The success of white clover and perennial ryegrass with 250kg of nitrogen per hectare at Clonakilty has been well documented. In a nutshell, the clover swards produced 1t DM/ha of grass and an extra 48kg MS/cow. That study ran from 2014 to 2017 but, by 2017, there was evidence that clover was running out of steam. The percentage of clover in the sward had declined from 37.1% in 2014 to 15.2% in 2017.
A new clover study at Teagasc Clonakilty commenced in 2019, this time looking at the impact of reduced nitrogen on grass and clover and grass only swards.
Aine Murray is the PhD student working with researcher Brian McCarthy and farm manager Fergal Coughlan.
There are four treatments: two nitrogen rates (150kg N/ha and 250kg N/ha) and two sward types, grass only and grass with clover. There are 30 cows on each farmlet, so 120 cows in total. The Irish Farmers Journal visited the west Cork research farm on a wet and windy day last week to see how the trial is progressing.
First off, maintaining clover in the sward has been a challenge for the Clonakilty team. Previous work has shown that average clover content needs to be between 20% and 25% of the total sward on average over the year in order to have sufficient nitrogen fixation and to have an impact on cow performance.
It reached its lowest point in 2018 at just 10% of clover in the sward.
The researchers have been looking at why the clover content reduced to just 10% in 2018 from a peak of over 37% in 2014. There appears to be two common factors and both are related to management. The first is closing cover – closing high clover content paddocks in early autumn reduces clover content. This is because the clover gets shaded out by grass over the winter months.
The second reason for the reduction in clover is due to silage cuts.
“Cutting fields with a high clover content for silage can lead to either clover taking over the field or clover dying out of the field, one or the other,” Aine says.
Re-establishing clover on the farm has been on-going since 2018.
Looking at the preliminary results from the current experiment, clover content continues to increase. The 150kg N/ha treatment had an average clover content of 14.4% in 2019 while this increased to 19.1% in 2020. The 250kg N/ha treatment had an average clover content of 11.1% in 2019 and this increased to 13.6% in 2020.
As expected, there is more clover in the paddocks getting the lower rate of nitrogen, but even at 19.1% for 2020, it is still lower than where the researchers would like it to be.
Oversowing and reseeding have been used to try and increase clover content. All the farm was reseeded in 2012 and 2013, prior to the original clover trial commencing, so none of the swards are more than nine years old.
Oversowing has proved to be more hit and miss than full reseeding. Twenty per cent of the farm was reseeded in 2019 and 15% was reseeded in 2020. The plan for 2021 is to reseed another 10% to 15% of the farm, but Aine is confident that clover content will increase further in 2021, saying it takes two years for clover to get fully established.
Preliminary results from the first two years of trial work are presented in Table 1.
At 150kg N/ha, clover contributes an extra 0.6t DM/ha of grass while at 250kg N/ha clover contributes an extra 0.5t DM/ha of grass.
In terms of milk solids, the cows grazing the clover swards at 150kg N/ha produced an extra 30kg MS/cow while those grazing clover swards at 250kgN/ha produced an extra 23kg MS/cow. The difference in both grass and milk solids production can be accounted for by the differences in clover content between the two treatments.
Can clover replace applied chemical nitrogen? Previous experiments at Moorepark and Solohead have shown that where clover content is adequate, it can replace approximately 100kg N/ha of chemical nitrogen.
Looking at the current Clonakilty experiment from 2019 and 2020, the grass-only sward with 250kg N/ha grew 15.1t DM/ha while the grass and clover sward with 150kg N/ha grew 14.4t DM/ha, a difference of 0.7t DM/ha. Clover content averaged just shy of 17% over the two years, which based on the preliminary results is insufficient to replace 100kgN/ha of chemical nitrogen.
The stocking rate on the experiment is 2.75 cows/ha. Less silage was produced and more silage was fed out to lactating cows in the low nitrogen treatments.
In reality, none of the treatment groups generated enough winter feed to fully support the herd without having to import silage from other parts of the farm. In other words, the current level of pasture growth does not support an overall stocking rate of 2.75 cows/ha. Meal feeding rate is 612kg/cow.
As of Monday 15 March, 55% of the farm has been grazed. All cows have been assigned to treatment groups and the herd is being fed 14kg of pasture and 3kg of concentrate per day. Planned start of the second rotation is approximately 3 April. Average farm cover and current growth rates are similar among all the treatments.
Divergence in grass growth is first seen in late May. Daily growth rates in the two low nitrogen treatments drop off quicker than the high nitrogen treatments and it is not until clover content starts to increase in June and July that growth rates in the grass and clover swards start to outperform the grass only swards. Most of the extra silage fed to the clover group is fed in the autumn to build up average farm cover and stretch the final rotations. Fertiliser plan for the treatments is outlined in Table 2.
Similar levels of nitrogen are applied in springtime, with a gradual reduction in chemical nitrogen as the season progresses.
The main varieties of clover at Clonakilty are Chieftan and Crusader, sown in equal amounts at a rate of 2kg per hectare when reseeding. In recent years, varieties such as Buddy and Coolfin have also been sown, but the researchers are not overly impressed with their performance and have reverted to Chieftan and Crusader for 2021. This is despite the fact that they have performed well in other experiments. In terms of grass varieties that are best suited as a companion to clover, Áine Murray says that perennial ryegrass varieties that have a moderate pre-grazing sward height and have a low post-grazing sward height are best as these prevent shading out of the clover plant. There have been no cases of bloat over the last three to four years. In high-risk times for bloat such as lush pastures and high clover contents, cows will be grazed in 12-hour breaks.