Grassland and forage crops

Fertiliser use this spring – what to consider

High fertiliser prices will be a real challenge for farmers, and many face a dilemma of how much and what to buy. Where there’s scope to reduce grass or silage demand, then there is some wriggle room but if demand is going to remain the same then the opportunity to reduce fertiliser volumes from previous years is limited. Farmers may be faced with prioritising nitrogen (N) this year and skipping or reducing phosphorus (P) and potassium (K) applications in a bid to offset higher fertiliser costs.

Much of the commentary on reducing the impact of higher fertiliser prices has focused on slurry N utilisation, and there is scope on some farms to shift more applications into spring, however, a large proportion of farmers won’t have access to low emission spreading equipment (LESS). This cohort of farmers will have very little opportunity to lower their fertiliser N use from previous years without lowering grass production.

The most consistent and, usually, the best response to N from a bag or from slurry is during April and May. A reduction in N rates for grazing needs to be evaluated as it means the area normally closed-up for silage is less, or alternatively, the total N rate for 1st cut silage is reduced from previous seasons. When we lower our N rate (slurry + bag) on 1st cut silage, we will reduce the cost on a per acre basis but not per tonne. We end up with less silage in the pit with no real savings.

Spring N for grazing

One of the most important factors affecting spring grass growth on farms is the timing and quantity of the first spring N fertiliser application. This early spring grass is extremely valuable as a means of increasing the proportion of grazed grass in the diet. The on-farm response to early N can be variable (5 – 18 kg/DM per kg N, source: Teagasc), and coupled with higher N prices, getting timings right is more crucial than ever to make sure we justify the cost.

There’s always an element of debate around the right approach to spring N management. As a rule of thumb, the timing of the first N application should coincide with soil temperatures reaching 5 - 6^oC. You can check soil temperature and soil moisture deficits (SMD’s) for different soil types on Met Eireann. If the SMD is negative, don’t apply fertiliser or slurry. Of course, a favourable weather forecast and good field conditions are also necessary when deciding when to spread.

For this first N application, we recommend using a product like YaraVera Amidas at a rate of 20 – 25 units N/acre. The second application should aim to deliver 35 – 40 units and be applied by the end of March on intensively stocked farms or mid-April on farms with a later turn-out date later and/or on heavier soils to take advantage of improving growing conditions. These rates are appropriate for newer swards with high perennial ryegrass content. On less intensively stocked farms or on swards that will be less responsive to N, then the above rates should be scaled back by 25%.

What about P & K?

Farmers may be considering a P & K ‘holiday’ this year in an effort to offset higher fertiliser prices. This needs to be considered carefully, particularly on farms that are highly stocked. The trouble is, while the cost-saving is known, the potential penalty in lost yield is uncertain so it’s a question of risk. This is in the short term, because longer-term, any negative balance of phosphorous and potassium where these are not applied this year, will have to be made up. You don’t get something for nothing.

We have not seen the same price rises for fertiliser P & K as N, and currently, NPK’s could be considered better value for money in comparison to straight N products. On dairy farms, it may be counterproductive not to maintain current soil fertility levels this year, at the expense of growing less grass. Also, there is no certainty that the cost of fertiliser P & K will return to early 2021 price levels.

For drystock farmers, it may be a case that N is prioritised, at the expense of P & K. This is not a situation that we want to see, but it may well be the reality on many farms. If only a percentage of the normal P & K rates can be spread, then late spring is the most opportune time to apply these on grassland approaching peak growth rates. Slurry should be prioritised for silage fields and the remaining if any for grazing fields/paddocks with low soil P & K fertility.

The great unknown is the weather and it ultimately has the greatest impact on fertiliser performance, so fingers crossed for an early spring where livestock can be turned out early and can stay out.