Grassland farmers can mitigate against impact on grass growth of applying less N

Improving efficiency through better soil, grazing and slurry management can help grassland farmers reduce the negative impact on grass growth from reducing nitrogen (N) inputs.

Fertiliser price rises will add 3p/litre to milk production costs this year, compared to 2021 – or £12 to the cost of producing a fat lamb, and £90 to a weanling, calculates independent grassland and soil specialist Chris Duller.

During a recent Farming Connect webinar, Mr Duller offered expert advice on how growers can mitigate against those higher input costs.

Fertiliser prices in 2020 cost grassland systems on average the equivalent of £47/ha in 2020, but that cost is now £134/ha: ammonium nitrate (34.5%N) is currently selling at a record £650/tonne (t), TSP (46%N) £555/t and MOP (60%N) £560/t.

Mr Duller said there were no market signals to indicate that fertiliser prices will fall in the near future: “The current prospect of significant price drops is low.”

Against that backdrop, he set out the options for farmers to consider.

“Losses in grass growth will be relatively small from reducing individual application rates by 5-10kgN, particularly in the spring,” he advised.

But significantly reduce rates and the difference will be very noticeable, he warned:

“If a farmer uses half the amount of N than they did last year because it is twice as expensive, they can expect to grow 20-30% less grass, especially if they choose to cut back in the peak grass growing months of May and June.

“And if nothing is done to improve efficiency, then they will end up with less output – for instance, milk or meat – and probably a higher concentrate bill.”

The best fields for cutting back on N inputs are the very poor fields where the response rate is low, and during the summer and autumn on fields where soil conditions and clover are good and likely to supply good levels of natural N, Mr Duller advised.

He cautioned against cutting back on recent reseeds; “They are the best responding fields,” he said.

Growing 12t dry matter (DM)/ha of ryegrass at an average crude protein of 18% has a requirement of around 346kgN, but this can come from multiple sources other than synthetic fertiliser and slurry; mineralisation, fixation from clover leys and deposition from the air all contribute.

There is an opportunity to buy forage now if less grass is likely to be grown, or to sell stock early – for instance, cattle destined for the store market could instead be sold as weanlings.

Taking on cheaper rented ground for grazing is another consideration.

“I am not normally an advocate of taking on additional land instead of maximising the potential of existing land, but you can produce 4tDM/ha from fairly poor ground,” said Mr Duller.

“If the rent is £200/ha and no NPK (nitrogen, phosphate and potash) is applied, it will work out at 5p/kgDM.

“To get double that output on home ground (8t/DM/ha) will need 200kgN to produce that extra 4tDM at a cost of £400/ha, and that will cost you 10p/kgDM.”

Mr Duller said establishing low N demanding crops might work for some systems.

To produce 12tDM from a three-cut silage system requires 250kgN, 80kg phosphorous (P) and 250kg potassium (K), but producing 12tDM of maize the nutrient requirement is lower – 50-100kg N and 55kg P and 175kg K.

But Mr Duller said that even at current prices, the response rate of N made it cost-effective to apply, compared to buying in feed.

With its response rate of at least 25:1, for every 1kg of N applied, 25kgDM of forage is produced. Factoring in spreading costs, at £650/t for AN (artificial nitrogen), it costs £2/kg N applied.

“Assuming an average response of 25:1, that puts the cost per kgDM at 8p, which is still favourable compared to feed at 28p/kgDM – but that assumes 100% utilisation,” said Mr Duller.

To get the best response from any N applied, soil chemistry should be good, soils warm and dry, and the day length longer. Good sward species and soils that are not compacted are important, too.

Avoiding those marginal conditions that hinder the response rate is another route for reducing N use, Mr Duller pointed out.

“Avoid conditions that will give poor response, and skip the worst fields altogether,” he recommended.

Grass losses can be negated in part by improving efficiency, such as the nutrient status of the soil.

At a soil pH of 5.5 and N use of 200kg/ha, the response rate will be 20:1, so each kgDM will cost 10p to grow. By increasing pH to 6.2 and factoring in a liming cost of £23/ha over eight years, N use can be reduced to 150kg/ha and the response rate will increase to 25:1, which means that the cost of growing each kgDM will reduce to 7p.

“Correct P and K status, address any soil compaction and drainage issues, tighten grazing or rake to remove thatch, increase ryegrass content and add regular manure inputs to soils with low organic matter,” Mr Duller advised.

Increasing usage through controlled or rotational grazing will reduce wastage: if 150kg of N is used to grow 6000kgDM/ha, but only 60% is utilised, that is the equivalent of only 3600kgDM/ha being grazed.

Other options to reduce N rates are to identify the fields where clover and good soil conditions can provide summer and autumn N, and calibrating fertiliser spinners and using precision application techniques.

Improving slurry storage and spreading is important too; low trajectory slurry application reduces ammonia loss by 70%.

“Slurry management can go some way to reducing, but probably not eliminating, bagged N,” said Mr Duller.

For farmers who are uncertain when they should place their fertiliser order, he urged them not to get caught out by supply issues.

“If you don’t buy now, you won’t have anything for March/April,” he said.

“It is OK to hold fire on buying fertiliser for May/June use, but don’t leave it too long.”

Farming Connect is delivered by Menter a Busnes and Lantra Wales and funded by the Welsh Government and the European Agricultural Fund for Rural Development.