Can DCD Increase Grass Dry Matter Production?

An extended grazing season is a key component of low cost animal production systems in Ireland. Increasing nitrogen (N) availability in spring can increase herbage supply for grazing. Spring N is usually supplied by chemical fertiliser or slurry. Urine and dung are also sources of N in grazed pastures (Figure 1). The N concentration under a single patch is high, equivalent to an application rate of up to 1,000kg N/ha (Whitehead, 1995). The quantity of N excreted exceeds grass demand for N and, therefore, the majority of this N is often lost by nitrate (NO3-) leaching or nitrous oxide (N2O) emissions.

Nitrification inhibitors

Nitrification inhibitors are being investigated in Ireland, New Zealand and elsewhere as a strategy to reduce NO3- leaching and N2O emissions. One such nitrification inhibitor is dicyandiamide (DCD), which slows the conversion of ammonium (NH4) to NO3- by temporarily blocking the enzymes on the Nitrosomonas bacteria responsible for the first step in the nitrification process (Serna et al., 1995).

NO3- is readily taken up by growing plants, but if surplus NO3- is available, such as under urine patches or when plant growth slows during winter, it is likely to be lost through leaching. DCD has been shown to reduce NO3- leaching and N2O emissions in New Zealand and in Ireland (Moir et al., 2007; Dennis et al., 2008). As well as reducing N losses, increased herbage dry matter (DM) production has been observed in New Zealand following the application of DCD in autumn and early spring (Di and Cameron, 2002; Moir et al., 2007). To date, no work has been undertaken in Ireland to examine the effects of DCD on herbage DM production.

The efficacy of DCD is influenced by soil temperature. At 8ºC DCD has a half-life of approximately 90 days, and at 20ºC or greater the half-life is reduced to approximately 20 days (Di and Cameron, 2004). As a result, under Irish conditions DCD is most likely to be effective during the late autumn to early spring period. DCD is ideally applied to swards as a fine particle suspension (FPS) to maximise spatial coverage.

Herbage DM production experiments Two experiments are being undertaken to examine the effect of DCD application on herbage DM production. The first experiment is examining the effect of applying DCD in late summer and early autumn (July, August and September) on herbage DM production at Moorepark (free-draining soil) and at Johnstown Castle (moderate drainage capacity). Artificial urine (urea and water mix) was applied to grass plots in July, August or September at a rate of 0 or 1,000kg N/ha.

DCD was applied at a rate of 0 or 10kg/ha as a single application within 24 hours of urine application. Herbage DM production was measured every four weeks from July 2009 to June 2010. Annual herbage DM production (July to June) was significantly (p<0.01) increased at Moorepark when DCD was applied to plots receiving urine in August and September by 15 per cent and nine per cent, respectively, compared to when DCD was not applied.(Figure 2)

There was no significant effect of DCD on herbage production at Johnstown Castle. High soil temperatures in July reduce the effectiveness of DCD in slowing the conversion of NH4 to NO3-, and naturally high grass growth rates at this time of year reduce the grass growth benefits that may occur from increased N availability. At Johnstown Castle, wet waterlogged soils in 2009 may have reduced grass growth, resulting in no effect of DCD or urine application.

The second experiment was undertaken at Moorepark on two contrasting soil types (free-draining at Moorepark and moderate to heavy soil at Ballydague Farm) over a two-year period. Artificial urine (as above) was applied to grass plots in September, October or November or not applied. DCD was applied at rates of 0, 5 or 10kg/ha as a single application within 24 hours of urine application, and a second application of DCD was applied to half the plots 90 days after the initial application. Herbage production was measured every four weeks from February to November.

In general, urine application increased spring (up to April) herbage DM production at both sites, and increased annual herbage DM production at Ballydague. DCD had little effect on herbage DM production during the experiment. Applying DCD in November only, and in November and again 90 days later, tended to increase herbage DM production in spring of Year 2 on both soil types compared to other application times, but there was no effect in Year 1. This may be due to low rainfall during the December to February period of Year 2, combined with cold temperatures, which increased the efficacy of DCD and, therefore, increased N availability at the commencement of grass growth in spring. There was no significant effect of DCD rate (5 or 10kg/ha) on herbage DM production.

In addition to measuring herbage DM production, in both experiments soil sampling to a depth of 10cm was undertaken on a weekly basis for the first two weeks post application and less frequently thereafter to day 84 post application to quantify the NH4:NO3- ratio in the soil. This data is still being analysed and, when this analysis is completed, will provide information on the efficacy of DCD in slowing the conversion of NH4 to NO3- in the top 10cm of the soil during a 2.5- to three-month period post application. Herbage chemical analysis will also give an indication of the effect, if any, of DCD on N uptake by the plant.

Conclusion

Reductions in N2O emissions and NO3- leaching observed by researchers at Johnstown Castle in other experiments (not reported here) indicate that DCD is effective in the soil. However, based on the herbage DM production data available from experiments to date, DCD does not have a large benefit in terms of herbage production (herbage production was only increased on a limited number of treatments on two experiments at three sites over two years).

High rainfall in autumn in Ireland may be washing the DCD and the NH4/NO3- on which it is acting beyond the rooting zone, and therefore the N is not available for plant growth. Soil analysis will provide information on the efficacy of DCD in the first 10cm of soil below the surface, a key area for N uptake by roots.