Crops for Nitrogen Capture

Feeding of livestock on kale or fodder beet in winter is a known hot spot for nitrogen leaching losses. But recent trials at Plant & Food Research in Canterbury show that leaching losses can be significantly reduced when a “catch crop” is established directly after grazing in these conditions. 

Grazing on winter forage crops is an important management strategy in many New Zealand regions where there is commonly a pasture feed deficit. 

However, the high stocking densities that are associated with these systems can have negative environmental effects, such as excessive nitrogen leaching losses to groundwater. 

Typically the land remains fallow for three to five months after winter grazing until a new crop is established in spring. The risk of N leaching is greatest during these fallow periods due to the absence of a plant “sink” to capture the residual N.

An earlier lysimeter study indicated that sowing a catch crop could reduce the amount of N leaching loss by 18-46% in a winter forage crop system. The trial also found that the earlier the crop was sown, the greater the reduction in leaching. 

This field study, undertaken by scientist Brendon Malcolm at Plant & Food Research Lincoln, investigated the potential biomass production and reduced risk of N leaching from establishing oats as a green-chop catch crop after winter-grazed forage kale. 

The aim of Brendon’s research was to answer some of the agronomic questions posed by earlier work on catch crops, such as, how early can a catch crop be established under field conditions during winter grazing and what is the production potential?   Additionally, what are the effects of establishing a crop in cool winter temperatures and unfavourable soil conditions - including pugging and left-over grazing stubble? 

Oat crops were direct-drilled early (1 July 2015) and late (1 August 2015).   They were managed under high and low N load conditions, representing urine patch and inter urine-patch areas. 

By establishing oats as a catch crop in winter soil profile, mineral-N was reduced by up to 86% in the high N treatments compared with just leaving the paddocks to lie fallow after winter grazing. 

These results indicate the technical feasibility of direct drilling oats as catch crops immediately after winter forage grazing, with likely additional production and environmental benefits such as the reduced risk of N leaching losses. 

Brendon says the amount of benefit will depend on the rate of growth of the catch crops - how quickly they get established and the seasonal rainfall patterns. 

He says they have discovered that establishing a catch crop is possible during a traditionally cool period and can offer yields of 6-12 t DM/ha by the end of November. 

A second round of trials are focusing on extending the knowledge gained from the 2015 trials. These trials are focusing on getting a greater understanding of the water and nitrogen movement throughout the soil profile – and how a catch crop mops up what is an initially high load of N. 

The other trial is looking at the practicalities of crop establishment in winter as well as looking at other catch crop options, aside from oats which have been used up until now.