Title: Renovation Crops and Pastures – Green Manuring and Mulching
Renovation crops and pastures are those which are returned to the soil with the key aims of improving soil fertility, reducing weed burdens and increasing soil organic matter. Crops may be returned to the soil by burial, mulching or desiccation.
Mulching involves mowing or slashing the crop or pasture and leaving the residue laying on the soil surface. This enables more soil contact with the crop residue and helps reduce soil moisture loss through evaporation. Residues may break down more rapidly as pieces are likely to be smaller than in brown manuring.
Manuring and mulching reduce viable seed-set, thereby controlling high weed numbers. Manuring can greatly reduce seed-set of all weeds.
Green manuring incorporates green plant residue into the soil with a cultivation implement. Most commonly conducted with an offset disc plough, cultivation aims to kill weeds and control seed-set while building soil organic matter and nitrogen status. Green manuring has a very long history of managing weeds and building soil fertility.
A number of legumes are relatively competitive with certain weeds such annual ryegrass making green manuring of high density plantings of legumes (eg arrowleaf, persian and berseem annual clovers) a reliable method of reducing the seed-set and subsequent germination of weeds in the following season.
Timing: Green manuring takes place early in the season as yield can be more easily compromised for weed control benefit, and so the seed-set of certain weeds such as annual ryegrass is effectively controlled.
Costs (income loss) from the renovation crop method can be offset by improved yield in subsequent cereal crops. Income loss from renovation crops can be mitigated in successive seasons by benefits such as increased wheat yields, improved grain quality and reduced weed control costs.
Grazing competitive forage with high-value stock prior to imposing the treatment can also make a difference to the first-year economics. These benefits may in some instances outweigh and exceed the loss of income.
Whole-farm benefits
There are additional benefits from renovation crops:
Manuring will have a beneficial effect on organic matter and soil nitrogen status. The benefit will be much greater if the crop or pasture being manured has a high legume content.
Green manuring of annual forages can be used to manage other crop pests and diseases.
Manuring must be carefully timed to prevent seed-set and addition to the seed bank. Strategies aimed at reducing weed seed-set must be carried out when the most advanced target weed is at the mid-flowering to early seed-set stage. Green manuring needs to be carried out a little earlier as seed can continue to develop and mature after the plant has been cut or incorporated with a plough. A general guideline is to green manure before the end of the milk stage of the most mature seeds.
Choice of renovation crop species will influence the competitiveness of the renovation crop.
Less weed biomass should restrict weed seed production but final levels of weed seed production in renovation crops and pastures will be dependent on the success of the manuring treatments. In general, renovation crop species that had good early vigour and were able to establish quickly (oats or oats/ peas mixes) were more effective at suppressing weed growth than those crop/pasture choices with slow early growth. Renovation crops which were excellent at suppression were barley, oats, triticale, wheat, field peas and mustard. Canola, lupins and vetch provided good weeds suppression and faba beans, medics, clovers, serradella, chickpeas, lathyrus and narbon beans provide only fair to poor weed suppression.
Economics in the year of manuring can be improved by planning the use of the method and by understanding and valuing subsequent benefits. Manuring a grain crop foregoes the income from grain in that season. However, where other strategies have failed and hay or silage are not suitable options, manuring does have merit. Where herbicide resistance in key weeds is problematic, the economics of completely stopping seed-set in one or more years makes renovation crop procedures more favourable.
Assess the economics of manuring on a rotational basis to capture ongoing benefits, as opposed to conducting single-year gross margins. The long-term benefits may outweigh the loss of income suffered by sacrificing the crop. High-value stock, such as prime lambs, used to graze a competitive forage crop prior to imposing the strategy can make a big difference to the first-year economics.
If grazing animals are not available, lupins, vetch or field peas may be a useful crop in which to conduct a green manure operation. These crops are reasonably inexpensive to establish, contribute a significant nitrogen boost and provide a useful disease break for subsequent cereal crops.
Use an autumn tickle at a later stage to encourage optimum seed emergence and reduction in weed seedbank. Monitor and manage regrowth. Monitor and control any regrowth or subsequent germinations. Control surviving weeds through grazing to eliminate subsequent weed seed-set.
Autumn tickling (also referred to as an ‘autumn scratch’ or shallow cultivation) stimulates weed seed germination by placing seed in a better physical position in the soil. At a shallow depth of 1–3 cm the seed has better contact with moist soil and is protected from drying forces. Because weeds that germinate after an autumn tickle can be controlled, such a process will ultimately deplete weed seed reserves.
An autumn tickle can be conducted using a range of equipment, including tyned implements, skim ploughing, heavy harrows, pinwheel (stubble) rakes, dump rakes and disc chains.
A well-timed autumn tickle will promote earlier and more uniform germination of some weed species for subsequent control.
Autumn tickle can be performed any time in autumn or winter, but post-cultivation erosion risk will be minimised when cultivation occurs closer to sowing.
Soil type is critical for a successful autumn tickle. Light textured (sand) soils, non-wetting soils and those where moisture has trouble penetrating the profile are poor candidates for autumn tickling. Where soils wet unevenly, weed seeds may be buried in pockets of dry soil. These pockets may become wet during the season, with seeds subsequently germinating to cause in-crop problems.
Use autumn tickling in non-crop situations to stimulate germination of weeds which can then be managed with grazing, flaming or cultivation.
Autumn tickling is a tactic best suited to weeds that are easily released from dormancy such as annual ryegrass.
Determine the suitability of autumn tickle as a weed management tactic by considering the following points:
Soil disturbance prior to sowing can reduce soil moisture, placing the sowing operation at risk in a dry season
Soil disturbance prior to sowing can incorporate stubble and, as a result, significant amounts of soil nitrogen will be tied up by microbes that proliferate to degrade the stubble
In the early stages of no-till adoption short-term nitrogen deficiencies are likely if stubble levels are high.
Whole-farm considerations
There are multiple issues to consider when deciding when and how to use a renovation crop or pasture:
Cultivation leads to losses of soil organic matter which needs to be considered when using green manuring.
Plan to graze soon after management to avoid the risk of forage quality loss due to weather damage. Strip grazing with an electric or movable fence can reduce trampling loss.