RESEARCH ARTICLES

 

Effect of the beta-adrenergic agonists, ractopamine hydrochloride and salbutamol, on the performance of finisher pigs

 

 

J.P. de Jager^I; W.J. Steyn^{I, II}; E.A. Akuru^I, ; R.J. Coertze^I; C. Jansen van Rensburg^I

^IDepartment of Animal Science, Faculty of Natural and Agricultural Sciences, University of Pretoria, South Africa
^IISwiNE Nutrition Management, Pretoria, South Africa

 

 

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ABSTRACT

The efficacy and safety of beta-adrenergic agonist use in modern swine production are controversial. This study aimed to determine the effect of dietary ractopamine hydrochloride and salbutamol on the performance of 440 male and female finisher pigs (Topigs Norsvin TN70), housed in 40 pens of 11 animals each, with five treatments and four replicates per treatment. The dietary treatments included a negative control, containing 16% crude protein and no feed additives; ractopamine-16 (negative control plus 6 mg ractopamine/kg); ractopamine-13 (13% crude protein and similar essential amino acid concentrations to the negative control, plus 6 mg ractopamine/kg); salbutamol-6 (negative control plus salbutamol 10% at 6 mg/kg); and salbutamol-4 (negative control plus salbutamol 10% at 4 mg/kg). At 6 mg/kg feed, ractopamine had no effect on pigs' growth and carcass indices compared to the negative control, regardless of the dietary crude protein concentration. However, gilt carcasses had lower lean percentages on the ractopamine-13 diet than on the negative control diet. Pigs fed 4 mg and 6 mg salbutamol/kg had higher lean percentages than the negative control group. The crude protein content of a diet containing ractopamine can thus be decreased from 16% to 13% for boars, whereas gilts may still benefit from a higher crude protein concentration. Salbutamol supplementation suppressed feed intake and growth in boars and enhanced feed efficiency and lean percentage in gilts. Hence, salbutamol supplementation at 4 mg/kg may be a viable alternative to ractopamine for finisher pigs, producing a higher lean percentage and return on investment, and reduced production costs, despite not enhancing growth.

Keywords: carcass modifiers, lean carcasses, nutrient partitioning, pork quality, sustainability

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Introduction

Worldwide, the pig industry is steering towards the production of heavier but leaner carcasses. In South Africa, for instance, the PORCUS classification system has been developed to categorise pig carcasses based on the thickness of the backfat and the percentage of lean meat. This system rewards the production of leaner pigs by awarding a higher classification and price to carcasses with the highest percentage of lean meat (Soji et al., 2015). However, the system fails to adequately address sustainability and safety concerns, thereby ignoring the increasing interest of consumers in safe products that also meet animal well-being criteria.

Beta-agonists such as ractopamine hydrochloride have been used as feed supplements to increase weight gain and feed efficiency, reduce fat tissue accretion, and improve carcass dressing percentage and quality. At higher dietary inclusion levels, they may also enhance carcass leanness by redirecting nutrients away from adipose tissue and towards muscle growth (Halsey et al., 2011; de Almeida et al., 2012; Marcolla et al., 2017; Ferreira et al., 2017; Costa et al., 2018; Abbas et al., 2022; Elanco, 2023). However, concerns remain regarding the safety of ractopamine for both the animal and the consumer (Zaitseva et al., 2014; Abbas et al., 2022), based on reports of increased excitement and aggression in animals fed this supplement (Marchant-Forde, 2003; Poletto et al., 2010). This has led to its ban in China and the European Union (Needham et al., 2017), despite reports of minimal effects on mortality rates (Ritter et al., 2017) and evidence that it increases lipolysis and reduces the intramuscular fat content of pork (Martins et al., 2015; Ferreira et al., 2017).

Despite these safety concerns and bans by some countries, ractopamine was the first beta-agonist receptor ligand to be approved for swine by the United States Food and Drug Agency (FDA) in 1999 (FDA, 1999; Abbas et al., 2022). On 5 July 2012, the first-ever maximum residue levels (MRLs) for ractopamine were adopted by the Codex Alimentarius Commission as 10 μg/kg for pork and beef (Alfredo et al., 2017). Hence, the use of ractopamine is approved in swine, cattle, and turkey diets in the United States, which is the third largest producer of pork worldwide (Arp et al., 2014; Centner et al., 2014; Abbas et al., 2022). Ractopamine use is also permitted in Canada, Japan, South Korea, Brazil, Mexico, and South Africa, as regulators in these countries have deemed meat produced by pigs and cattle fed dietary ractopamine safe for human consumption (de Almeida et al., 2012; Pacelle, 2014; Jaurez et al., 2016; Ritter et al., 2017; Park et al., 2019; Abbas et al., 2022). Ractopamine is currently authorised as an acceptable feed additive in the pig and cattle industries in many countries, based on the recommendations of the joint Food and Agriculture Organization (FAO)/World Health Organization (WHO) Expert Committee on Food Additives and Scientific Evidence (Needham et al., 2017; Abbas et al., 2022), with some countries stipulating MRLs (FAO, 2012; Needham et al., 2017; Abbas et al., 2022).

In South Africa, beta-agonists have been used to improve meat production for the past 25 years (Webb et al., 2018). Ractopamine is registered in South Africa for use in swine and cattle under the trade name Paylean™ (Elanco, 2023), and Needham et al. (2017) reported improved average daily gains (ADG) in male pigs when 10 mg ractopamine/kg was added to the finisher feed. A significant interaction between ractopamine inclusion and dietary crude protein (CP) concentration was also noted, with medium (20% CP, 9.8 g/kg standardised ileal digestible lysine (SID Lys)) and high (25% CP, 12.07 g/kg SID Lys) CP diets resulting in improved feed conversion efficiency compared to a low (16% CP, 7.5 g/kg SID Lys) CP diet. Considering the market value and current authorisation of the use of ractopamine in 27 countries, ractopamine inclusion in pig diets still has the potential to enhance the efficiency of large-scale commercial pork production (Needham et al., 2017).

Salbutamol, a prescription drug used to relieve breathing difficulties in humans, also acts as a repartitioning agent in pigs (Marchant-Forde et al., 2012). Existing literature has demonstrated the positive effects of R-salbutamol supplementation at inclusion levels as low as 2 mg/kg on growth and carcass characteristics in pigs (Marchant-Forde et al., 2012). Salbutamol was registered for use in swine diets in South Africa in 2021 as Salbutamate 10%, having previously been registered in 2014 and 2017 for use in cattle and sheep diets, respectively. The South African supplier of salbutamol (Animate Animal Health, 2021) claims that it has no adverse effects on meat quality and has the potential to ensure sustainable and high-quality meat production. The supplier recommends an inclusion level of 2-6 mg/kg feed during the last 28 days of the finishing phase, with no withdrawal period required in pigs. Several studies have been conducted on the supplementation of salbutamol to South African feedlot cattle (Webb et al., 2023) and sheep (de Klerk et al., 2016; du Toit et al., 2017), but no studies on South African swine could be found.

The FDA approved the use of ractopamine supplemented at 4.5-9.0 mg/kg in feed that contains at least 16% CP for finisher pigs heavier than 68 kg for the last 20-40 kg of weight gain prior to slaughter (FDA, 2006). However, over recent years, the pork industry has made significant genetic advancements (Wu & Bazer, 2019), and it is therefore possible that current commercial pigs, with their high potential for fast lean tissue deposition and high feed efficiency, do not benefit from the continued use of expensive beta-agonist agents. It is therefore pertinent to ascertain whether the use of beta-agonists is still relevant in the nutrition of the modern pig. Furthermore, lower CP diets are often implemented to reduce production costs and minimise environmental pollution. These are generally nutritionally adequate, provided that the requisite amino acids are present in appropriate and balanced proportions (Åkerfeldt et al., 2019; Rocha et al., 2022). Rocha et al. (2022) showed that when diets were balanced for all essential amino acids, the growth performance and feed conversion efficiency of finisher pigs could be sustained, if the dietary CP concentration was maintained above 11.6% and 11.4%, respectively. However, ractopamine enhances lean meat accretion, which naturally increases the requirement for amino acids. Many studies have shown that ractopamine-supplemented pigs often benefit from a higher dietary lysine concentration (Andretta et al., 2012; Needham et al., 2017). However, no study can be found that addresses whether the recommendation of 16% dietary CP with ractopamine supplementation is still valid, or whether the total CP concentration, and thus the concentrations of the non-essential amino acids, can be reduced if the concentrations of essential amino acids are kept sufficiently high.

Considering the beneficial effects of ractopamine and salbutamol supplementation in pigs, it was hypothesised that dietary ractopamine and salbutamol have the potential to elicit significant improvements in the performance parameters of pigs, and that dietary salbutamol can be considered as a viable alternative to ractopamine in pig diets. It was also hypothesised that the recommendation of 16% total CP inclusion in a diet containing ractopamine is outdated, and that the CP concentration could be reduced if the levels of essential amino acids are kept constant.

To test these hypotheses, this study was conducted to assess the suitability of the continued use of ractopamine and salbutamol in enhancing performance and modifying carcass parameters in the modern commercial pig, specifically Topigs Norsvin TN70. The study also aimed to determine the impact of reducing the dietary CP concentration from the recommended inclusion level (16%) to a lower level (13%), while maintaining equal levels of essential amino acids, on the performance of finisher pigs supplemented with ractopamine. Furthermore, in view of the recent approval of salbutamol use in pigs in South Africa, the effects of ractopamine and salbutamol on the growth performance and carcass characteristics of finisher pigs were compared.

 

Materials and methods

The study was conducted at the Baynesfield Swine Research Unit (BSRU) on the Baynesfield Estate Piggery, in KwaZulu-Natal, South Africa. The study location is known for its tropical and humid climate. The area experiences high rainfall between October and March, which are summer months, and has a daily temperature range of 20 °C to 35 °C. Experimental procedures were in line with the ethical approval obtained from the Animal Ethics Committee of the University of Pretoria, with ethical clearance number: NAS128/2022.

Four hundred and forty (440) 70-day-old Topigs Norsvin TN70 x Tempo grower pigs, with equal numbers of boars and gilts, were sourced from the weaner unit at Baynesfield Estate Piggery and moved to a pig grower house on the same site. The house had open sides with automatically controlled curtains that opened and closed to achieve the desired temperature and air flow. There were 40 numbered group pens, divided into four equal blocks with 10 pens in each block. Boars and gilts were randomly divided into separate pens with 11 pigs per pen at a stocking density of 1 m² per pig. Each pen served as an experimental unit and all pigs were ear-tagged with a unique number. Feed and water were provided ad libitum by one Rotechna TR2 feeder per pen and two water nipples per pen. Each TR2 feeder had two 28 cm wide outlets. The same feed was fed to all the pigs from the point of placement at 70 days old until the end of the grower phase at 120 days old (Table 1). The initial body weights of the pigs at 120 days of age did not differ significantly between any of the pens, indicating that the pigs were not influenced by any factors during the pre-trial phase, before the initiation of the dietary treatments.

Five treatment diets (Table 2) were introduced at the start of the finisher phase (at 120 days of age) and were fed until the pigs were slaughtered at 154 days of age. The five dietary treatments fed to the finisher pigs were as follows:

1. Negative control (NC): A diet containing no additives and 16% CP, with 9.9 g SID Lys/kg.

2. Ractopamine-16: The NC diet (16% CP) plus ractopamine included at 6 mg/kg of feed.

3. Ractopamine-13: A finisher diet containing 13% CP and essential amino acid concentrations similar to those of the NC diet, with ractopamine included at 6 mg/kg of feed.

4. Salbutamol-6: The NC diet (16% CP) plus salbutamol 10% included at 6 mg/kg.

5. Salbutamol-4: The NC diet (16% Cp) plus salbutamol 10% at 4 mg/kg.

The inclusion of R-salbutamol sulphate (Salbutamate® 10%) was based on the supplier's (Animate Animal Health) recommendation that the supplement should be included in the diets of pigs at 5-18 mg/pig/day (or 2-6 mg/kg of feed), with higher efficacy linked to higher inclusion rates. In previous studies, R-salbutamol was fed to finisher pigs at 2 mg/kg, 4 mg/kg, and 8 mg/kg (Marchant-Forde et al., 2008, 2012). In the present study, the chosen inclusion levels (4 mg/kg and 6 mg/kg) were within the recommended levels and aligned with the levels reported in previous studies. Future studies can explore higher levels of salbutamol inclusion in finisher pigs' diets. All treatment diets were similar in nutrient specifications, except for the ractopamine-13 diet, which had a lower CP concentration of 13% but similar essential amino acid concentrations to the NC diet (Table 2).

During the trial, the performance of the pigs was measured at the end of the grower phase/start of the finisher phase (120 days of age) and the end of the finisher phase (154 days of age). The feed fed weekly per pen was weighed and divided by the number of animals per pen to determine the mean feed intake (in kg). Pigs were weighed individually once a week, and the ADG was then calculated. Mortality records were kept throughout the trial. The feed conversion ratio (FCR) was calculated by dividing the feed intake of a pen over a period by the total weight gain of pigs in that pen over the same period. The FCR was corrected for mortalities.

At the end of the trial (day 154), all pigs were euthanised and slaughtered at Frey's Food Brands abattoir, Cato Ridge, KwaZulu-Natal. Warm carcass weight, cold carcass weight, carcass moisture loss, lean meat percentage, and dressing percentage were recorded. The Hennessy Grading Probe, a durable, handheld electronic and optical device connected to a computer system that uses reflectance spectroscopy, was used to measure the lean meat percentage. Fat thickness and eye muscle thickness were measured on a hanging carcass with the probe positioned between the second- and third-last ribs, 45 mm from the mid-back line, as described by Steyn et al. (2012). The following formula, proposed by Visser (2004), was used to calculate the lean meat percentage:

The carcass classification system used in this study was based on the South African PORCUS classification system used at the time of the trial. The return on investment (ROI) per pig was calculated using the following formula:

The feed cost was calculated per phase and summed to calculate the total cost per treatment. Revenue was then determined by multiplying the average cold carcass weight by the value of the meat (ZAR/kg) per treatment. The ROI (ZAR/pig) was calculated by subtracting the total cost of feed from the revenue (ZAR/pig) and was calculated per pig per treatment.

Data were analysed using the Proc Mixed model (Statistical Analysis System, 2021) for the average effects. Means and standard errors were calculated, and the significance of the difference (P <0.05) between means was determined using Fisher's test (Samuels, 1989). Repeated measures analysis of variance with a mixed model was used for repeated week or period measures. The following equation describes the linear mixed model used:

where: Y_ijk = variable studied during the period, μ = overall mean of the population, T_i = effect of the ij^th treatment, S_j = effect of the j^th sex, B_k = effect of the k^th block (four equal blocks per treatment), TS_ij = effect of the ij^th interaction between treatment and sex, and e_ijk = error associated with each Y.

The male and female pigs were individually weighed, divided, separated, randomised, and blocked into 40 numbered group pens of 11 animals each, with four replicates per sex for each of the five dietary treatments. Each pen served as an experimental unit. The pens were divided into four equal blocks, with each block consisting of ten pens of five treatments with two sexes per treatment. Sex, treatment, and block were the fixed effects, while pen was the random effect. The averages of the pens were used to determine differences, and testing was done within the sexes between the treatments and not between the sexes.

Carcass classification data were analysed statistically using the Proc Freq model (Statistical Analysis System, 2023) for the chi-square test, frequencies were calculated, and the significance of the differences (P <0.05) were determined using Fisher's test (Samuels, 1989).

 

Results

Results in Table 3 show that the body weight gain and ADG of boars fed the 16% CP and 13% CP ractopamine diets were comparable (P >0.05) to those of the boars fed the NC diet. Final body weights did not differ (P >0.05) between groups, regardless of the dietary treatment. The same trend of comparable (P >0.05) dietary effects was observed for the feed intake and FCR of gilts and boars fed the ractopamine-16, ractopamine-13, and NC diets. Gilts and boars fed the salbutamol-6 diet had lower (P >0.05) feed intakes than the NC pigs, whereas comparable (P >0.05) FCR values were recorded for the two salbutamol dietary groups. Body weight gain and ADG were lower (P <0.05) in boars fed salbutamol-6 than boars fed the NC diet, whereas performance indicators were comparable (P >0.05) between pigs fed diets containing high and low salbutamol concentrations.

Compared to the NC diet, reducing or increasing dietary CP levels had no effects (P >0.05) on the carcass performance indicators of either gilts or boars fed ractopamine hydrochloride (Table 4). No differences (P >0.05) between the treatment groups were observed in the warm and cold carcass weights and dressing percentages (Table 4). Gilts fed the ractopamine-16 diet had a comparable (P >0.05) lean percentage to those fed the ractopamine-13 diet, whereas the gilts in the NC group had higher lean percentages (P <0.05) than those in the ractopamine-13 dietary group. The lean percentage in boars did not differ between the high and low CP ractopamine diets (P >0.05). Gilts fed the 6 mg/kg salbutamol diet had similar (P >0.05) lean percentages to those fed the 4 mg/kg salbutamol diet and the NC diet.

The results of the probability of a significant difference in the frequency of carcass classification are shown in Table 5.

The P and O classes and the R and C classes were grouped, as these classifications tend to receive the same price per kilogram. Pigs fed the low and high concentrations of salbutamol had more carcasses classed in the P+O grouping (80 and 77, respectively) than pigs in the NC group (68). This test was significant and showed that the ractopamine-16 treatment yielded 87.1% P+O carcasses and 12.9% R+C carcasses, compared to the 93.0% P+O carcasses and 7.0% R+C carcasses recorded for the salbutamol-4 group. For ractopamine-13 pigs, 87.8% of carcasses received a P+O classification and 17.2% received an R+C classification, compared to 91.7% and 8.3%, respectively, for salbutamol-6 pigs.

The effects of ractopamine hydrochloride and salbutamol on the ROI are shown in Figure 1. The 4 mg salbutamol/kg diet yielded the highest and best ROI, compared to the ractopamine-16 group (P = 0.0135). The other dietary groups also had higher ROI values than the ractopamine-16 dietary group.

 

Discussion

The swine industry has made significant progress in nutritional technologies, which has led to a ca. 30% increase in the growth efficiency of pigs (Wu & Bazer, 2019; Kim et al., 2024; Monteiro et al., 2025). These significant advancements have provided a better understanding of how feeds are formulated, how feeding strategies are adopted, and the nutritional requirements of animals during any given phase (Pomar et al., 2021). Similarly, higher consumer demands have arisen following these outcomes, resulting in increased preferences for affordable and high-quality pork that is healthy and free from residues (Costa et al., 2018). In this regard, feed additives or feed supplements with carcass-modifying effects have become relevant. According to Kim et al. (2024), efficient utilisation of feed additives is necessary to increase the production efficiency of pigs and the profitability of pig production enterprises.

Feed additives are non-nutritive feed components that can enhance performance indices, increase feed utilisation efficiency, boost metabolism and health in animals, and preserve feeds (Abbas et al., 2022). Ractopamine hydrochloride and salbutamol are beta-agonist feed additives with growth-promoting and carcass-modifying benefits, capable of meeting the modern consumers' demand for lean, tender, and high-quality pork with a low-fat content (Costa et al., 2018). By stimulating beta-adrenergic receptors and the mTOR pathway (Ferreira et al., 2017), ractopamine hydrochloride and salbutamol have been associated with altered nutrient partitioning, increased protein synthesis and muscular growth, reduced fat deposition, and enhanced feed efficiency (Hansen et al., 1997; Mills et al., 2002; Carr et al., 2005; Bohrer et al., 2013; Marcolla et al., 2017; Panisson et al., 2020; Abbas et al., 2022). However, the high number of pigs that become fatigued and/or injured at packing plants as a direct consequence of ractopamine use is concerning (FDA, 2002; Ritter et al., 2017), and safety and regulatory concerns persist regarding the use of ractopamine and salbutamol in pig and livestock nutrition. In some jurisdictions, such as China and the European Union, these concerns have led to the banning of beta-agonists in food-producing animals. These bans have been supported by reported cases of adverse health effects in humans following the consumption of products contaminated with beta-agonist residues, including food poisoning and cardiovascular or central nervous system disturbances (Cheng et al., 2013; Mi et al., 2014).

Salbutamol has been shown to accumulate more in edible organs such as the liver, kidneys, lungs, and intestines, rather than in muscle and fat tissues (Zhang et al., 2017; Yang et al., 2018). Sun et al. (2021) reported that, although residues in muscle and fat were undetectable or low after a seven-day withdrawal period, substantially higher concentrations persisted in edible organs. Similarly, ractopamine concentrations were reported to be higher in the lungs and kidneys than in muscle and fat tissues (Huang et al., 2016). Given that edible offal and organ meats are commonly consumed in China and other regions, potential dietary exposure to beta-agonist residues may be elevated (Sun et al., 2021). In addition, challenges associated with regulating illegal use and overdosing of beta-agonists further support the continued bans in these jurisdictions (Huang et al., 2016).

Nevertheless, ractopamine is licensed by the FDA for use in finisher pigs during the last 28 to 42 days before slaughter (Davis & Belk, 2018). Moreover, both ractopamine and salbutamol are approved for use in the South African pig industry, with both supplements having no withdrawal periods when fed as feed additives to pigs following the manufacturers' prescribed inclusion levels (Needham et al., 2017; Animate Animal Health, 2021). Qiang et al. (2007) supported the established zero-day withdrawal time set by the FDA in a residue depletion study of ractopamine and associated metabolites in the tissues, urine, and serum of pigs fed ractopamine-supplemented diets for 28 days. The study revealed that the concentrations of ractopamine in the edible tissues of the pigs following a zero-day withdrawal period were lower than the tolerance value established by the FDA and the MRL listed by the joint FAO/WHO Expert Committee on Food Additives. Furthermore, the ractopamine, salbutamol, and clenbuterol residue levels detected in fresh meat products (pork, beef, and lamb) in Jilin province in China were reportedly very low and the meat products were considered safe for human consumption (Xu et al., 2019).

The beneficial role of beta-agonists as growth stimulants and carcass modifiers, particularly concerning sustainability and intensified animal production, cannot be overlooked (Abbas et al., 2022), although their continued use in the modern pig with its high genetic potential for fast lean growth has been questioned. This study aimed to determine the efficacy of the beta-adrenergic agonists, ractopamine hydrochloride and salbutamol, as performance enhancers and carcass modifiers in the modern commercial pig, specifically Topigs Norsvin TN70. The study also aimed to ascertain whether lowering the CP concentration from the currently recommended 16% to 13%, while still meeting essential amino acid requirements, would impact the performance of finisher pigs fed supplemental ractopamine hydrochloride.

Results showed that ractopamine hydrochloride supplemented at 6 mg/kg feed did not affect the feed intake, ADG, or FCR of Topigs Norsvin TN70 finisher pigs, regardless of the total CP content of the feed. Other studies similarly found no effect on feed intake when ractopamine was included in the diet at 7.4 mg/kg (Gerlemann et al., 2013; Ritter et al., 2017) and 10 mg/kg (Needham et al., 2017; Panisson et al., 2020). In contrast, Armstrong et al. (2004) and Panisson et al. (2020) found an increase in the average daily feed intake when finisher pigs were fed a diet containing 5 mg ractopamine/kg. A decrease in the daily feed intake was noted by Herr et al. (2001), who supplemented ractopamine at a high dosage of 20 mg/kg, although others who used this same dosage reported no effect on intake (Armstrong et al., 2004). The effect of ractopamine on average daily feed intake is thus not clear, but it appears that a dosage of 5-10 mg/kg generally has no effect on feed consumption in finisher pigs.

The positive effect of ractopamine use on the ADG of commercial finisher pigs has been well-established (Herr et al., 2001; Armstrong et al., 2004; See et al., 2004; Gerlemann et al., 2013; Needham et al., 2017; Panisson et al., 2020). Ractopamine is known to improve growth response by promoting protein deposition, inducing metabolic changes, and increasing muscle protein accretion by increasing the proportion of glycolytic muscle fibres, thereby allowing for the occurrence of enhanced muscle hypertrophy (Schinckel et al., 2003; de Almeida et al., 2012). Ractopamine activates protein kinase B, also known as Akt, which activates the mTOR pathway, thereby increasing muscle growth by stimulating protein synthesis (Ferreira et al., 2017). Proteolysis is also reduced by ractopamine through the inhibition of the ubiquitin ligases, muscle atrophy F-box (MAFbx), and muscle RING finger (MuRF) (de Almeida et al., 2012; Bodine et al., 2014).

Additionally, beta-agonists utilise other pathways, such as the cAMP/PKA/CREB pathway, which also contributes to increased protein synthesis and muscle growth. In a recent study that lasted for 21 days, Reeves Pitts et al. (2023) observed that ractopamine supplementation at 20 mg/kg did not affect the feed intake or ADG, and thus feed efficiency, of unimproved Mangalica pigs. However, ractopamine hydrochloride supplementation might not always be effective, as Rickard et al. (2012) showed in a study in which PIC 337 barrows responded with only a numerical but not significant increase in ADG when fed a diet containing 7.4 mg ractopamine/kg.

As was observed for growth rate, feed efficiency was mostly improved in previous studies where ractopamine was included at various concentrations in the diets of finisher pigs. In the meta-analysis by Apple et al. (2007), it was noted that when significant effects on FCR were found, it was only up to an inclusion rate of 5 mg/kg, with further increases not providing any additional benefit. However, in a more recent study by Panisson et al. (2020), the FCR further improved as the ractopamine inclusion rate increased from 5 mg/kg to 10 mg/kg. In the current study, the FCR was not affected by ractopamine supplementation at 6 mg/kg at either 13% or 16% CP concentrations, despite gilts having slightly but significantly fatter carcasses at slaughter when fed the 13% CP diet, compared to the NC treatment.

In this study, salbutamol supplemented at 6 mg/kg depressed feed intake significantly, compared to pigs fed the NC and ractopamine-supplemented diets. Boars were less sensitive, with the 4 mg/kg salbutamol treatment only causing a numerical but not significant decrease in feed intake, whereas gilts' feed intakes were decreased by salbutamol supplementation at both inclusion rates. The lower feed intake of the 6 mg salbutamol/kg group resulted in a decreased growth rate in boars, but not in gilts, while the FCR was not affected. None of the growth parameters of the pigs that received 4 mg salbutamol/kg differed significantly from those of the NC pigs.

Beta-adrenergic agonists have been shown to suppress appetite in food-producing animals. There is evidence that salbutamol causes a dose-dependent decrease in feed intake by activating the central beta-adrenergic sites (Borsini et al., 1982; Marchant-Forde et al., 2012), as well as peripheral mechanisms affecting hunger and satiety signals, suggesting that salbutamol may cross the blood-brain barrier (Marchant-Forde et al., 2012). Marchant-Forde et al. (2012) reported a significant decrease in feed consumption with the use of 8 mg salbutamol/kg in a finisher pig diet during the last four weeks before slaughter but did not find this decrease when supplementing salbutamol at 2 mg/kg or 4 mg/kg. Furthermore, Marchant-Forde et al. (2012) reported improved ADG values in commercial-type finisher pigs after four weeks of salbutamol supplementation at 2 mg/kg and 4 mg/kg, but not at 8 mg/kg, although feed efficiency was significantly improved by all three dosages. Fawcett et al. (2004) also reported that salbutamol had appetite-suppressant effects at higher dosages, when reductions in weight gain and feed intake were observed in male and female broiler chickens fed higher dietary levels of R-salbutamol.

In the present study, the appetite-suppressing effect of 6 mg salbutamol/kg led to a pronounced reduction in feed intake and a decline in growth rate. Gilts also experienced suppressed appetites at both 4 mg and 6 mg salbutamol/kg, without an effect on growth performance. The reason for this salbutamol x sex interaction is not clear but may be related to the anabolic effects of androgenic steroids such as testosterone in boars, which increase their protein deposition capacity, proportion of lean tissue, feed efficiency, and growth rate. Boars are therefore very reliant on a high supply of amino acids to sustain their high production potential (Xue et al., 1997; Millet et al., 2011; Dubois et al., 2012; Elsbernd, 2014; Patience et al., 2015). Gilts, in contrast, have slower growth, more subcutaneous fat, and increased intramuscular fat contents (Beattie et al., 1999), and thus have lower requirements for dietary amino acids. As both sexes in this study received the same diets, amino acids were in oversupply for the gilts, and lower feed intakes consequently did not affect their growth.

The CP concentrations of the ractopamine-supplemented diets tested this study did not affect carcass weight, dressing percentage, or lean percentage. The lean percentage of gilt carcasses was, however, significantly lower for the 13% CP ractopamine-supplemented diet than for the NC, which contained 16% CP and no ractopamine. Ractopamine inclusion at 6 mg/kg in a feed containing 16% CP had no effect on the warm and cold carcass weights, dressing percentage, or lean percentage of the carcasses compared to the NC group. In the meta-analysis by Apple et al. (2007), it was shown that the lean percentage of carcasses mostly increased when 5 mg, 10 mg, and 20 mg ractopamine/kg was added to finisher diets, although some exceptions were reported. Gerlemann et al. (2013) similarly found that continuous dosing with 7.4 mg ractopamine/kg for 31 days before slaughter significantly decreased fat depth and increased lean percentage. In a more recent study by Soares et al. (2022), ractopamine supplemented at 20 mg/kg feed resulted in a significantly larger loin muscle area but did not affect the backfat thickness, as measured over the longissimus thoracis muscle, 6 cm lateral of the midline. Overall, the growth performance and carcass quality of the ractopamine-supplemented pigs in this study were not superior to those of the pigs fed the NC diet, with the ADG, FCR, average daily feed intake, hot carcass weight, cold carcass weight, and dressing percentage being comparable between the groups.

A previous study showed that ractopamine supplementation at 7.4 mg/kg feed increased the loin eye depth, but backfat depth was only reduced at the location of the tenth rib, and not at the first or last rib, suggesting that ractopamine may affect carcass composition differently at different sites on the carcass (Rickard et al., 2017). March-Forde et al. (2012) noted the same effect for salbutamol at 2 mg/kg feed, with the loin muscle area increasing and backfat thickness at the tenth rib and last lumbar vertebra decreasing significantly, while the backfat thickness at the last rib was not affected. Furthermore, increasing the salbutamol dosage in the feed beyond 2 mg/kg did not result in any further effect. While the mechanisms of beta-agonists' effects on protein synthesis and muscle growth are well explained, their role in fat accretion is less conclusive and not fully understood (de Almeida et al., 2012). Liu et al. (1994) reported that muscle tissue is more responsive than adipose tissue to beta-agonist supplementation, and its ability to reduce adipose accretion may depend on the potency of the agonist, its dose, and the treatment regimen. Salbutamol and other beta-agonists stimulate lipolysis, break down stored fat reserves, and inhibit lipogenesis, which involves the synthesis of new fat molecules (Mitchell, 2009). As a result, pigs supplemented with salbutamol exhibit reduced fat deposition and improved carcass composition, leading to a greater proportion of lean muscle tissue relative to fat tissue (Lu et al., 2017). In the present study, gilts fed salbutamol-supplemented diets had significantly improved lean percentages and outperformed the ractopamine-supplemented groups. This enhanced performance suggests that salbutamol was able to elicit a significant effect on this carcass trait owing to its effect on lipolysis and muscle enhancement. Furthermore, supplementation with salbutamol resulted in significantly more carcasses falling within the P+O carcass class and fewer carcasses falling within the R+C class, compared to the carcasses of pigs that were in the NC and ractopamine-fed groups.

The PORCUS classification system used in South Africa favours leaner carcasses to meet the demands of the consumer, who is looking for healthier, leaner pork. Producers are paid a higher price per kilogram for a carcass with a P or O classification, which partially explains the better ROI (Figure 1) yielded by the treatment group that received the lower dose (4 mg/kg) of salbutamol. The lower feed consumption and subsequently lower feed cost also likely contributed to the higher ROI noted for pigs that were supplemented with 4 mg salbutamol/kg. The other treatment diets (NC, ractopamine-13, and salbutamol-6) also had higher ROI values than the ractopamine-16 group. Economic viability in pork production relies on complex interactions between feed costs, performance outcomes, market prices, and production efficiencies, and will likely vary for a different set of conditions to those prevalent during the current study.

 

Conclusions

The results of this study showed that ractopamine hydrochloride supplementation at 6 mg/kg did not affect the feed intake, ADG, or FCR of Topigs Norsvin TN70 boars and gilts, regardless of the total CP content of the feed. At 6 mg/kg, the ractopamine-supplemented diet containing 16% CP had no effect on the carcass indices of boars and gilts. However, the lean percentage of gilt carcasses was lower for the 13% CP ractopamine-supplemented diet, compared to the NC, which contained 16% CP and no ractopamine. It was also evident that the gilts' appetites were suppressed at lower salbutamol doses than the boars'. However, these decreases in feed intake resulted in decreased growth rates in boars but not in gilts, resulting in enhanced feed efficiency rates in gilts fed diets containing low doses of salbutamol. Gilts fed salbutamol-supplemented diets had carcasses with improved lean percentages, compared to the pigs that received ractopamine-supplemented diets. Dietary salbutamol supplementation also resulted in more carcasses that fell within the P+O carcass class and fewer in the R+C class, compared to the carcasses of pigs that were in the NC and ractopamine-supplemented groups.

It can thus be concluded that decreasing the CP concentration of ractopamine-supplemented diets from the recommended 16% to 13% has no effect on the growth indicators of either gilts or boars. However, supplementing ractopamine in a diet containing 13% CP has the potential to reduce the carcass lean percentage in gilts. Hence, the CP content of a diet containing ractopamine can be decreased from 16% to 13% for boars without any negative consequences, whereas gilts may still benefit from a higher dietary CP concentration, if the production of lean carcasses is a priority.

Considering the established safety of salbutamol use in pigs, the inclusion of salbutamol at 4 mg/kg has the potential to yield a higher ROI, when compared to ractopamine and non-supplemented diets. Further investigations into the economic benefits of salbutamol use in pigs are warranted.

 

Authors' contributions

This research was conducted to fulfil the requirements of an MSc (Agric) Animal Science degree for J.P.D.J., under the supervision of C.J.V.R. and in collaboration with W.J.S. C.J.V.R., W.J.S., and J.P.D.J. conceptualised and designed the experiment. J.P.D.J., W.J.S., R.J.C., and J.J.R.M.M. conducted the experiment. R.J.C. and J.J.R.M. analysed the data. E.A.A. and C.J.V.R. wrote the draft manuscript. E.A.A., C.J.V.R., and W.J.S. revised, corrected, and edited the manuscript before submission.

 

Conflict of interest declaration

The authors have no conflicts of interest to declare.

 

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Submitted 20 June 2025
Accepted 13 January 2026
Published 13 March 2026

 

 

# Corresponding author: eunice.akuru@up.ac.za