For organic farmers, the first step in putting in any crop involves manure, either brown or green.
Animal manures offer a wide range of benefits from basic plant nutrients to micronutrients, as well as a cascading benefit derived from its microbial content.
However, that kind of manure is often available only in limited quantities, is expensive to transport, and if it’s not composted properly, could contain weed seeds.
The green kind, whether from terminating a forage legume stand after a number of years, or plowing under an annual crop, can offer more flexibility and lower costs, said University of Manitoba plant sciences professor, Martin Entz.
The weed-suppression effect of green manures, he added, is particularly important when growing top-grade, high-value forage seeds.
“There’s no shortage of legumes that you can use to bring nitrogen into the soil,” said Entz, at a MAFRI-organized workshop on laying the foundations for forage seed crop production at the Portage Food Development Centre.
Green manures for every situation are available for acid, dry, or wet soils, as well as short-season or long-season needs.
Hairy vetch is a versatile option, while faba beans can work in the wet, and lupins for acidic conditions. If quack grass needs to be worked under first, tropical warm-season legumes such as cowpeas, mung beans or even the exotic lablab can be seeded as late as July.
“Lablab is my favourite midsummer legume. It grows so fast. The problem is there is no seed source,” said Entz, who joked that “We go to the Bulk Barn (bulk food store) to buy our seed. You don’t want to do that. If you back up your tandem truck, they don’t have an auger.”
Green manure legumes are best grown in mixes because using diverse species have different nitrogen-release patterns that can sustain a crop for longer. Also, diversity helps ensure better field coverage without gaps. Gaps in alfalfa, for example, tend to fill in with weeds.
At the U of M’s Carman research farm, Entz has good success in experiments seeding barley and hairy vetch together. The barley is killed with a blade roller at the flowering stage, which allows the vetch to flourish until frost. After the winter, the field is covered with a thick “mat” of flattened leftover crop residue mulch — about 8,000 pounds per acre — that can be seeded with a zero-till disc drill.
The barley residue is key to the weed-suppression goal because it decomposes much more slowly than the legume leftovers, creating a more optimal carbon:nitrogen ratio.
“The soil and the rumen function exactly the same,” said Entz. “If you feed livestock wheat straw, it all moves through them very slowly, and the cow patties are like pyramids. If you feed it all legumes, you don’t want to stand behind the animal.”
Leaving the green manure on the soil surface instead of plowing it under increases volatilization losses slightly, but it helps to release the nitrogen more gradually — a plus for growing perennial forage seed crops over multiple years.
“You put in your green manure, you build up your nitrogen, then you use it over the next three years in your forage seed crop,” said Entz.
Ruminants used for grazing forage seed residues also help to speed up nutrient cycling, he added.