If you’ve ever been on to FeedXL or read information about ‘bighead’ and oxalates in pastures, you’ll have come across the terms “C3” and “C4” grasses.

But what do these terms really mean?

It’s all about the way they photosynthesize – use the energy from sunlight to convert the carbon dioxide from the air and water from the soil into energy for plant growth. The by-product of photosynthesis is (luckily for us) oxygen. The vast majority of plants on earth use C3 photosynthesis.

C3 plants store their energy as sugar (fructan) whereas C4 plants store starch rather than sugar.

C4 plants are higher in fibre than C3 plants because C4 photosynthesis must occur in the ‘dark’ and requires a ‘wall’ between different parts of the plant’s energy making factory. Therefore the cells inside C4 plants contain more fibrous walls and some cell structures not present in C3 plants.

C4 plants have an undeservedly poor reputation in the horse world as some C4 grasses contain high levels of oxalates that bind calcium. This can put horses at risk of developing diseases associated with calcium and phosphorous/magnesium imbalances such as ‘bighead.’ However, adequate calcium and magnesium supplementation can overcome the oxalate problem to allow horses to safely graze low to moderate level oxalate pastures long-term.

C4 plants are more energy efficient than C3 and grow much faster. The optimum temperature for growth of C4 plants is over 30 degrees, whereas C3 plants prefer 15 to 25 degrees Celsius. C4 plants have a higher water use efficiency than C3 plants.

C3 plants perform best in cooler, moist conditions with moderate sunlight and tend to be temperate plants (ryegrass, clover, wheat, barley, oats) and C4 plants evolved in warm, strong sunlight areas of the tropics (sugarcane, corn, sorghum, Rhodes grass, paspalum).

So really, it’s a matter of horses for courses. Your climate and geographical location will be a major factor in determining whether you can grow C3 or C4 grasses at different times of the year. The higher fibre levels in C4 plants generally correlates with a lower level of calories per kilogram – an advantage for feeding easy-keepers, but a disadvantage if you’re trying hard to put weight on a horse. C3 plants tend to pose more of a problem for laminitis prone horses – but diligence is always required for founder management no matter what type of pasture you have.

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