To pre-mow or top? That is the question
Published 01/07/2015 | 02:30
A repeated comment on farm as June draws to a close is that many paddocks are looking rather hairy. This means farmers are struggling to keep pasture quality on track.
Grazing management has a direct influence on profitability, and dairy farmers who actively manage grazing through weekly pasture walks know the value of pasture quality.
However, the old debate as to what the appropriate course of action should be to manage pasture quality in summer continues to rumble on, especially with conditions becoming drier.
Dairy farmers manage pasture quality by four methods:
• Grazing the cows to consistent post grazing residuals of 3.5-4.5cm, or around 0-200kg/ha drymatter (DM);
• Mowing the paddock to conserve surplus pasture into bales, or pit silage;
• Pre-mowing and feeding the wilted pasture to the cows in situ;
• Post-grazing topping.
So I thought it was time I briefly review the science associated with these techniques.
Achieving a grazing residual of 3.5-4.5cm - combined with the assessment of pasture availability and a swift removal of surpluses in bales - can reduce reliance on the need to pre-mow or top pasture.
This philosophy has been widely encouraged by research in New Zealand between 2003 and 2011
Results at Lincoln University dairy farm (LUDF) have shown that these techniques consistently produced high levels of performance. This sparked a renewed interest in this practice in Ireland and was backed up by research in Moorpark.
However on farm, it's easy to take this practice and get over zealous about grazing residuals and forget the impact on the cow. Driving a cow to graze to a residual of 3.5-4cm where there was a poor residual in the previous grazing, where the pre-grazing yield is too heavy, or grazing below 3cm will impair cow performance.
The decisions associated with grazing management are a fine balance between the grass quantity and quality and cow performance.
To complicate things further the grass plant also likes to present challenges by putting up the seed head and little else, especially in periods of moisture and nutrient stress.
When the seed head is emerging, taking obvious grass surpluses as bales or pit silage can maintain pasture quality.
However, stocking rate, management and the weather may require other methods of mechanical intervention. Hence the repeated debate on whether to pre-mow or top.
LUDF has demonstrated a slight shift in their philosophy, on pre-mowing in recent years, possibly driven by the high milk price of €5.24/kg milk solids (MS) of recent years.
Since 2011 LUDF has reduced the stocking rate from 4.2 cows/ha to 3.9 cows/ha, increased pre-mowing. This has increased performance per cows from 410kgMS to 480kgMS.
Results concluded that pre-mowing is a tool that can be used in period of good grass growth to increase cow intake on high quality pastures. This can speed up the grazing round as an alternative to making some silage.
Most importantly, pre-mowing should not be used if pasture supply is tight, when a faster rotation will drop farm pasture cover and increase the feed deficit.
Pre-mowing can complement good grazing management but cannot replace it. Other suggestions when using pre-mowing are to apply 25-40kg/ha (20-32units/ac) nitrogen behind the mower.
Grass covers that are 200-300kgDM/ha higher than grazing standing grass can be mowed to counteract losses and increase cow intakes.
Mow no more than 2.5pc of the farm on any one day. This means that it takes at least two grazing rotations to mow the whole farm - otherwise growth rates will fall.
Finally, it's essential to mow tight to achieve the desired post-grazing residual.
When debating the merits of topping verses pre-mowing, there no clear answers. Some research has suggested that topping results in grass with a higher energy content than pre-mown grass in summer.
But because it's a slightly cleaner practice and is more reliable in achieving the desired residual, pre-mowing seems to be winning this debate.
Do not be tempted, however, to use this technique where pasture is in short supply or conditions are getting too dry.
Mary Kinston is a discussion group facilitator and consultant, and farms with her husband in Co Kerry