Diploid vs Tetraploid: Why Chromosome Count Changes How a Pasture Grass Behaves

Ever looked across a paddock of Rhodes grass thick with seed heads, flowering and running to head no matter what time of year it is — then driven to the next paddock and found none of that? Wider, thicker leaves, no seed heads in sight, and it only seems to throw a flush of flowering once the days start getting shorter. Same species, same rainfall, maybe even sown from the same seed source a few years apart. The difference usually isn’t the variety name on the bag. It’s ploidy.

What Ploidy Actually Means

Every plant cell carries a set number of chromosomes. Most plants are diploid — two sets (2n). Some cultivars are tetraploid — four sets (4n), created by doubling the chromosome number through breeding. Same species, same genus, but the doubled chromosome count changes how the plant grows, flowers and gets used.

It’s not better or worse. It’s a different tool for a different job.

What Changes When You Double the Chromosomes

A few practical differences show up consistently between diploid and tetraploid cultivars of the same species:

• Flowering response: diploids tend to flower right through the season regardless of daylength. Tetraploids often need shortening daylength to trigger a flush of flowering.
• Palatability at maturity: tetraploids are generally more readily eaten by stock once mature. Diploids can mature quickly and reduce nutritive value.
• Fertility requirement: tetraploids typically need higher fertility to perform. Diploids tend to persist/perform on more marginal country, although still a good choice for high production country as well.

None of this is universal across every species, but it’s consistent enough to build cultivar choice around. Rhodes grass is the clearest tropical example of it in action — but as you’ll see further down, the same split shows up in temperate ryegrass systems too.

A Tropical View: Rhodes Grass, Same Species, Two Different Grasses

Rhodes grass (Chloris gayana) is a clear sub tropical or tropical example, because Australian breeding has deliberately gone both ways.

Pioneer and Katambora are the two long-standing public diploid types, selected for early and reliable flowering, easy seed set, and persistence on lower-fertility, lower-rainfall country.

Callide is the classic public tetraploid — later flowering, bigger leaves, and it responds strongly to higher fertility and irrigation, holding feed quality well if it’s managed to match that input.

Beyond public types, PBR (Plant Breeder’s Rights) cultivars are also available through Pasture Portal. On the tetraploid side, Epica (Selected Seeds PBR variety) is a newer release bred specifically for high dry matter production and salt tolerance. On the diploid side, Finecut now a common cultivar a good hay variety. Reclaimer, a Selected Seeds PBR variety, was selected as a dual-purpose hay and grazing type; and Gulfcut, also a Selected Seeds PBR release, is promoted for its fine leaf and stem — credited with drying around a day quicker than standard diploid types.

This image shows Finecut, a Rhodes diploid, with fine leaf and stem.

This image shows Reclaimer, a Rhodes diploid, with fine leaf and stem.

That’s the agronomic reasoning behind the old rule of thumb: diploids west of the Great Dividing Range (Maranoa, Condamine, Central QLD), tetraploids east (Northern Rivers, South East QLD, Burnett etc.). It’s not a hard boundary, but it lines up with where each type’s requirements — fertility, rainfall, persistence — are actually met.

Diploid vs Tetraploid in Ryegrass: The Same Rule for the Temperate Market

Rhodes grass isn’t the only pasture grass carrying this split — it’s the same genetic principle behind the two types of ryegrass grown across temperate Australia and New Zealand. Perennial ryegrass is naturally diploid, with 14 chromosomes. Tetraploid types are bred by doubling that to 28, and the plant responds the same way it does in Rhodes grass: bigger cells, more water content, bigger leaves — but fewer tillers per plant.

What that means in the paddock:

• Diploid ryegrass tillers densely, handles continuous grazing, pugging and higher stocking rates better, and holds up longer under pressure. The trade-off is slightly lower metabolisable energy and a tendency to shade out clover in the sward.
• Tetraploid ryegrass carries more sugar and is more palatable — stock will select it over a diploid sward every time — and typically runs about 0.25 MJ ME/kg DM higher. The more open sward also gives clover more room to establish, generally around 10% more clover in a mix. The cost is fewer tillers, less persistence under heavy treading, and a bigger call on grazing management, particularly in wet conditions.

Seed size follows the same pattern — tetraploid ryegrass seed is nearly double the size of diploid seed, so sowing rates need to go up to hit the same plant population.

On-farm, the production upside of tetraploids is well documented: dairy farmers commonly see milk production lift on tetraploid paddocks, and lamb finishers get faster liveweight gain. But it’s not a straight upgrade — persistence issues turn up fast if grazing residuals aren’t managed, and Argentine stem weevil has a marked preference for tetraploid over diploid ryegrass.

The common practical answer in temperate systems is to not pick one. Many growers sow diploid/tetraploid blends, taking the density and robustness of the diploid and the palatability and quality of the tetraploid in the same sward.

What This Means for Hay and Grazing Intensity

Ploidy on its own doesn’t create a big yield gap in Rhodes grass. A DPIRD irrigation trial in the Kimberley found a diploid type and the tetraploid Callide both sat among the best performers, both capable of 35-40 t/ha DM/year under intensive management — cut every 28 days with adequate nitrogen. Well-managed diploid Rhodes on its own commonly runs 18-25 t/ha DM/year (the figure usually quoted for Katambora). The real yield lever is fertility and cutting frequency, not ploidy. What ploidy changes is how much input it takes to get there — tetraploids need the higher fertility and rainfall to hit that top end, diploids get closer to it on less.

Where ploidy does show up clearly is drying time. Diploid Rhodes grass types have been specifically bred over the years for finer stems and a higher leaf-to-stem ratio, which gives quicker, more even drying and a higher-quality bale. Callide carries more stem bulk, so the stem lags behind the leaf when curing — more risk of the leaf shattering from over-drying before the stem’s ready, or the bale going in with a wet stem still inside it.

Nutrition tracks growth stage more than ploidy. CP runs roughly 15-17% in young leafy growth and falls to as low as 3% in old, rank material — Callide has been measured at 9% CP with 52% leaf at 7 weeks regrowth, down to 3% CP with 20% leaf at 27 weeks. Digestibility (IVDMD) sits 70-80% in vegetative growth and drops toward 40% once it’s gone stemmy. That’s the case for keeping any Rhodes grass — diploid or tetraploid — leafy through regular defoliation rather than letting it run to head.

For grazing intensity, frequent defoliation cuts production in any Rhodes grass regardless of ploidy — cutting every 14 days instead of 28 measurably reduces yield. But the later-flowering tetraploids give you more flexibility to keep pasture leafy for longer before it needs a spell.

Which One Should You Plant?

There’s no universal answer — it comes down to your rainfall, fertility and whether you’re chasing hay, grazing, or both. Run your paddock through the Pasture Picker to see which cultivars — diploid or tetraploid — actually suit your conditions.

Further Reading

For those who want to go deeper on the species and the science behind this:

• Chloris gayana — Tropical Forages fact sheet: https://tropicalforages.info/text/entities/chloris_gayana.htm
• NSW DPIRD — Rhodes grass varieties: https://www.dpird.nsw.gov.au/agriculture/pastures-and-rangelands/species-varieties/rhodes-grass
• Development of genomic resources for Rhodes grass (Chloris gayana): https://pmc.ncbi.nlm.nih.gov/articles/PMC10507473/
• IHSG 2025 Conference Proceedings: https://ihsg.org/wp-content/uploads/2025/12/IHSG-2025.pdf
• Dairy Australia — Pasture Options: https://www.dairyaustralia.com.au/feeding-and-farm-systems/pastures/pasture-options
• FutureBeef — Fodder research yields big opportunity for small-scale irrigation in northern WA: https://futurebeef.com.au/resources/fodder-research-yields-big-opportunity-for-small-scale-irrigation-in-northern-wa/