Tha Importance of Nitrogen Supply for Grass Silage
The Importance of Nitrogen Supply for Grass Silage
By Mark Scott, Greenmount Campus, CAFRE and James Black, John Thompson & Sons Ltd
This article is one of a series prepared jointly by CAFRE and John Thompson & Sons, focusing on the key grassland management issues facing dairy farmers approaching the start of the 2008 season. The articles draw on experiences from the management of the Future Herd at Greenmount Campus, CAFRE managed in partnership with Thompsons, and the Thompsons Milk Manager data from farmers across N. Ireland.
Fertiliser Requirements
The cost of nitrogen fertiliser has increased substantially from last year. Many dairy farmers are questioning fertiliser usage because of this. However it is essential for the production of quality grass but certainly efficient use must be made. Fertilizer prices may be very expensive but do still give a return of £3 for every £1 spent.
It is essential to match grass nitrogen requirement to demand so to maximize crude protein content of the grass ensiled. Through the growing process the grass plant will take up nitrogen from the soil and convert it to protein in the leaf of the plant. When grass is then consumed by a dairy cow, either grazed fresh or as grass silage, the nitrogen is consumed in the form of protein.
The simple grassland utilization rule of 2 units N/acre/day (2.5KgN/ha/day) is still relevant and particularly so in a late spring. It is important to plan cutting date when fertilizer is being applied so that it is efficiently utilized by the plant.
Protein Feeds for Winter 2008
In October 2007, cereal prices were up to 70 percent higher compared to the previous year, whilst protein sources such as Rapeseed and Soya increased on average by 40 percent. Unfortunately for dairy farmers, the global market has again shifted and protein sources have increased considerably in cost. This will have a significant impact on herds confined all summer and on those herds that make and feed lower quality grass silage and alternative forages with low protein contents during winter 2008.
Protein is an essential ingredient in the dairy cow diet, both as it is a nitrogen source for rumen microbes and for direct use by the cow. In addition it is a key driver of dry matter intake (DMI). Protein levels in the total diet must be optimal for maximum performance. Quality forage is therefore essential.
Table 1 illustrates the differences in the amount of concentrates and also protein contents required between good and average silages made in 2007. The increased protein required with average quality silage will be considerably more expensive for the 2008 winter.
Table 1. Concentrate amount and Crude Protein per cent required to sustain 30 litres when complementing good and average quality silages
| Good Silage | Average Silage | Difference | |
|---|---|---|---|
| ME (MJ/KG/DM) | 12.1 | 10.6 | +1.5 |
| Crude Protein (% DM) | 15.5 | 12.6 | +2.9 |
| Dry Matter Intake (KG/Day) | 12.2 | 9.8 | +2.4 |
| Concentrates Required % | 7.5 | 11 | +2 |
The aim for all dairy herds should be to maximize the crude protein content of forages used so to minimize the requirement for purchased protein. Making high quality grass silage for next winter must be the goal. This will allow the achievement of maximum crude protein levels in forage diets. Where alternative forages are being grown, it is even more important that the grass silage fed to the dairy herd has a protein content which compensates as much as possible for the lower crude protein levels of these crops.
Nitrogen Fertiliser For 1st Cut Silage
Quality is always paramount when making grass silage but particularly so in 2008 with the circumstances surrounding concentrate price. The grass crop must therefore be fertilised to give adequate yields of high quality material.
Quite simply, if you do not supply the grass plant with sufficient nitrogen it will run out of fuel before your expected harvest date. But if you oversupply the crop with nitrogen you will end up cutting very heavy crops of low quality material or clamping poorly fermented, low intake silage. This adds significantly to farm costs on two fronts. Firstly money has been wasted and secondly expensive poor quality silage has been made, reducing profit margins next winter.
The value of slurry should not be underestimated, especially in the current climate of high inorganic fertiliser prices. Where slurry has been applied the nutrient content must be taken into account when calculating crop requirements.
Plan now to supply sufficient nitrogen to grow an adequate yield of 1st cut silage with the ultimate aim of ensiling high protein, high energy grass. Ensiling under good dry conditions will ensure high quality silage, resulting in a more profitable winter of milk production.
Example Fertiliser Recommendation for Grass Silage targeting 21 May Harvest
- Total nitrogen recommendation for first cut grass silage – 120kg N/ha or 100 units N/acre
- Assuming 3000 gallons per acre slurry has been applied in mid February (33.715 m3/ha)
- Total nitrogen from slurry = 3kg/m3 or 101kg
- Available nitrogen from slurry = 35% or 35kg/ha
- 120 less 35 = 85 kg/ha or 70 units per acre required from inorganic nitrogen
- 12th April to 21st May = 39 days
- Grass can utilize 2.5kg/ha/day or 2 units/acre/day of nitrogen therefore the sward currently has the ability to utilize approximately 97kg N/ha or 78 units N/acre.
- In this example the 85kg/ha or 70 units/acre will have been utilized by the grass by the time of planned harvesting.
- If slurry or fertiliser has not been applied before 12th April, the maximum amount of fertiliser that can be applied for cutting on 21st May is about 100kg N/ha or 80 units N/acre