In this article, David Mackey of CAFRE explores the question on many farmers’ minds – when should I cut grass for silage?
Making good quality silage is key to the feed efficiency of your herd through the winter, but what makes good silage, and when should you cut?
Cutting date is important as the developing seed head, with its higher fibre content in the stem, reduces silage quality.
For high-quality silage, with a ME of 11.5 MJ/kg dry matter (DM) or more, you must cut your silage well before 50% ear emergence.
To assess the level of ear emergence, walk your silage swards. Pulling up a grass plant and splitting the sheath allows you to determine how far up the stem the seed head is and plan your cutting date accordingly.
As a general guide, early heading perennials reach 50% ear emergence around May 10th, mid-season varieties around May 20th and late heading in the first days of June, but this is weather dependent and can vary a few days either way.
Each week of delay after 50% ear emergence results in a 0.4 MJ/kg DM decline in silage ME, requiring an extra 1.2 kg of concentrates to achieve the same daily milk yield.
Good weather conditions make for good silage. Bright sunshine increases the sugar content of grass leading to a better fermentation.
Sunshine also promotes a more rapid wilt reducing the amount of water ensiled. Ideally, grass should be mowed dry.
Cutting whilst wet, even with dew, means a longer wilt with a reduction in nutrients and a poorer fermentation.
Unused fertiliser or slurry
Unused fertiliser or slurry nitrogen can adversely affect fermentation, resulting in poorer silage quality.
Nitrogen uptake by grass is typically about 2.5 kg per hectare per day (2 units per acre per day).
This is weather dependent and could be less in cold, wet, cloudy weather when the grass is growing less actively.
Unused nitrogen decreases the sugar content of grass, leading to higher ammonia concentrations in the silage.
If you have concerns, consider getting a grass ensilability test carried out to check how well the grass will preserve.
Harvesting and clamp management
The aim of silage production is to preserve grass whilst avoiding loss, maintaining feeding value and promoting intake. There are several phases in the production of silage. They are:
Harvesting – aim to ensile grass at 30% DM. This is best achieved through a rapid wilt, where the grass is spread out over the entire field straight after mowing.
A rapid wilt minimises sugar and protein losses and, in ideal weather, can be achieved within eight hours.
At 30% DM, short chop length promotes better clamp consolidation but should not be less than 25 mm to maintain fibre for rumination in the winter diet.
Consider longer chop lengths in wetter crops, closer to 20% DM.
Have a wilting plan. Assess the starting DM of grass. Standing crops can range from 14% DM (lush, early crops) to 23% (maturing, lighter crops in exceptionally dry weather).
In average conditions, a crop of grass can increase in DM by 1.0% per hour during the day. Early, lush crops will struggle to achieve 30% within a day.
Rowing a crop greatly reduces, if not stops, wilting. If a crop starts to get over-dry, get it rowed up! Make sure the rake is set correctly.
Machines that are set too low will pull up debris and dead material from the bottom of the sward, reducing the ME and possibly increasing contamination. Rakes set too high will produce high in-field losses.
If you are not sure about the starting DM, carry out a microwave drying test to assess DM more accurately.
Filling the clamp
This should be done quickly with grass distributed evenly in shallow layers and rolled continuously to eliminate oxygen and start the fermentation process.
Pay particular attention to the shoulders of the pit, which have the greatest potential for loss. Seal the pit with at least one cover to make it airtight, and weigh down the cover with tyres or mats.
Fermentation – microorganisms in the grass produce lactic acid, which is the primary acid responsible for lowering pH, producing silage, and making it stable.
Undesirable microorganisms can dominate if the pH does not drop rapidly.
This is more likely with lower DM grass and with high levels of residual nitrogen present. Soil contamination and poor sealing can also be factors.
Use of an additive
Effective silage fermentation produces high levels of lactic acid, reducing the crop pH. Silage additives can help this process.
Various additives are available, including bacterial inoculants, enzymes, acids, and sugar sources.
It is important to emphasise that none of these are a substitute for good silage-making techniques and management, but they should help make a good situation better.
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