Firstly...

These terms are relevant to ryegrasses, all four types: Westerwolds, Italian ryegrasses, hybrid ryegrasses and perennial ryegrasses.

The Scientific Answer

“Ploidy” refers to the number of chromosomes in a ryegrass’s cells. A normal ryegrass plant contains two sets of chromosomes per cell, 14 in total, and is referred to as diploid.

By inhibiting cell division (usually with the use of the chemical colchicine) at a very early stage, plant breeders can create tetraploid plants, which contain four sets of chromosomes, 28 in total. Tetra comes from the Greek for four. This process has long been part of a breeder’s repertoire in the search for large productive ryegrass varieties for use in the agricultural forage market.

Diploids have smaller cells and so more structural cell wall material compared to the tetraploid cells which are larger, and so overall the tetraploid plant is higher in water soluble carbohydrates (sugar).

The Practical Answer

The Key benefits of diploids

• Diploids provide a denser crop, and they do this in two ways. Firstly, compared to a tetraploid, a diploid always has more seeds per kilogram of weight. For example, a perennial ryegrass diploid on average will supply on average 600,000 seeds per kg compared to a perennial ryegrass tetraploid which will supply on average 290,000 seeds per kg. Secondly, a diploid has the genetic capacity to provide more tillers per plant, up to 60% more in perennial diploids compared to tetraploids.

This increased sward density helps a grass crop to:

• be more competitive against weeds
• be more forgiving under grazing stress environments
• improve persistence

The Key benefits of tetraploids

• Some people say tetraploids are better nutritionally; however, do not think that diploids are a poor dietary choice – they are not! Because tetraploids are higher in sugar, they potentially could be more palatable; this in turn leads to higher intakes by grazing livestock. The higher cell content and lower percentage of structural cell wall material leads to improved digestibility and utilisation by livestock. These factors in turn, mean increased potential meat and milk output. This increased sugar content can also lead to improved fermentation in silage crops.
• Tetraploid swards are more open, due to less seeds per kilogram and lower tillering ability. Their growth habit is also more upright meaning that there can be more clover in a sward with higher tetraploid content because the clover plants have more space to move and grown amongst the grass and less competition.
• For the more the image conscious amongst us, tetraploids are visually more impressive. They appear bigger and bolder with a more upright growth habit, a broader leaf and tend to be darker green when compared directly to a diploid as seen here in this image:

“So what do I do?” I hear you ask...

Both types of grass are important and that is why we use mixtures allowing grass farmers to get the benefit of both with balance.

For those with a shorter-term rotations and/or more requirement for silage than grazing, a higher proportion of tetraploid can be used. Bear in mind that increased silage pressure on a plant which is already more open, can reduce overall persistency. Increase sowing rates and/or utilise grazing to improve sward density. Never sow anything with more than 50% tetraploid for a short-term rotation and ideally, reduce that percentage for a medium – long rotation. Be prepared to overseed should a sward become very open.

Tetraploids are ideal for overseeding because their bigger seed has more resources for a faster establishment making them more competitive with existing, established plants. Use specialist mixtures and techniques for overseeding.

For long term rotations or permanent grass, a greater emphasis on diploids is necessary. Their ability to form a dense sward is essential for long term persistency and resilience. A balance of 2/3 diploid to 1/3 tetraploid is sensible where swards are required to last for many years.