperatures need to be over 2°C and it is best to sow them as soon as winter grazing has ceased and when other weather, soil and operational conditions allow.

Research shows that sowing oats immediately after grazing has the greatest impact on reducing nitrogen losses. With every month that sowing is delayed, there is a notable reduction in their effectiveness.

While July-sown oats have been sown to be very slow to come away, they are still capturing a significant amount of nitrogen through their root system, despite the small amount of above-ground foliage during the key risk period (winter-early spring).

At paddock scale, oat crops capture up to 100 kg N/ha by the end of the leaching period – this is nitrogen that might otherwise be lost.

While our research has found catch crops to be beneficial to some farm systems, they are not a silver bullet solution.

For example, not every farm is suited to cultivating an oat crop and taking it through to green-chop silage (the ideal harvest timing to maximise both yield and quality; typically 8–10 t DM/ha, at approx. 11 MJME/kg DM), as they can delay the sowing of subsequent winter forage crops. However, the research has found that this is balanced out as, overall, a paddock will grow more feed in total over a 12-month period.

A wet winter-spring can also mean that some paddocks cannot be accessed to plant a catch crop. In addition, they tend not to be well suited to a 2-year fodder beet rotation, as the optimum harvest time of the catch crop is past the essential sowing window for fodder beet, meaning the productive potential of the catch crop is not harnessed. It is about finding the right balance to suit conditions and the farm system.

We recommend you sow catch crops early, as soon after grazing as possible, and well before mid-October when draining stops. Soil temperature must be 2°C or more and we recommend you use a minimum or no-till technique (depending on the soil conditions) to establish the crops and ensure good soil-to-seed contact at drilling. A desirable plant population target is 300 plants per square metre (a seeding rate of 110-120kg/ha for oats).