Agronomy Advice
January 10, 2021

Influencing earlier spring grass

By: Philip Cosgrave

Earlier grass is becoming more valuable than ever with winter forage reserves running low on many farms. Increasing the supply of early spring grass so that livestock can start grazing sooner has many benefits, not least saving money that would otherwise be spent buying feed. Nutrition can play a vital role in influencing early growth.

Influencing earlier spring grass
Influencing earlier spring grass
Spring nitrogen drives early grass growth

The most important factor affecting spring grass growth on farms is the date and quantity of spring nitrogen (N) and sulphur fertilizer application. Experiments have shown that early spring applications of N resulted in at least 10 kg DM per kg N applied. If you apply N late in the spring, you could be delaying grass growth by 3 weeks. This in effect means that if you don’t apply N early you could be faced with an avoidable extended housing period. The value of this early grass is significant, but for a spring block calving dairy herd every extra day you can have cows out at grass in the spring increases net profit by €2.20 per cow (Teagasc, 2016).

Why is this early nitrogen critical? 

As day length and soil temperatures begin to increase in spring, the grass plant is at its lowest ebb in terms of energy. It needs N to trap sunlight and therefore grow, if we supply nitrate that is easily taken up by the root system then photosynthesis and grass growth will begin earlier. Soil sulphur levels are also at a low ebb at this time of year and any available sulphur remaining from last year has been leached at this stage. Nitrogen works better when there is sulphur available so the use of an N & S product will boost grass growth but also protein and sugar levels.

What form of nitrogen is best?

A choice must be made on the form of N for this first N application. AN is the most reliable and the nitrate portion is ready for immediate uptake by grass and the remaining Ammonium-N fraction is stable and is not prone to leaching or ammonia loss through volatilisation. Urea needs to be applied two weeks before growth begins and is prone to ammonia-N loss unless rainfall (5mm) follows application. Urea applied in cold dry weather will have significant ammonia-N loss as there will be enough moisture to initiate the chemical process which drives this N loss.

YaraBela Nutri Booster (25% N, 2% S and selenium) is an ideal product for early spring application. It contains nitrate nitrogen, sulphate (rather than elemental sulphur) and sodium selenate. The nitrate and sulphate can be immediately taken up by the roots, and together with sunlight will kick start grass growth while the sodium selenate in every granule will provide selenium in every bite of grass.

What about cattle slurry?

Cattle slurry (2,500 gallons/acre) should be applied on the third of the paddocks with the lowest covers or on silage ground with low K indexes. Remember N utilisation in slurry is highest in spring and is further increased if applied by trailing shoe or shallow injection. Make sure you have your soil samples taken before application. Aim to apply 25 - 30 kg/ha (20 -4 units/acre) of nitrogen to the remaining paddocks with highest covers. 

How much early nitrogen do I need ? 

Yara recommend 25 – 30 kg N/ha (20 – 24 units/acre) for intensively stocked grazing systems. On less intensively stocked farms the rate can be reduced but even on these farms early nitrogen will promote grass growth. When? As soon as field conditions exist, especially in milder parts of the UK.

Phosphate availability in spring limits growth

Phosphate is a key nutrient for grass, and its role in energy supply, root growth and tillering makes its availability crucial for grass growth in the spring. The plants requirement for phosphate is small when compared to nitrogen but its availability is essential.

Phosphate availability is reduced at low temperatures in spring but grass uptake in April and May can reach 0.25 kg P per day. At this rate of uptake the release of phosphate from the soil reserve is not sufficient, therefore mineral phosphate is necessary to top-up soil available phosphate to maximise yield and herbage phosphate concentration.

Typically the phosphate in fertilizer is 100% water soluble; this however creates its own problems. As soon as you apply water soluble phosphorus to a soil, this soluble phosphorus becomes slowly fixed by iron and aluminium.

The phosphate contained in YaraMila Silage Booster (20-2-12+3%S+Selenum) and Super Booster (25-2.2-4.2+2%S+Selenium+Sodium) is a mix of water soluble phosphate and Di-Calcium Phosphate (DCP). This DCP is not fixed by the soil but becomes available as it is triggered by weak acids from grass root exudates. This ideal combination of two phosphate fractions rather than one results in superior availability of phosphate during April and May.

The recommended maintenance requirement for P on grazed swards is 8 kg/ha and for 1st and 2nd cut is 18 and 10 kg/ha respectively. Soil test results are invaluable for optimising phosphate applications.

What is the effect of nitrogen and sulphur on silage yield and quality 

The accompanying graph illustrates the effect of nitrogen rate on 1st and 2nd cut silage. As nitrogen rate increases, so does the crude protein percent. Nitrogen is required to build crop biomass but is also integral to building plant protein.

Sulphur has a key role to play in grass crop nutrition, and should no longer be seen as only necessary on light and sandy soils. Obviously the yield response is greatest on these soil types but we are now seeing time and again that grass protein and energy are improved where sulphur is applied at the recommended levels.

Effect of nitrogen rate on percent crude protein 

What is this extra quality worth?

The independent nutritionist Wesley Habershon from the farm consultancy group calculated that increasing silage protein levels from 11% to 14.5% could save a 120 cow dairy herd £32,000 annually on purchased feed.

Optimum N & S rates for a 1st cut silage are 120-140 kg N/ha and 14-18 kg S/ha.

If we build yield early and cut in early May, we can take advantage of quicker re-growths which can be over 100 kg DM/ha/day and consequently improve 2nd cut quality by cutting earlier.

Find more information on grassland nutrition

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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