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Mechanisms and effects of ractopamine hydrochloride on fat and muscle tissue deposition in finisher pigs

Rikard-Bell, Charles (2012) Mechanisms and effects of ractopamine hydrochloride on fat and muscle tissue deposition in finisher pigs. PhD thesis, Murdoch University.

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The series of experiments presented in this dissertation were conducted to evaluate the optimal responses to dietary RAC in the Australian pig industry. The unifying hypothesis was proposed in two parts: first, optimal responses to dietary ractopamine (RAC) depends on factors that include the level of dietary lysine, level of dietary RAC and gender, whilst the inclusion of specific metabolic modifiers porcine somatatropin (pST) and anti-GnRF immunization vaccine (Improvac) has synergistic effects on growth and performance. Second, the β-adrenoceptors (β) in adipose and skeletal muscle respond differently to RAC dose and therefore the down regulation of specific βs mediates the responses observed in fat and muscle tissue deposition rates.

Experiment 1 (Chapter 3) was conducted to compare current commercial applications of dietary RAC and dietary RAC+pST and their responses in growth performance as well as lean and fat tissue deposition at day 0, 14 and 28 in boars, gilts and boars immunized against GnRF. The study was also designed to confirm whether RAC decreases fat deposition in boars and boars immunized against GnRF. The results of Experiment 1 were surprising in the fact dietary RAC and RAC+pST had no effect on average daily gain (ADG) (P=0.543) or feed conversion ratio (FCR) (P=0.255) on these fast-growing, high-health-status finishing pigs. The effect of dietary RAC on tissue deposition rates were also unexpected as a reduction in fat deposition rate occurred (P=0.064) but no effect on lean deposition rate was observed (P=0.642). The results of experiment 1 influenced the design of the experiments that followed in which factors that affect responses in production and tissue deposition responses to dietary RAC were examined.

Experiment 2 (Chapter 4) comprised of two RAC dose studies to determine the response in light, medium and heavy initial-weight gilts (study 1) or boars (study 2) fed four levels of dietary RAC (0, 5, 10, and 20mg/kg) over a 28-day feeding regime. The hypothesis examined was that light, medium and heavy initial-weight pigs have similar responses to increasing levels of dietary RAC. The major findings were:

In gilts for all initial weight categories:
• Dietary RAC improved ADG, FCR compared to controls (P=0.023 and P=0.029, respectively).
• A linear relationship was observed for ADG (P=0.006) and FCR (P=0.003) with increasing dose of RAC
• Carcass weights improved linearly (P linear =0.006) and tended to improve dressing percentage (P=0.098) with increasing dose of dietary RAC, and
In boars for all initial weight categories,
• Incremental increases of RAC resulted in linear increases for ADG (P=0.003), HSCW (P=0.018) and dressing percentage (P=0.045).
• Dietary RAC did not alter FCR (P=0.289), however there was a tendency (P=0.082) for FCR to decrease linearly as dosage level of dietary RAC increased.
Experiment 3 (Chapter 5) was conducted to investigate whether there are interactions between RAC dosage levels and two levels of dietary lysine (low and high, 0.56 and 0.65 g of available lysine / MJ DE, respectively) on growth performance and carcass characteristics of finisher boars and gilts using dual energy X-ray absorptiometry (DXA) to measure body composition. The hypothesis examined was that low lysine diets of 0.56 g available lysine/MJ DE are sufficient to optimize the response in feed efficiency, growth rate and tissue deposition in boars and gilts fed high (20 mg/kg) or low (5 mg/kg) levels of dietary RAC at initial weights of 65 kg. The major findings were:

• There were significant interactions (P = 0.023 and P = 0.025) between dietary RAC and lysine levels in the first seven days for ADG and feed efficiency (FE) respectively, such that pigs fed high-lysine diets supplemented with dietary RAC had improved ADG and FE, whereas pigs fed low-lysine diets did not respond to RAC supplementation in the first seven days.
• Over the study duration dietary RAC improved daily gain (P=0.026).
• As RAC dose increased ADG increased in a linear (P = 0.072) and a quadratic (P = 0.041) manner.
• In the first seven days dietary RAC improved FE (P = 0.002), but not over the study duration (P = 0.555).
• Dietary RAC reduced change in P2 backfat (P=0.002) as well as indicating a linear (P = 0.033) and quadratic (P = 0.003) reduction with increasing dose in both boars and gilts over the duration of the study.
• A lysine x sex interaction (P=0.043) indicated that lean deposition rate increased in boars but not gilts when fed the high lysine diet.
• Dietary RAC tended (P < 0.1) to increase lean deposition rate only in boars fed high lysine diets.
• Dietary RAC tended to alter lean tissue deposition rates over the initial 14-day period (P = 0.067) and as RAC dose increased lean tissue deposition increased over 14 (P = 0.035) and 28 days (P = 0.044).
• Fat deposition tended to be reduced in a quadratic manner (P = 0.074) as dietary RAC increased over 28 days.
Experiment 4 (Chapter 6) was conducted to investigate the responses of finisher pigs offered a wider range of dietary lysine levels (0.40, 0.48, 0.56, 0.64 and 0.72 g available lysine/MJ DE) and three levels of dietary RAC (0, 5 and 10 mg/kg) over 28 days duration, to determine the optimal level (critical value) of dietary lysine with or without RAC, after which the response to increasing dietary lysine is insignificant in male and female finisher pigs. The major findings were:
• The critical value of dietary lysine for gilts without RAC supplementation for ADG and FCR was 0.54 and 0.52 g available lysine/MJ DE, respectively.
• The critical value of dietary lysine for gilts offered 5 mg/kg RAC supplementation for ADG and FCR was 0.51 and 0.49 g available lysine/MJ DE, respectively. Increasing RAC supplementation to 10 mg/kg increased the critical values to 0.51 and 0.52 g available lysine/MJ DE for ADG and FCR respectively
• A response plateau was not calculated for control boars because the data set did not display diminishing responses for ADG or FCR over the range of dietary lysine concentrations offered.
• Boars offered the 10 mg/kg RAC diet had improved response plateaus for ADG and FCR compared to boars fed the 5 mg/kg RAC diet, however, the improvements required higher critical lysine levels of 0.65 versus 0.56 and 0.59 versus 0.54 g avail Lysine/MJ DE for ADG and FCR respectively.
Experiment 5 (Chapter 7) consisted of two combination studies. The first study examined the combination of dietary RAC and porcine somatatropin (pST) and the second study examined the effect of anti-GnRF immunisation vaccine (Improvac) and dietary RAC. The hypothesis tested for the first study was that the combination of a 28-day dietary RAC regimen with the addition of daily injections of pST in the final 14 days will have additive effects on pig performance when compared to a 28 day dietary RAC regimen. The major findings were:
• Over the study duration FCR was reduced by the RAC + pST treatment (P<0.05) and tended (P<0.09) to be reduced by the RAC only treatment.
• The ADG of the RAC and the RAC+pST treated gilts increased (P<0.05) compared to the control gilts, whereas the ADG of the treated boars did not differ from control boars (Treatment x sex interaction, P=0.025)
• In the second half of the study both dietary RAC and the combination treatments increased (P<0.001) lean tissue deposition in gilts by 165 and 286 g/day respectively.
• The RAC + pST treatment increased lean tissue deposition in boars by 202 g/day when compared to respective controls.
• The RAC + pST treatment also reduced (P<0.001) fat tissue deposition by 87 g/day (gilts) and 118 g/day (boars) when compared to controls in the final 14 days.
• The dietary RAC treatment did not alter fat deposition significantly in gilts and boars.
The hypotheses to be tested for the second study were that 1) anti-GnRF immunization would increase average daily feed intake (ADFI) around 2 wk after secondary vaccination, and that 2) a simultaneous step-up in dietary RAC concentration would allow the additional energy intake to be deposited as lean tissue rather than fat. The major findings were:
• Boars immunized against GnRF had greater ADG and ADFI, but a reduced Gain:Feed (G:F) than the entire boars (P<0.001) over the study duration.
• Pigs fed RAC had greater ADG and Gain:Feed (G:F) (P< 0.001) and tended to eat less (P<0.076) than the controls.
• The change in ultrasound P2 backfat during the study was greater (P<0.001) in boars immunized against GnRF and tended to be reduced (P=0.076) by RAC in boars immuniszed against GnRF.
• Percent lean in the half carcass was increased (P=0.006) by dietary RAC, conversely, percent fat in the half carcass was decreased (P=0.004) by dietary RAC.

Experiment 6 (Chapter 8) was conducted to examine the β gene expression using PCR techniques in adipose and skeletal muscle tissues of boars and gilts fed dietary RAC at either a low (5 mg/kg) or high (20 mg/kg) level compared to control pigs fed (0 mg/kg RAC).. The pigs used in this study were from Experiment 3. The hypotheses was that adipose and skeletal muscle tissue respond differently to dose level of RAC with respects to down (or up) regulation (as measured by the abundance of mRNA transcripts) of the specific βs in finisher boars and gilts. The major findings were:
Within adipose tissue:
• The β1-AR gene was not affected by duration of treatment (P=0.88) or sex (P=0.54), however the addition of dietary RAC reduced the expression (P=0.04).
• The β2-AR gene was not affected by dietary RAC (P=0.66), or sex (P=0.22), however day of treatment increased β2-AR expression at day 29 of treatment compared to day 15 (P=0.06).
Within skeletal muscle tissue:
• The β1-AR gene was not affected by sex (P=0.43) or day of the study (P=0.69) but increased expression with the addition of dietary RAC (P=0.04).
• A sex x RAC dose x Day interaction was observed (P=0.12). Only the boars fed the low RAC diet increased β1-AR expression at day 15 whereas gilts fed high RAC at day 15 and the low RAC at day 29 had increased β1-AR expression compared to controls.
• A sex effect was observed for β2-AR expression (P=0.05), in that the control gilts had a greater expression of the β2-AR gene after 29 days than control boars.
• The addition of dietary RAC tended to reduce the expression of the β2-AR gene (P=0.07) particularly at the high inclusion level (20 mg/kg) of dietary RAC.
From the results obtained in this thesis, I conclude that:
1. RAC dose rates are effective in improving production indices and carcass traits in light (65 kg), medium (80 kg) and heavy (95 kg) initial-weight boars or gilts.
2. Dietary lysine levels are critical in the first 7 days of the dietary RAC regimen.
3. The lysine requirements for boars are higher than gilts between 65 and 95 kg live weight in order to maximise growth and lean tissue deposition.
4. The combination of pST in the final 14 days of a 28-day RAC feeding regime was synergistic in gilts for FCR, ADG and lean deposition, whereas boars further declined their P2 backfat levels with the addition of pST.
5. The additional growth in boars immunized against GnRF was as fat tissue, and that this could be attenuated by supplemental dietary RAC.
6. The β2-AR has a more critical role as the age of the finisher pig advances as expression of the gene increases over time whereas the β1-AR expression is constant.
7. The β2-AR was less sensitive to dietary RAC in fat tissue than the β1-AR gene which down regulated independent of RAC dose at day 15 and may explain the lipolytic response observed in Experiment 3, Chapter 5.
8. The down regulation of β2-AR expression observed in muscle by pigs treated with RAC suggested that the β2-AR may mediate the decline in response observed for ADG (8.6% to 3.7%) and FE (7.1% to 5.8%) in Chapter 5.
9. Low level of dietary RAC has a stimulatory effect on the expression of the β1-AR gene. The up regulation of β1-AR may explain why only small differences between responses in lean meat deposition rates occurred for pigs fed either a high (20 mg/kg) or low (5 mg/kg) RAC diet.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary and Biomedical Sciences
Supervisor: Pluske, John and Dunshea, Frank
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