Ractopamine in finishing pigs: Australian trial data on feed efficiency gains

Why the Australian pig industry needed its own data#

Ractopamine hydrochloride is a beta-adrenergic agonist that binds to receptor sites on muscle and fat cells, a mechanism described by Watkins and colleagues and summarised in this paper. It raises protein synthesis and reduces protein breakdown, which pushes growing pigs to lay down more lean tissue and less fat. By 2003 it had already been registered and used commercially in the United States for several years, with more than twenty studies there showing it could lift daily gain by 9 to 12% and feed efficiency by around 15%, while cutting feed intake by 5 to 6%, according to the review by Schinckel and colleagues cited in the paper.

The problem for Australian producers was that those American results came from a different kind of pig industry. Production systems in the USA favour heavy castrated males and females paid on carcass lean indices, whereas most Australian pigs at the time were entire males and females sold at lighter weights. Smits and Cadogan set out to test whether the American response pattern, including the well-documented decline in effectiveness over time as adipose and muscle receptors desensitise, would hold up under Australian genotypes, housing and market specifications before the additive, sold as Paylean, became available to local producers.

How the Corowa trials were run#

The first and largest experiment was carried out at a research unit belonging to QAF Meat Industries at Corowa, run under a short-term approval granted to support Australian registration of the product. Because it was a registration trial, the authors used a high inclusion rate of 1 kg of additive per tonne, supplying 20 ppm ractopamine, fed for the maximum intended duration of six weeks. One hundred and thirty two pigs, split into entire males, females and castrates in a 2 x 3 factorial design, were offered either a control grower diet or the same diet with ractopamine added, delivered through automated feeding stations beginning when the animals were 18 weeks old. Pigs were weighed and scanned for P2 backfat at the start, then followed individually through the abattoir and boning room, where the four primal cuts, leg, foreleg, loin and belly, were weighed before and after boning, rind removal and fat trimming.

Two further experiments examined a step-up feeding strategy in a larger group of female finishing pigs housed in groups of 20 with electronic feeders. Experiment 1 began at 70 kg liveweight, comparing a constant 5 ppm inclusion against 5 ppm stepped up to 10 ppm after 14 days, over 28 days. Experiment 2 began at 90 kg liveweight, comparing a constant 10 ppm inclusion against 10 ppm stepped up to 20 ppm, also over 28 days. Both were benchmarked against an unsupplemented commercial finisher diet, and carcasses were processed through the boning room against a Japanese customer specification.

What the registration trial found#

Over the 42 day feeding period, ractopamine significantly improved feed efficiency and carcass leanness without significantly changing final liveweight or carcass weight. Daily feed intake fell from 2.14 to 1.98 kg per day and feed conversion ratio improved from 2.97 to 2.74, both highly significant results. Dressing yield rose from 79.4% to 80.9%, lean yield rose from 59.3% to 61.2%, and ultimate pH increased slightly from 5.41 to 5.50. Backfat at the P2 site and leg fat measurements were not significantly altered. The authors estimated that the treatment produced an extra 2 kg of lean meat per carcass, a meaningful gain when pork is sold boned out and fat trimmed.

Splitting the result by sex showed castrates and females responding best. Feed efficiency improved by 8% in females and by 16% in castrates, and lean meat weight in castrates rose by 6% over controls, reflecting both higher lean yield percentage and heavier carcasses. Most of the extra lean meat came from a heavier, leaner hind leg, with a smaller contribution from the loin; the belly, which is harder to trim because of its layered fat, did not show significantly higher yield, though belly weight tended to be heavier in treated carcasses.

Testing a step-up feeding strategy for ractopamine in finishing pigs#

Purdue University work by Herr and colleagues, summarised in the paper, had suggested that gradually increasing the ractopamine dose every two to three weeks, rather than feeding a constant rate, gave the best and most efficient growth response over an initial 28 day period. Smits and Cadogan tested this idea directly in female finishing pigs at two starting weights. In the lighter pigs (starting at 70 kg), daily gain improved from 0.689 to 0.767 kg/day with constant ractopamine and to 0.781 kg/day with the step-up strategy, with feed conversion ratio improving from 2.68 to 2.46 and 2.39 respectively.

In the heavier pigs (starting at 90 kg) the step-up approach produced the clearest advantage, lifting daily gain to 0.841 kg/day and lowering feed conversion ratio to 1.99, against 3.12 for unsupplemented controls. The authors note that growth performance across both experiments was below expectations because of a severe summer, but the treatment effects on gain and efficiency remained statistically significant. Carcass weight and P2 backfat were not significantly improved in either weight group, though there was some evidence of lower P2 relative to carcass weight in the heavier treated females.

Carcass and boning room outcomes#

The boning room data reinforced the live-animal findings. In the heavy-pig experiment, hind leg yield rose from 50.3% in controls to 53.1% with constant ractopamine and 52.8% with the step-up strategy, while loin yield rose from 63.8% to 65.9% and 65.4% respectively. Hind leg weight and loin weight both increased significantly with treatment. Using digital image analysis, the proportion of intermuscular fat in the belly, the fat between muscle layers, was lower in both ractopamine treatments (8.6%) compared with controls (11.7%), even though belly yield as a trimmed percentage did not differ significantly.

The authors link the strength of these carcass responses to how fat the starting population is. They note that other researchers, including Schinckel and colleagues and Herr and colleagues, recorded larger backfat reductions of 15 to 20% in trials where control pigs were notably fatter, around 17 to 20 mm at heavier carcass weights, than the Corowa animals. Because the QAF pigs were a modern lean genotype with lower starting backfat, the scope for further fat reduction at the P2 site may simply have been smaller, even though the lean tissue response was still clearly present.

How much confidence the results deserve#

Taken together, the three experiments give a fairly consistent picture: feed efficiency improved by around 15% and carcass leanness by around 5% across the studies, with the biggest responses in heavier, leaner-genotype castrates and females rather than entire males, matching the pattern Dunshea and Gannon had reported from earlier reviews. The step-up strategy appeared to sharpen the response in heavier pigs destined for export-weight carcasses, while a constant low rate was simpler to manage for lighter market pigs. We would treat the backfat findings as the weakest part of the evidence, since none of the three Australian experiments found a statistically significant reduction in P2, in contrast to some US trials, and the authors themselves attribute this to the already-lean starting genotype rather than to any failure of the additive.

It is also worth remembering that this is a single research group working with one genetic line of pigs at one site, albeit across three separate experiments and sex groups. The Purdue step-up data it draws on, and the broader US literature reviewed in the paper, add useful context, but the specific numbers for Australian castrates under a step-up program were not tested experimentally; the authors say explicitly that they expect larger benefits there based on other published work, not their own direct measurement.

Commercial considerations raised by the authors#

Smits and Cadogan are candid that a positive feed efficiency and lean yield response does not automatically mean a straightforward return on investment. The base diet needed to support ractopamine, with higher available lysine specifications of 0.60 to 0.70 g per MJ DE, cost $30 to $40 per tonne more than a standard presale diet at the time of writing, and feed prices were already at record highs. The profitability of using the additive would depend on how pigs are paid for, whether on liveweight, carcass weight and P2, or on a premium for lean yield, and on whether the business captures integrated boning room benefits.

The paper also flags practical questions around silo management, the choice between constant and step-up feeding programs, and the duration of feeding, all of which interact with how much of the laboratory-scale response actually survives in a commercial shed. It notes, without further elaboration, that public perception of treated pork in both export and domestic markets was a live consideration for adoption, separate from any question of meat quality or residues. Related Australian work on lean deposition strategies, including the combined betaine and ractopamine trial in finisher pigs and the ractopamine trial in entire and immunocastrated males, sits alongside this paper in the same research tradition documented in the RAAN Conference Proceedings collection.

Sources and further reading#

• Trove library search: find a library that holds the paper
• Feedipedia animal feed database: an open-access database of animal feeds
• Australian Meat Processor Corporation: research for red meat processors

Questions#

— The Livestock Library team