Here’s how you harvest subterranean clover seed in Australia

When you think of the legumes typically grown for forage in the US, alfalfa, white and red clover, and even soybean often come to mind. In many parts of Western and Southern Australia, however, subterranean clover “sub clover” is king. So much so, in fact, that over 23 million hectares of the region is planted with the stuff. (If you want to learn more about this intriguing forage legume, click here for a highly informative fact sheet on “sub clover” written by Phil Nichols of the Department of Primary Industries and Rural Development.)

With so much of Australia being planted to sub clover each year, those seeds have to come from somewhere, right?

Well yes, they do. And this week I was fortunate to visit one of the premier sub clover seed production operations in Australia. The farm is located in Pingelly, Western Australia (about 155 km southeast of Perth). The trip was organized by my friend and colleague, Kevin Foster of UWA, and joining us on the trip were Phil Nichols and Andrew Guzzomi, an agricultural engineer at UWA (actually, it was their trip and Kevin and his colleagues were kind enough to let me tag along–thanks, guys!).

So, here are four things that you need to know about sub clover. First, sub clover, like all other legumes, can be a fantastic component of a cropping system because apart from being a nutritious forage for livestock, it provides a biological source of nitrogen that can be used to fertilize subsequent grain crops. This is due to the legume’s ability to form a symbiotic relationship with a specific type of bacteria. The bacteria convert the nitrogen in the air into a form that plants can actually utilize, while the plant provides the bacteria with shelter and carbohydrates. This biological nitrogen can reduce or eliminate the need for synthetic fertilizer inputs.

Second, sub clover is called subterranean for a reason. It produces its seeds in small burrs that the plant either fully or partially buries under the soil. While this trait enables sub clover to be used as a self-seeding forage crop (i.e., you plant it once, graze it almost down to nothing, and then it grows again next year from the seed that is already in the soil), this trait also makes sub clover an incredibly challenging crop to harvest for seed.

Third, sub clover is not native to Australia. The plants were accidentally introduced from the Mediterranean and Western Europe around the late 1870s, although there are no exact records. It has thrived here in Australia due to the similarities in climate and soil conditions. Since the 1950s, Australia has had a significant sub clover breeding program involving collection missions to the Mediterranean region. As a result of these breeding efforts, Australia now has sub clover genotypes capable of growing well under many of Australia’s climate zones and soil types.

Fourth, like many Mediterranean annual legumes, sub clover produces “hard seeds” (i.e., seeds that won’t absorb water and germinate). This protects most of the seed from germinating out of season (but not always, see photo below). Over the summer, the seed coat softens, enabling sub clover to germinate in the late fall or early winter. Of course, not all of the hard seeds break down enough to germinate the next winter, some remain in the soil for multiple years.

Quick aside: because it readily reseeds, sub clover can be used as the foundation of either a permanent or semi-permanent pasture system, despite the fact that it has an annual life-cycle. Most other annual forage crops would need to be resown each year–a potentially costly endeavor. The self-seeding trait (along with producing hard seeds) also enables farmers to use sub clover within a cereal rotation (e.g., wheat, oats, or barley) where several years of cereal production are followed by a year or two of sub clover that germinates from the seeds that are already in the soil from the previous sub clover crop. This rotation is the basis of the “ley farming system”, an integration of livestock and cereal  production that has been used successfully in southern Australia for over 80 years. Also, a sub clover pasture can help build soil structure, and if the grasses are controlled, can help reduce disease in the following cereal crops. Of course, the fact that it reseeds itself also poses challenges (but perhaps also opportunities?) from a weed management perspective–namely because sub clover has the potential to become a weed in any subsequent crop if not properly controlled. But I digress. . . .

So, given sub clover’s somewhat unique tendency to bury its seeds, how exactly does one harvest sub clover seed on a commercial scale? Here’s how its done:

First, it is grazed. In this case, by sheep. Because of sub clover’s prostrate growth habit, it can be heavily grazed without damaging the seeds. This is great for the sheep. It is also great for the seed producer because grazing it down to the soil also means there is less plant material that eventually needs to be separated from the burrs.

Next, when conditions are right (i.e., the remaining residue is dry and crispy, the top soil is relatively free of moisture, and the winds are low), a shallow cultivation (or two or three) is conducted to loosen the ground and free the seed-containing burrs from the soil.

Last, and perhaps the most interesting step in the process, the burrs, along with some not insignificant amount of the topsoil, are literally vacuumed into a large machine that then threshes the seeds from the burrs.

The eventual result is a harvester full of nearly clean sub clover seeds.

Interestingly, the technological side of harvesting sub seeds has not evolved very much since these harvesters were first developed here in the 1960s. Prior to that, I’m told that Australia’s sub clover seed producers wrapped a sheep skin around a large barrel and rolled it across the field. The burrs would get caught in the wool of the sheep skin, which would then need to be brushed in order to remove the burrs for eventual threshing.

As you might imagine, there is a fair amount of interest in improving on the current approach to harvesting these seeds, namely to reduce the amount of topsoil that ends up making its way off the paddocks. Kevin, Phil, and Andrew are hoping to come up with some creative ideas for doing just that–hence the reason they visited this farm. In the future, farmers may be harvesting sub clover in a different way. On this day, however, we were learning from the experts–the farmers–who have figured out how to optimize their systems within the constraints of the climate and the soils that dominate this region of the country. And what a privilege it was to see.

My education in Australian agriculture continues. . . .

***I owe a special thanks to Kevin Foster (AHRI/UWA) for proof-reading this post and adding additional details about sub clover biology/agronomy and Australia’s sub clover breeding program.