DCDs wanted back
FERTILISER COMPANY Ballance says it shares farmers’ sentiment that nitrate inhibitors containing DCD should be allowed back on the market.
THE WITHDRAWAL of DCD has removed one of the tools in the box for reducing nitrate (NO3) in water courses and emissions of nitrous oxide (N2O). Ideal conditions for N2O emissions are high concentrations of NO3 in the top 20 cm of wet soils with restricted aeration. High concentrations of NO3 are the result of high inputs of N.
Reduce NO3 in soil.
The most obvious strategy to reduce soil NO3 is to reduce total N input. A reduction in fertiliser N will reduce NO3. But grass yield will also be decreased if this strategy is adopted over the whole season. Tactically, it is advisable to reduce applications of N in spring, so that more effective use is made of mineralised soil organic N, and also to reduce fertiliser N from late-summer onwards to make efficient use of N deposited in dung and urine during previous grazing cycles. Another option is to sow grass cultivars which grow well in early spring and are capable of utilising soil N at relatively low temperatures.
Avoid anaerobic soils.
N2O emissions are highest from soils with restricted aeration. Under these conditions soil bacteria strip oxygen from NO3 by denitrification. Areas around gateways and water troughs are ‘hotspots’ for N2O emissions so mechanical aeration of compacted areas is desirable.
Reduce N excretion by the animal.
The more N that is deposited on pasture in animal manure, the higher the concentration of NO3 will be in the uppermost layer of soil. Excretion of N in urine and faeces is directly related to dietary N intake, so a reduction in N intake will reduce soil NO3. Higher sugar grasses tend to have lower concentrations of protein than conventional grasses; they have a dual role in not only limiting total N intake but also in enhancing efficiency of use of dietary N in the rumen by increasing the supply of readily fermentable sugar to the microbes.
The concentration of crude protein (CP) in grazed pasture is often substantially higher than that required to support milk production. For example, the average concentration of CP in weekly pre-grazing grass samples at the South Island Dairy Development Centre in 2011/12 was 24.3% of the dry matter with a range from 16.8% to 33.1%. The lowest value in the range is about right for a dairy cow diet. Interestingly, the lowest pre-grazing CP concentrations were in early spring, possibly reflecting lower soil N concentrations than later in the season.
New grass varieties bred for lower CP concentration and/or higher efficiency of N use are options for the future. Another possible option is to increase pasture covers, i.e. offer more mature herbage than the recommended three-leaf stage of growth – 4500kg DM/ha instead of 3500kg DM/ha. This option may only work in early spring when there is little stem in the sward and when efficiency of pasture utilisation is high. When stem is present and when grazing residues are high the animal selects leaf in preference to stem and total N intake is higher than the average for the pasture. This grazing selection differential for CP means that at an average CP of 24% the CP in herbage eaten may be anything between 27% and 36%, lower at high efficiency of pasture utilisation and vice versa.
An effective way of reducing total intake of CP and N excretion is to offer dairy cows about 5kgDM per head of a low protein supplement daily.
Grazed pasture intake will be reduced so stocking rates should be adjusted accordingly. Maize or whole-crop wheat silages are the most appropriate supplements. Palm kernel and soyabean meal are not suitable as pasture supplements because their concentrations of CP are far too high and there will be no reduction in total daily intake of CP.
Therefore a suitable strategy for life after DCD is to reduce fertiliser N input and provide a low protein supplement throughout the grazing season, at the same time maximising efficiency of pasture use by careful and frequent adjustment of stocking rate.
