Catch crops and feeding strategy can reduce the risk of nitrogen leaching in late lactation fodder beet systems

Autumn grazing of high-yielding forage crops such as fodder beet (FB) has increased in popularity in many regions of New Zealand. During grazing, large amounts of urinary nitrogen (N) is returned to the soil which is at risk of leaching. This study evaluated using two different catch crops of oats or Italian ryegrass (‘Italian RG’) to reduce the risk of N leaching both after FB grazing by lactating dairy cows and after lifting (i.e. un-grazed FB) in autumn. During FB grazing, cows grazed either (i) high [40% of diet dry matter (DM), with pasture; FB40] or (ii) low (25% of diet DM, with pasture; FB25) FB allocations. Both Italian RG and oats reduced soil mineral N (0–90 cm depth) by 16%–37% by August, and 53%–58% by October, compared with a fallow control (115 and 100 kg N ha⁻¹ for August and October, respectively). The greatest reductions in soil mineral N were from oats, largely attributed to higher DM yields resulting in greater N uptake. Furthermore, the total amounts of soil mineral N in the soil after grazing differed between FB allocation treatments and lifted FB (FB25 > FB40 > lifted FB), in proportion to urinary-N inputs and associated dietary N intake.