Does sowing an oats catch crop reduce nitrate leaching from urine deposition following simulated winter forage grazing? - a growth chamber experiment
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Growing winter forages, such as brassicas and fodder beets, is a common feed strategy to build body condition for non-lactating cows, prior to calving, in the temperate regions of New Zealand. However, the large quantities of feed grown, ~14-20 tonnes ha-1, and resulting high stocking rates, means large volumes of urine are deposited during grazing, over relatively small areas, on cool, bare and often wet soils. These conditions create a high potential for large N leaching losses. Sowing a catch crop might be one way to mitigate this potential but most catch crops are sown in late-summer/autumn, not late winter/spring, so this study’s objectives were to examine the potential of a cold-tolerant cereal, oats, to establish and take up soil N over this period.
A study was undertaken using free-draining repacked soil cores in two climate-controlled growth chambers at 6° and 10°C, and at two lighting intensities, 5 and 10 MJ m-2, reflecting the mean soil temperature (0-100 mm) and light levels experienced in Canterbury, New Zealand, around late winter and mid spring, respectively. The cores all received an application of 15N-labelled urine (350 kg N ha-1) and two weeks later, oats were sown in half the soil cores, with the remainder left fallow.
Sowing oats at 6° and 10°C both proved highly effective in reducing nitrate leaching loss by around 33% and 75%, respectively, compared to fallow treatments. Soil temperature, however, was the chief driver of oats development, growing an order of magnitude more rapidly at 10°C and reducing nitrate leaching loss quickly through a combination of increased N uptake and reduced drainage. However, ammonia oxidising bacteria (AOB) abundance in 10°C fallow treatments increased more rapidly after urine application than at 6°C, meaning soil nitrate concentrations rose quickly, leaching twice as much nitrate as the 6°C fallow treatments after 90 days. Greater plant growth (10-times more) at 10°C than at 6°C, 75 days after sowing, was offset to some degree by greater N% and N content in the plant stems and roots at 6°C (2-4 times more). Higher light intensity increased evaporative water loss in both 6° and 10°C chambers, reducing drainage and nitrate leaching, and increasing the rate of oats development at 10°C.
The sowing of oats as a catch crop following simulated winter forage grazing conditions shows promise as an effective means to limit nitrate leaching. Further research, however, is needed to show that a catch crop can be sown successfully in most years at farm paddock scale across a range of soil conditions.
KeywordsSoil nitrogen Crop development Solar radiation Fodder crop Avena sativa
The authors wish to thank the Pastoral 21 consortium partners (DairyNZ, Fonterra, Dairy Companies Association of New Zealand, Beef + Lamb New Zealand and the Ministry of Business, Innovation and Employment) for funding this study. We also wish to thank Mr. Simon Kelly, Ms Jie Lei, Ms. Shelagh Bassett, Mrs. Jeneth Hendry, Mrs. Emily Huang, Mrs Carole Barlow and Mr. Barry Anderson for skilled technical assistance.
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