We are grass farmers!

When we own horses, we sign up to become the caretakers of large herbivorous animals that have adapted to eating a plant-based diet primarily made up of grasses. This is why we, as horse owners, should think of ourselves as ‘grass and forage’ farmers for our horses, even on the smallest of properties. 

The more you learn about how plants in general and grasses in particular work, the better equipped you will be to make informed decisions about the diet and, therefore, long-term health of your horses. 

Why are we grass farmers?

As horse owners, we create as best as we can husbandry and pastoral systems that can cater for the needs of our horses. However, we soon find out that managing horses in these systems is not without effort; it needs continuous understanding and action.

To make your pasture systems really work, it helps to have some basic knowledge on how grasses and legumes develop and grow, the effect of grazing pressure, what leaf area should remain after grazing, and how all this effects the recovery of your plants and, accordingly, your grazing planning and paddock rotation. In short, you need to think of yourself as grass farmers! (See the previous article in the March 2016 issue of Horses and People).

Even if you agist your horse and don’t have direct control over grazing and pasture management, it is never too late to get involved, become grass farmers and get to know your horses better!

If we manage horses on pastoral systems, it is important you get to know the plants your horses may consume. However, there are thousands of grass species out there and it won’t be possible to provide you a complete list in this magazine that gives you all the in’s and out’s of your pasture. The aim for this article is to discuss the differences between grass species, and what are the most common species we encounter in temperate and (sub) tropical environments here in Australia.

A large family

The grass family, known scientifically as the Poaceae or Gramineae, is one of the four largest families of flowering plants, with approximately 600-700 genera and 10,000 species.

Grasses are included with lilies, orchids, pineapples and palms in the group known as the monocotyledons, which includes all flowering plants with a single seed leaf.

Until recently, fossil findings indicated that grasses evolved around 55 million years ago. Recent findings of grass-like phytoliths in Cretaceous dinosaur fossilised faeces have pushed this date back to 65 million years ago.

Grasses range from tiny inconspicuous herbs less than an inch high to the giant bamboos that grow to 130 feet tall. The family is undoubtedly the most important flowering plant family to animals and humans, directly or indirectly providing more than three quarters of our food.

Grasses are also a major producer of our oxygen and provide a large component of the earth’s environmental filtering processes, due to their enormous geographic range, spatial coverage and biomass. Grasslands are estimated to compose 20% of the vegetation cover of the Earth. Poaceae live in many other habitats, including wetlands, forests and tundra.

The economic importance of grasses can hardly be overstated. Grasses are the greatest single source of wealth in the world. They provide much of the starch (rice, cornmeal, bread, cereal, pasta, and so on) and much of the protein in most human diets.

Although a few grasses can significantly accumulate heavy metals and other harmful substances from the soil, and a few others have potentially poisonous cyanogenic compounds in their shoots and leaves, the overwhelming majority are not poisonous.

Grasses sweeten what you drink and eat with cane sugar, molasses and high fructose corn syrup. Corn by-products also provide the raw material for many chemicals used in industry.

Grasses provide the raw material for most alcohol products (sake from rice, rum from sugar cane, beer from barley, bourbon from corn, and whiskeys and other spirits from wheat and rye). Besides food and fuel, they also provide building materials, such as bamboo and thatch.

What are C3 and C4 grasses?

Grasses can be classified as either C3 or C4 plants, sometimes referred to as cool season and warm season plants.

The terms C3 and C4 refer to the different pathways that plants use to capture carbon dioxide CO2 during photosynthesis. The names are simply derived from the number of carbon atoms in the first product of photosynthesis (C3 = 3 carbon compound and C4 = 4 carbon compound). C3 and C4 grasses have different growth requirements.

Cool season, warm season

Cool season, C3 plants are adapted to cool season areas and grow in either wet or dry environments. C3 plants are more efficient than C4 plants under cool, moist conditions and at lower light intensities. ‘Temperate’ pasture species are all C3 plants and they grow actively in Winter when the C4 plants are dormant or only able to grow slowly.

Warm season, C4 plants are more adapted to warm or hot seasonal conditions under moist or dry environments. The C4 plants have some important advantages over C3 plants in hot, dry conditions. They are more water and nitrogen efficient, but require more energy (light) as there is an additional step in the photosynthetic pathway.

The C4 plants only need to keep their stomata (the tiny openings that allow gasses and water to enter the plant) open for short periods, so they lose much less water (transpiration) for the same amount of CO2 fixed by photosynthesis. This is a significant advantage under hot, high light intensity and moisture-limiting conditions.

Carbohydrate storage

Both grass types have internal structure to support the different types of metabolism. C4 grasses form starch and C3 grasses generally form fructan as they store carbohydrate, but there are some C3 grasses that evolved and store starch instead of fructan. There a number of Australian native grasses, such as Weeping grass and Wallaby grass, that are of the C3 type, but produce starch and no fructan.

Both C3 and C4 grasses can accumulate significant amounts of soluble sugars, non-structural carbohydrates (NSC) or starch during active growth periods, which can put some horses at risk of developing metabolic disorders, such as laminitis. However, between C3 and C4 species there is no difference in how much NSC they tend to accumulate in optimal growth conditions.

C3 grasses and legumes, such as ryegrasses, cocksfoot, Yorkshire fog and clover, and C4 grasses, such as, paspalum, kikuyu, couch grass, early growth Rhodes grass and panic grass, have a high potential to accumulate NSC.

Many Australian native grasses are lower in sugar and fructan but, because of this, they are also more sensitive to overgrazing. These include Red grass, Windmill grass, Weeping grass, Speargrass, Kangaroo grass and Wallaby grass.

Oxalate content in grasses

Oxalate content of grasses is especially relevant to those who keep horses in tropical and sub-tropical climates because the oxalates upset the uptake of calcium in the body and are associated with conditions, such as ‘big head’.

Various tropical grass species, including Kikuyu grass (Pennisetum clandestinum), Seteria (Setaria sphacelata), Panic grass (Panicum maximum), buffel grass (Cenchrus ciliaris) and pangola grass (Digitaria decumbens) contain significant levels of soluble oxalates that react with Ca to form insoluble Ca oxalates, reducing the Ca absorption in the digestive tract of grazing animals. This leads to a disturbance in the absorbed calcium:phosphorus ratio, resulting in mobilisation of bone mineral to alleviate the hypocalcaemia.

Prolonged mobilisation of bone mineral in horses consuming these tropical grasses results in nutritional secondary hyperparathyroidism (NSH) or osteodystrophy fibrosa (‘big head’). Cattle and sheep are less affected, but not unaffected, because they can degrade oxalates in the rumen.

Grasses with more than 0.5% oxalate or calcium:oxalate ratios of less than 0.5 result in a negative calcium balance and are capable of inducing hypocalcaemia in horses. Levels of 2% or more soluble oxalate can lead to acute toxicosis in ruminants.

It is important to note the levels of oxalate in the plant depend on the species, cultivar, time of the day, season, and soil conditions (nutrient availability). Nevertheless, when horses graze on pastures where grasses known to have high levels of oxalates predominate, it is important to balance their rations with a dietary calcium and phosphorus (mineral) supplement.

What grass species should I have in my pastures? 

The presence of both C3 and C4 species can be desirable in a pasture as they can occupy different niches. For example, C3 species are often more abundant under the shade of trees and on southerly aspects, while C4 species often dominate full-sun conditions and northerly aspects. This results in providing greater grass cover across a range of conditions that exist within a same area.

It is very common to find both C3 and C4 species in one paddock, and this has the additional advantage of providing a broader spread of production throughout the year (Winter and Summer) for both our domestic grazing stock and the animals native to the area.

Download and read this article “Equine Permaculture: Part 3 We Are Grass Farmers” here [wpdm_package id=52380 template=”link-template-button.php”]