Efficient nutrient use can enhance production and environmental targets

The adoption of practices that enhance the efficient use of nutrients on sheep farms will have positive implications in increasing farm profitability while also enriching the sheep sector’s positive sustainability and environmental credentials.

This was the overriding theme of a presentation delivered by David Wall, Teagasc, at Tuesday evening’s Teagasc lowland sheep conference in Killarney. Wall started by setting the current scene where agriculture is facing huge external pressures to reduce greenhouse gas (GHG) emissions and ammonia emissions, while also improving water quality.

Main offenders

Nitrate (NO3-N) leaching from agricultural soils is highlighted as an important contributor to water quality. The gas, nitrous oxide (N2O), is described as a potent greenhouse gas which originates primarily from organic and chemical fertilisers. This gas, along with excreted nitrogen (N) inputs to soils, is said to account for almost one-third of agricultural emissions.

Ammonia emissions, meanwhile, must be reduced by 5% by 2030 with 2005 levels taken as the base

Ammonia (NH3) volatilisation reduces the efficiency of nitrogen usage and is linked to a deterioration in regional air quality and over enrichment and acidification of natural ecosystems.

Ireland is committed to reduce GHG emissions by 2030 and agriculture has been tasked to achieve a 10% to 15% reduction.

Ammonia emissions, meanwhile, must be reduced by 5% by 2030 with 2005 levels taken as the base.

Wall says that adhering to these targets will require a new focus on improving the efficiency of N on all farms. Sheep production systems generally fare out well in this regard but there will be pressure on all systems given the scale of the targets. There is also a positive story for the sheep sector to demonstrate its environment credentials when marketing produce.

Measuring N efficiency

Wall said that to reduce N efficiency, we need to be in a position to measure and analyse the situation on every farm. He explained that the efficiency of N use can be defined as the efficiency in which inputs are turned into saleable end products.

For sheep systems, this includes livestock (meat or live animals) and grass including silage and hay.

He said that a farm-gate balance that analyses the flow of N in and out of the farm is useful.

He outlined the concept of a nitrogen balance which is calculated on a per-hectare basis and used as an indicator to denote the potential N surplus which, in turn, estimates the risk of nutrient loss.

The whole farm nitrogen balance is calculated by subtracting the total kilos of N exports of the farm from the kilos of N imported and dividing this by the grassland area.

The higher nitrogen surplus, the less efficient the farming system is deemed to be in terms of N use

Wall says the balance is typically a positive number that indicates how much surplus N is being inputted per hectare.

The higher nitrogen surplus, the less efficient the farming system is deemed to be in terms of N use.

This is demonstrated in Figure 1, which also details another new concept termed nitrogen use efficiency (NUE).

Nitrogen use efficiency

Wall defines NUE as the proportion of N retained within the farming system. It is similar to the nitrogen balance but he says it allows comparisons to be made across different farm types. It is calculated by dividing the total N imports by the total N exports and representing as a percentage.

He says that for grazing systems, the NUE is typically low at less than 30%. The NUE for sheep farms in Ireland is also shown in Figure 1.

Farms are grouped into top, middle and bottom performing thirds based on their gross margin per hectare from the 2019 Teagasc National Farm Survey.

Explaining the representation, Wall said the top-performing farms (based on profitability) use more N fertiliser compared with the middle and bottom performing farms.

A happy balance needs to be achieved that also takes into account how nutrients can be used more efficiently

The bottom-performing farms can be categorised as more extensively managed with very low N fertiliser use and hence these are displayed the lowest N balance surplus and NUE.

While this may be positive in terms of N use, it is unsustainable in the long term if farms are struggling to generate a profit.

Therefore, a happy balance needs to be achieved that also takes into account how nutrients can be used more efficiently.

Substantial gains in farm profitability can be achieved on most farms through increasing the volume of grass grown

Wall highlighted that sheep production systems will continue to be dependent on highly productive pastures that are utilised by high-performing ewes and lambs.

He said that substantial gains in farm profitability can be achieved on most farms through increasing the volume of grass grown.

In general, lower N fertiliser use and concentrate feed imports – combined with greater exports of agricultural products (livestock, meat and forage) and relatively high stocking rates – will drive an improvement in NUE on sheep farms.

For example, he listed a reduction in N fertiliser of 10kg/ha as reducing GHG emissions by 1% and improving income by €10/ha.

Improving N efficiency

It would be wrong to surmise that sheep systems can become more environmentally sustainable by simply reducing fertiliser use. This is especially important given the average low levels of fertiliser usage in the sector.

Huge improvements can be achieved in the efficiency of nutrient use and in turn allow chemical fertilisers to be used more strategically by focusing on four main areas – applying lime where required, utilising organic manure N sources efficiently, using the optimum N fertiliser at the right time and correct rate and lastly improving grazing management and incorporating clover when reseeding poorly performing soils.

Lime application

Maintaining grassland soils at the optimum pH range of >/=6.3 creates the ideal environment for earthworms and micro-organisms to thrive and breakdown plants residues, organic manures and organic matter.

This helps to release soil nutrients with grassland soils receiving regular lime application, for example, shown to release up to 80kg N/ha compared with soils with a low pH.

Maintaining soils at the optimum pH also underpins persistency in plants such as ryegrass and clover which will not tolerate acidic soils (pH less than 6.0).

Wall presented research which showed how lime can greatly enhance the efficiency of applied phosphorus (P) fertiliser.

As detailed in Figure 2, the application of 5t/ha of ground limestone to soils with a low pH produced approximately 1t DM/ha additional grass and delivered similar results to the application of 40kg P/ha.

The largest benefit was combined lime and P application which delivered 1.5t grass DM/ha more than the low pH soil.

Wall told farmers that in addition to a phosphorus and potassium release from the soil, a release of nitrogen worth €80 may also be achieved to boost spring growth.

Utilising this extra grass has the potential to reduce farm feed bills by about €160/ha each year with lime generating a return on investment of 6:1 at a cost of €25/t.

Slurry nitrogen efficiency

The manner in which slurry is applied will have a significant effect on available N. Low-emission slurry spreading machines such as the dribble bar, trailing shoe and injection system improve the efficiency of N within slurry by three units of N per 1,000 gallons compared with the conventional splash-plate. Spreading in spring rather than summer will also deliver major gains.

Optimum N fertiliser

In talking about N fertiliser, Wall focused on the potential gains that can be achieved by using protected urea.

He explained that protected urea has the same granule as normal urea that has been on the market for years.

The difference is a urease inhibitor has been added to the granule, which reduces ammonia-N gas emissions from the urea.

This, in turn, means more of the applied fertiliser is available for grass growth. Recent Teagasc research was highlighted, which found protected urea had 71% lower GHG emissions than CAN fertiliser and 79% lower ammonia-N emissions than traditional urea.

Protected urea was highlighted as being suitable to spread throughout the grazing season.

The cost of protected urea was reported at €0.95 per kg N compared to €1.05 per kg N for CAN.

The fertiliser is available at present as a straight nitrogen compound (46%) or in a compound with potash and/or sulphur with phosphorus compounds currently not available.

With regards to early season nitrogen application, David Wall advised farmers that careful consideration of soil conditions and forecast weather should form the basis of a decision on when to apply nitrogen. Soil temperatures of 5oC to 6oC are required for a sustained period to commence grass growth while the stocking rate and grass demand should influence the timing and rate of nitrogen application. Table 1 details a suggested fertiliser plan for nitrogen based on three varying stocking rates.

Account also needs to be taken on phosphorus (P) and potassium (K) application. Wall says there is a low-to-medium demand on grazing areas as the majority of P and K is recycled back into pasture from grazing livestock.

He said the most suitable blend of P and K for grazing ground tends to be in a ration of 1unit P to 2 units k with fertilisers such as 18:6:12 and 10:10:20 highlighted as supplying the recommended ratio.

Wall advised that in the region of 50% of recommended P and K should be applied in spring once significant grass growth commences which tends to be in March or April depending on soil type.

The remainder of P should be applied in two or three applications in May or June while maintenance rates of K should be applied in spring/summer with build-up rates applied in autumn. Table 2 details a recommended fertiliser programme based on variable soil fertility.