Agronomy Advice

How to avoid compromising silage quality

By: Philip Cosgrove

Following a cold spring, many grass fields may be behind where they would normally be and first cut silage yields could well be back. So what is the best advice to avoid compromising both quality and yield?

Avoid compromising silage yield and quality following the cold wet spring
Avoid compromising silage yield and quality following the cold wet spring

Even though forage may well be in short supply and carryover stocks low it is important not to delay the first cut in the hope of 'catching up' as this will reduce the quality of first cut silage and will compromise the second cut yields too. The best advice is to take the first cut close to the normal date and to optimise the fertilisation for the second cut

Take the first cut close to the normal date to avoid a loss in quality

Silage nutritional quality, intake potential and live weight gain or milk production are governed primarily by the digestibility (D Value) of the grass being ensiled. A drop of one unit of D represents a 5% decline in animal productivity. Grass digestibility can decline by up to three units/week with the onset of stem formation and heading. Once 50% of the sward has started to head the D value is likely to be below 67D.

As digestibility decreases protein content will also decline, by around 3% a week. Grass protein levels are influenced by nitrogen applications as well as the stage of maturity. Uptake of nitrogen and protein formation can also be restricted by soil deficiencies in potash and sulphur.

The table below highlights the effect of cutting dates on 1st and 2nd cut yield and energy content. Later 1st cut silage leads to grass tillers dying and this increases sward recovery times and ultimately reduces 2nd cut yield and digestibility by up to four units.

Effect of cutting date on silage energy content and dry matter

To maximise second cut silage balanced nutrition is essential. Slurry applications alone will not supply sufficient nutrients and must be balanced with an appropriate compound fertiliser. It’s always worth calculating what fertiliser your second cuts require after accounting for the nutrients in applied slurry. 

Achieving good second cut yields will be more important than ever this year, with poor first cut yields and many farms won’t have the luxury of carrying over silage stocks from last year. Also, some farms have had to graze their first cut silage area. With favourable weather conditions, these lower than anticipated first cut yields can be made up with some good second cut management.

Optimise fertilisation for second cut to maximise silage yields

Nitrogen drives growth and yield. Second cut silage is capable of very high yields, with 15 – 17 tonnes of grass per ha achievable on swards which have decent yield potential. Where swards are only a couple of years old, then fertilising these swards to 100 kg/ha of nitrogen (N) is cost-effective. Older swards with a high proportion of perennial ryegrass (PRG) can be fertilised to 90 kg/ha N and old meadow swards which don’t contain much PRG should receive 70 – 80 kg/ha N. Where soil moisture is moderate, then every kg of applied N should return 14 – 16 kg of grass dry matter (DM).

This fertiliser N rate should be calculated by subtracting the available N in slurry from the crop requirement as outlined above. So, every cubic metre/ha of cattle slurry (6 % DM) applied by splash plate during summer months, will contribute 0.3 kg/ha of available N. If the same quantity is applied by a low emission spreading equipment then it provides 0.6 kg/ha available N.

Every kg of applied N should return
14 – 16 kg of grass dry matter

Sulphur is also required on these second cuts, to increase nitrogen use efficiency. Low soil sulphur supply restricts N uptake, which results in more unused N in the soil which is prone to leaching and denitrification. Sulphur is essential for nitrogen to be used efficiently, but once again the latest survey shows that many growers are still not applying sulphur at all or at too low a rate. This will be reducing both yield and quality of silage and could be more important than ever this year. Yara recommends you apply 16 kg S/ha (13 units S/acre) with each cut.

Phosphorus should not be overlooked for second cut silage. With 48 % of grassland soil samples showing that they require phosphate applications, make sure to check recent soil analysis and then calculate what phosphate your second cut requires. Each tonne of fresh grass requires 1.0 kg of P, so if this is scaled up to a 15 – 17 tonne crop the P requirement is 15-17 kg/ha.

Each tonne of fresh grass requires 1kg of phosphorus

Phosphate allows the plant to process light energy into biomass, so any soil availability shortfall is going to hamper grass growth rates. Also, grass phosphorus levels tend to drop in June, and it’s not clearly understood why this happens. By applying fresh phosphate, this dip can be avoided. If there is no slurry applied or low volumes on account of the risk of slurry contaminating second cut grass at harvest, then a phosphate fertiliser should be applied up to 17 kg/ha. If the soil P index is 1 or 2, then these fields will require a top-up of 20 and 10 kg/ha respectively if it hasn’t been applied earlier in the year.

Potassium should not be forgotten either, potassium offtake at 25%DM is 6kg/tonne fresh grass, so a 20t/ha (8t/acre) crop will remove 120kg/ha (96 units/acre) and this must be replaced. Teagasc soil analysis data from 2019 shows that 41% of grassland soil samples from dairy farms are now below the optimum soil potassium index 3. Potassium applications should take into account that silage crops have a high requirement for potassium, running at 6 kg per tonne of fresh grass silage. Potassium response trials have shown that for every 1 kg of potassium applied results in 5 – 7 kg of grass DM (25 – 30 kg of fresh grass). If you want to maximise yield potential on these 2nd cut’s, then 100– 116 kg/ha of K is required. This would require 28-33 cubic metres of cattle slurry per ha. If this volume of slurry isn’t being applied, then use a fertiliser with potassium.

Don't delay application to avoid losses

Don’t delay applying slurry and fertiliser, as delays will reduce yields and silage quality. Trials have shown an 8-day delay to cause a 6% reduction in yield. The slurry should be applied immediately after the first cut is harvested and then apply the fertiliser 5 – 7 days later. If slurry isn’t being applied, then get the fertiliser out ASAP.

Find more information on grassland nutrition

Recommended grassland fertilisers

The following compound fertilisers are recommended for grassland and supply combinations of nitrogen plus sulphur (NS) or nitrogen, phosphate, potash and sulphur (NPK+S) in appropriate ratios for grassland and with the added benefit of selenium for improved animal health.

Latest advice background
Latest advice background
Grow the future | Grassland Sulphur Grow the future | Grassland Sulphur

Spreading nitrogen and sulphur at the same time means more grass

Applying fertilisers containing nitrogen and sulphur means the grass uses nitrogen more effectively, you get more kgs of dry matter per kg of nitrogen that you apply

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Read about improving nutrient efficiency