First cut silage - take the guess work out of when to take the first cut

24 April 2018

The use of pre-cut grass sampling will help ensure better quality forage for the coming winter 




Dr Liz Homer, Ruminant Technical Development Manager with Trouw Nutrition GB believes that if farmers use the new tools at their disposal they will be able to improve timing of first cut and produce a better quality forage as a result.


“When grass is at its optimum quality in the field before first cut it will have an energy content of around 12.8MJ/kgDM.  Yet when the clamps are opened, silage energy contents will be closer to 10.8MJ, representing a loss of around 16% of the available energy.


“In part this will be due to factors at play during silage making, such as respiration losses while the crop is lying in the field and fermentation losses, but in many cases the biggest loss is a direct consequence of cutting grass too late, when it has gone beyond the optimum condition and is starting to deteriorate.”


She says that many farmers cut too late based on outdated and unscientific approximations of grass quality.  Following the rule of thumb of cutting when 50% of the sward is at ear emergence is to accept that half the grass at least has already gone over.


“As grass develops the ear stem, it goes through a process of increased lignification which reduces the digestibility of the whole plant and so leads to a decline in energy status.  In simple terms, D value typically declines by around three units per week and this leads to a reduction of 0.5MJ in energy content, but the rate of decline may be faster or slower depending on the season.  So if we want to improve the energy content of silage it is necessary to use a more scientific approach to timing of harvest.”


Pre-cut testing of grass has been widely used to determine how well it will ferment.  Dr Homer explains that fermentation quality is directly affected by sugar and nitrate levels, both of which can be checked by pre-cut analysis.


“To achieve an effective fermentation the target should be a sugar content of greater than 10% in the dry matter.  This will be affected by the weather and how rapidly grass is growing.  Targets for nitrates should be less than 1000mg/kg in the freshweight.  If fertiliser and slurry was applied late and if grass is growing slowly the plant will not have had time toassimilate nitrates into protein.  This may become more common as more farmers move to so called multi-cut systems.


“While this information gives a valuable insight into how well grass may ferment, it tells us nothing about the nutritional quality of the grass being harvested.”


Dr Homer advises having a more complete analysis carried out on pre-cut grass, specifically having it analysed for NDF content.  She says NDF is a measure of fibre levels and an indication of plant maturity.


The graph shows how NDF content in pre-cut grass evolved in 2015, 2016 and 2017 and demonstrates the different pattern between the years.  In 2015 NDF levels were higher from late April and rose gradually while in 2016 there was a rapid increase from the 8th -22ndMay but from a much lower start point.


“Over this period, the rapid increase in NDF levels over the typical cutting dates in 2016 was enough to reduce grass ME content by 1.2MJ/kgDM, representing a huge cost of delayed cutting.  For an average first cut of four tonnes of dry matter per hectare, the cost of delayed cutting would be 4800MJ/ha, equivalent to enough energy to produce 900 litres of milk/ha, indicating a clear benefit to cutting sooner.”


“2017 however, was different again.  Here the NDF levels remained low until the second half of May before accelerating away as grass matured later, meaning that harvest could have been taken anywhere from the 1st to the 17th of May with no detrimental impact on quality.  However, as in the other years grass was around 42% NDF by the end of May by which time quality will be much lower.”


Dr Homer says that further analysis shows that the difference between the years can be explained by the winter weather, as the table illustrates.  2014 and 2016 were characterised by unusually warm winters while 2015 was particularly cold and this affected overwintered grass growth.  In 2017; December was fairly mild compared to previous years, but January was colder which meant an effective winter kill was achieved.


In warm winters there was less winter kill and as a result more, older material was harvested in the first cut leading to higher NDF, reduced D value and poorer ME content.


“Pre-cut analysis including NDF content will give a much more precise indication of when to cut grass to achieve the balance between silage quantity, quality and fermentation stability.


“We would advise weekly sampling of at least two fields with different sward characteristics, starting four weeks before the anticipated cutting date with the target of cutting at 38-40% NDF provided nitrate and sugar levels are optimal too.  This will be equivalent to cutting grass at a D value of 70.”


Dr Homer stresses that while pre-cutting can improve the quality of harvested material, it is still vital to ensure proper wilting, clamp consolidation and sheeting to ensure the crop is effectively fermented.  Unless good silage making practices are applied, it will not matter what the NDF of the initial material was.  


“Monitoring trends is important if cutting dates are to be optimised.  A single result in each of 2016 and 2017 would not show the different ways the crop was developing and the impact on cutting date.  They were both around 36% NDF on May 1st but by May 15th, one was still around 36% while the other was over 42%.


“The time invested in pre-cut sampling, replacing visual assessment with science could have a major impact on the milk production potential of this year’s first cut,” she predicts.