Livestock weight gain while grazing novel legume species in spring was similar in research trials to subterranean clovers and medics, but offers additional, unique agronomic advantages and some variation in nutritional value.

Research by CSIRO, the South Australian Research and Development Institute (SARDI) and the New South Wales Department of Primary Industries has been identifying the benefits of novel pasture legumes for livestock production as part of the national Dryland Legume Pasture Systems (DLPS) project.

The ability of a pasture to produce animal liveweight gains is a function of the amount of herbage present, the stocking density applied to the pasture, the quantity consumed by the animals and the quality of the herbage.

Hard-seeded legumes such as French serradella and bladder clover (and biserrula in NSW) can be summer-sown and, if there is sufficient moisture, they can produce high quantities of biomass early in the growing season.

Their deep root systems mean they tolerate dry conditions and produce plenty of seed. In Western Australia and NSW, their superior growth compared to other species and good nutritional value means they can carry more stock and produce more meat and wool per hectare.

The research also found that some of the novel legumes have improved capacity to survive ingestion by livestock, leading to lower depletion of the seedbank during summer grazing.

More than 80 per cent of biserrula seeds pass through sheep and remain viable. For French serradella and bladder clover, the feed quality of pods suggests they might contribute to improved liveweight gain for sheep grazing these residues over summer compared to other species.

The goals of the DLPS livestock research component were to:

1. quantify the on-farm production gains that could be achieved on high-quality legume pastures;
2. ensure novel species being considered for commercial release are safe for livestock and do not cause any taint flavour to meat;
3. understand the potential impact of grazing mature, aerial-seeding legumes on livestock productivity and seedbank dynamics; and
4. to conduct plant analysis across DLPS trials in all states to optimise understanding of nutritional value across species, sites and seasons.

Grazing studies

Based on grazing experiments in WA and South Australia and associated meat quality tests, Trigonella balansae has proven its worth for further development for new crop-pasture sequences.

Pasture legume grazing trials have been carried out by the Dryland Legume Pasture Systems project as a duty of care requirement before new species can be commercially released. Pictured here are alternating large plots of trigonella, French serradella and subclover at Mokine, WA. Photo: Anvil Media

As part of a duty-of-care assessment in WA in 2020, a replicated large-scale grazing experiment was undertaken near Mokine comparing the liveweight gain, condition score and wool production of 14-month-old Merino wethers grazing subterranean clover (cv. Dalkeith), French serradella (cv. Erica) and trigonella at a stocking density of 12 dry sheep equivalents per hectare over a 50-day grazing period.

All three legume pastures produced comparable animal performance. After grazing, the sheep had similar liveweight gain, condition, wool growth and yield, hot carcass weight, cold carcass weight, carcass fat score, crude fat in muscle and ultimate pH of muscle.

At conclusion of the field trial, eight animals from each legume treatment were processed at a commercial abattoir and the backstrap muscle was vacuum-sealed and aged at 4oC for four days before freezing. Meat sensory evaluation was then conducted by 48 consumer panellists. The eating quality of the lamb grown on the trigonella pasture did not differ from the other pastures, with panellists reporting similar tenderness, juiciness and flavour to other lamb evaluated.

Trigonella is a relative to fenugreek and its leaves have a curry-like aroma and taste; however, clearly this did not impact the flavour of the meat.

In another grazing study, at Minnipa in SA, the sheep feed value of trigonella was compared to legume species adapted to alkaline sandy loam soils commonly found in Mallee regions. The newly released powdery mildew tolerant strand medic (cv. Seraph) was included with the widely naturalised strand medic variety Harbinger, common vetch (cv. Volga ) and rose clover (cv. SARDI rose). The 36ha trial was sown in 2019 and grazed by one-year-old Merino ewes, cropped in 2020 and allowed to regenerate in 2021. Wheat stubbles were also grazed in early 2021.

In 2019, trigonella pasture digestibility and crude protein levels were similar to other species and it produced comparable liveweight gains per head in sheep.

Banking seed

Compared to subterranean clover, aerial-seeded legumes require more careful grazing management, as the seed can be consumed by livestock, especially in the establishment year. The high feeding value of mature stands of some pasture species provides an opportunity to extend grazing, but it is important to ensure that enough seed enters the seedbank for future regeneration.

This component of the project used field and animal house studies to get a better understanding of trade-offs between late-season grazing, seed set and seed survival after ingestion.

In field studies with French serradella, stands were mowed at a height of three centimetres to simulate heavy grazing, either three weeks prior to flowering, at the start of flowering, or three weeks after flowering. Mowing three weeks prior to flowering had no significant effect on seed-set, while mowing at flowering reduced seed production.

As expected, defoliation three weeks after flowering had a massive impact on seed production and is not recommended if seed production is important.

The other critical parameter measured was how ingestion of the seeds by sheep affected seed viability post-ingestion. Pod and seed heads containing mature seed samples from 16 legume and four weed species were fed to Merino sheep in metabolism crates to determine seed survival and germinability after ingestion, and the mature legume seed digestibility (Table 1).

Table 1: Seed size, survival after ingestion by sheep and potential nutritional value (dry matter digestibility DMD, crude protein CP) of the pods.

Source: CSIRO

As a general rule, bigger seeds were more likely to be digested, but there was some variability within and between species. Seed survival after passage through the sheep ranged from eight per cent (yellow serradella) to 88 per cent (biserrula).

Within the serradellas, there was no relationship between seed size and survival after passage, as the structure of the pod is likely to affect how easily the sheep can chew the seed.

French serradella pod samples had much higher nutritional value for dry matter digestibility (DMD) and crude protein (CP) than yellow serradella. In theory, for sheep grazing a mature French serradella pasture, only 16 to 28 per cent of ingested seed returns to the seedbank in a viable form, so care should be taken not to overgraze the residues.

Mature bladder clover seed heads had moderate nutritional value, while subterranean clover in burr was relatively poor quality . Trigonella pods had 57 per cent DMD, 17 per cent CP and 27 per cent of seeds survived passage through sheep.

Of the weeds tested, 94, 70 and 85 per cent of the turnip, wild radish and annual ryegrass seeds, respectively, were unviable after ingestion by sheep. Hence, heavy grazing of legume pasture residues with established seedbanks may contribute to reductions in weed populations. This is particularly useful if herbicide resistant weeds are an issue.

This research was part of the national Dryland Legume Pasture Systems project supported by the Australian Government Department of Agriculture, Water and Environment (DAWE) Rural R&D for Profit program and the Grains Research & Development Corporation, Meat & Livestock Australia and Australian Wool Innovation.

More information: Dr Hayley Norman, hayley.norman@csiro.au