Diploids versus tetraploids – what’s the difference?
After a decision is made to reseed a field, the next big decision to be made is what varieties to reseed with. However, before going straight to the Recommended Lists, there are a number of things that need to be established. Principal among these is knowing what type of grass is most suited to that field.
Perennial ryegrass is obviously the preferred type of grass, but not all perennial ryegrass is the same. Varieties differ in terms of heading date and ploidy. Ploidy relates to whether a variety is classed as a diploid or a tetraploid. Whichever ploidy is chosen will have long-term implications on the success or otherwise of the reseeding job as it affects persistency.
The difference between diploids and tetraploids is down to their plant breeding, with tetraploids manipulated to have four chromosomes per cell, whereas diploids have two. This makes tetraploid cells larger. Tetraploid varieties tend to grow taller and more aggressively than diploids and animal intake is often higher on tetraploid swards. Tetraploids have lower dry matter characteristics and tend to be more open, making them unsuitable for heavy land that is prone to poaching.
For this reason, most grass seed mixtures are dominated by diploid varieties. In recent years though, there has been increased interest in tetraploid varieties, particularly on drier farms where an open sward would be less of a problem.
To increase our understanding of the differences between diploid and tetraploid swards, an experiment was established in Clonakilty Agricultural College in 2013 to examine the implications of ploidy on grass growth and milk production in a seasonal spring calving system.
Treatment groups
Sixty cows were randomly assigned to two treatment groups after calving. The tetraploid treatment grazed four tetraploid varieties (Aston Energy, Kintyre, Tymax and Dunluce) and the diploid treatment grazed four diploid varieties (Tyrella, Aberchoice, Glenveagh and Drumbo). Each variety was sown as a monoculture or single variety.
Cows on the experiment were stocked at 2.75 cows/ha, 250kg/ha of nitrogen was applied and cows received 320kg of concentrate in 2014.
The main findings from the first full grazing season on the farm show no significant differences between the tetraploid and diploid varieties on either grass growth or milk solids production.
In terms of grass growth and grazing characteristics, the diploid treatment had higher dry matter percent (21.3 versus 19.8%), higher pre-grazing yields (1,831 versus 1,720kg DM/ha) and had a higher post-grazing height than the tetraploid treatment (4.51 versus 4.24cm). The tetraploid treatment grew slightly more grass over the full season, 14.9t DM/ha versus 14.8t DM/ha for the diploid treatment.
When it comes to milk production, cows on the tetraploid treatment produced 11kg more milk solids (MS) than the diploid group (414 versus 403kg MS/cow). Milk solids production was similar between the two groups up until August, at which point the tetraploid group began to outperform the diploid group due to higher fat and protein percentages.
Commenting on the results, researcher Brian McCarthy said that they were in line with expectations.
“In an analysis of previous research work on the effects of ploidy, it was found that tetraploid varieties increased milk solids production by about 4%, but most of these experiments were conducted over shorter periods, on average 101 days and were conducted during the main grazing season from April to September. If run over the full season, like in the Clonakilty experiment the results would be similar to what we found.”
While the results from this experiment are not groundbreaking, they do form part of a bigger experiment into clover which we reported on recently (7 March 2015). An 11kg increase in milk solids per cow on the tetraploid treatment, while not massive, should not be dismissed.
It is also worth remembering that we are only reporting from the first full year of the Clonakilty experiment with three more years left to go, so no definitive conclusions should be drawn at this point.
For me, the decision on which ploidy to choose comes down to soil type. On heavier soils that are more prone to poaching, I would avoid tetraploids altogether and focus entirely on producing dense diploid swards with good ground cover better capable of withstanding damage.
On drier farms, I think tetraploids do have a role to play. Of course, a lot depends on whether a seed mixture or a monoculture is being sown.
Where a monoculture is used the risk is greater but so too is the potential reward. Even on dry farms, I would be slow to have more than 60% of the farm sown to tetraploids as these will be more prone to poaching. So to avoid grazing these in wet weather, a proportion of the farm should remain in diploids, like at the Greenfield Farm in Kilkenny.
The decision on what ploidy to choose depends largely on soil type. Tetraploids are more open so may be less persistent on heavy soils.After one full year of the Clonakilty experiment, no real differences were observed in grass growth between diploids and tetraploids.Cows grazing the tetraploid varieties produced more milk solids, with 11kg more milk solids per cow produced in 2014.
Diploids versus tetraploids – what’s the difference?
After a decision is made to reseed a field, the next big decision to be made is what varieties to reseed with. However, before going straight to the Recommended Lists, there are a number of things that need to be established. Principal among these is knowing what type of grass is most suited to that field.
Perennial ryegrass is obviously the preferred type of grass, but not all perennial ryegrass is the same. Varieties differ in terms of heading date and ploidy. Ploidy relates to whether a variety is classed as a diploid or a tetraploid. Whichever ploidy is chosen will have long-term implications on the success or otherwise of the reseeding job as it affects persistency.
The difference between diploids and tetraploids is down to their plant breeding, with tetraploids manipulated to have four chromosomes per cell, whereas diploids have two. This makes tetraploid cells larger. Tetraploid varieties tend to grow taller and more aggressively than diploids and animal intake is often higher on tetraploid swards. Tetraploids have lower dry matter characteristics and tend to be more open, making them unsuitable for heavy land that is prone to poaching.
For this reason, most grass seed mixtures are dominated by diploid varieties. In recent years though, there has been increased interest in tetraploid varieties, particularly on drier farms where an open sward would be less of a problem.
To increase our understanding of the differences between diploid and tetraploid swards, an experiment was established in Clonakilty Agricultural College in 2013 to examine the implications of ploidy on grass growth and milk production in a seasonal spring calving system.
Treatment groups
Sixty cows were randomly assigned to two treatment groups after calving. The tetraploid treatment grazed four tetraploid varieties (Aston Energy, Kintyre, Tymax and Dunluce) and the diploid treatment grazed four diploid varieties (Tyrella, Aberchoice, Glenveagh and Drumbo). Each variety was sown as a monoculture or single variety.
Cows on the experiment were stocked at 2.75 cows/ha, 250kg/ha of nitrogen was applied and cows received 320kg of concentrate in 2014.
The main findings from the first full grazing season on the farm show no significant differences between the tetraploid and diploid varieties on either grass growth or milk solids production.
In terms of grass growth and grazing characteristics, the diploid treatment had higher dry matter percent (21.3 versus 19.8%), higher pre-grazing yields (1,831 versus 1,720kg DM/ha) and had a higher post-grazing height than the tetraploid treatment (4.51 versus 4.24cm). The tetraploid treatment grew slightly more grass over the full season, 14.9t DM/ha versus 14.8t DM/ha for the diploid treatment.
When it comes to milk production, cows on the tetraploid treatment produced 11kg more milk solids (MS) than the diploid group (414 versus 403kg MS/cow). Milk solids production was similar between the two groups up until August, at which point the tetraploid group began to outperform the diploid group due to higher fat and protein percentages.
Commenting on the results, researcher Brian McCarthy said that they were in line with expectations.
“In an analysis of previous research work on the effects of ploidy, it was found that tetraploid varieties increased milk solids production by about 4%, but most of these experiments were conducted over shorter periods, on average 101 days and were conducted during the main grazing season from April to September. If run over the full season, like in the Clonakilty experiment the results would be similar to what we found.”
While the results from this experiment are not groundbreaking, they do form part of a bigger experiment into clover which we reported on recently (7 March 2015). An 11kg increase in milk solids per cow on the tetraploid treatment, while not massive, should not be dismissed.
It is also worth remembering that we are only reporting from the first full year of the Clonakilty experiment with three more years left to go, so no definitive conclusions should be drawn at this point.
For me, the decision on which ploidy to choose comes down to soil type. On heavier soils that are more prone to poaching, I would avoid tetraploids altogether and focus entirely on producing dense diploid swards with good ground cover better capable of withstanding damage.
On drier farms, I think tetraploids do have a role to play. Of course, a lot depends on whether a seed mixture or a monoculture is being sown.
Where a monoculture is used the risk is greater but so too is the potential reward. Even on dry farms, I would be slow to have more than 60% of the farm sown to tetraploids as these will be more prone to poaching. So to avoid grazing these in wet weather, a proportion of the farm should remain in diploids, like at the Greenfield Farm in Kilkenny.
The decision on what ploidy to choose depends largely on soil type. Tetraploids are more open so may be less persistent on heavy soils.After one full year of the Clonakilty experiment, no real differences were observed in grass growth between diploids and tetraploids.Cows grazing the tetraploid varieties produced more milk solids, with 11kg more milk solids per cow produced in 2014. 
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