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South African Journal of Animal Science

Print version ISSN 0375-1589

S. Afr. j. anim. sci. vol.39 no.4 Pretoria  2009

 

The effect of porcine somatotropin (pST) and gender on production parameters and tissue yield of pigs slaughtered at 135 kg live weight

 

 

E. PieterseI,II,#; I.C. SwartsI; L.C. HoffmanI

IDepartment of Animal Sciences, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa
IIAgricultural Research Council, Livestock Division, Private Bag X2, Irene 0062. South Africa

 

 


ABSTRACT

Eighteen F1 crossbred (commercial-type terminal crosses) pigs (boars, barrows and gilts) with an initial weight of 27.2 ± 2 kg were used to investigate the effect of porcine somatotropin (pST) administered for six weeks prior to slaughter on production parameters and tissue yield in the South African scenario. Pigs were grown to 135 kg live weight, which is heavier than the average weight at slaughter in South Africa of 80 - 100 kg. Porcine somatotropin had no significant effect on average daily gain or feed intake. However, pST administration caused a significant increase in feed conversion ratio (kg feed/kg gain) of treated boars, indicating that boars converted their feed less efficiently when treated with pST. This contradicts most of the findings in the literature. The effect of pST on the different carcass cuts was not significant, except for the percentage loin back, which was higher for pST-treated animals and percentage middle back of boars and barrows, which was slightly higher. No significant pST effects were found for live weight, carcass weight, % bone, % fat or % lean meat, but a significant increase in percentage skin was found.

Keywords: FCR, P2 back fat, pST, porcine somatotropin, pork, tissue yield


 

 

Introduction

The production of acceptable animal-derived products in a sustainable manner has been the aim of farmers since they started domesticating meat animals. The emphasis has, however, changed as consumer demands have changed from people who do physical labour to health conscious consumers demanding low fat, healthy food. Therefore, in recent times, the consumption of leaner meat has become the norm.

Despite the advances made in terms of genetics, associated problems with breeding lean pigs, such as the occurrence of pale, soft and exudative (PSE) meat, has slowed down the progress in breeding leaner animals. The production of recombinant porcine somatotropin (pST) has made it economically viable to produce leaner animals at higher bodyweights, with better carcass characteristics (McNamara et al., 1991), or to produce animals at similar bodyweights with better carcass characteristics (Thiel et al., 1993; White et al., 1993). This was also noted for both boars and gilts (Klindt et al., 1995).

The advantages of pST treatment of animals grown to normal slaughter weights (90 kg) are well documented in terms of increased average daily gain, decreased back fat thickness, feed conversion ratio and meat quality characteristics (Carter & Cromwell, 1998; Campbell, et al., 1990; Bidanel et al., 1991; Hagen et al., 1991; Klindt et al., 1992; 1995).

A number of studies investigating the influence of pST on carcass composition and carcass characteristics of animals grown up to 90 or 100 kg live weight (Thiel et al., 1993; White et al., 1993) have been reported, but few studies have been reported where animals were fed up to 135 kg. McNamara et al. (1991) treated animals up to 136 kg and found significant effects on the reduction of fat and increase of protein in the carcasses of treated animals, but they found only a small effect on bodyweight.

The aim of the current study was to ascertain whether pST treatment of animals used in commercial production in the South African scenario would have a positive effect on the performance and tissue yield (composition) of animals grown up to a bodyweight of 135 kg. Carcasses were also divided into commercial cuts that were dissected to ascertain how the distribution of body tissue is affected by pST treatment as has been noted previously (Thiel et al., 1993, White et al., 1993; Fabry et al., 1991). The effect of the sex type of the treated animals was also studied on all these parameters.

 

Materials and Methods

Eighteen crossbred animals (commercial-type terminal crosses, deemed by the South African Pork Producers Organisation to be representative of the South African slaughtering) were used. They were individually marked and housed in individual pens. The animals were equally divided into three sex types: boars, gilts and barrows with three animals per sex type receiving the treatment whilst the other three represented the control. The trial started with animals of 9-10 weeks of age, weighing 27.2 ± 2 kg. Pens were equipped with a self-feeder and automatic water nipple. The facilities comprised of a commercial-type temperature controlled grower house with self opening curtains.

Porcine somatotropin was administered to animals randomly allocated to the pST treatment group after they reached an average bodyweight of 95 kg. A daily dose of 1 mL (5 mg rpST) reconstituted Reporcin® (Alpharma Animal Health, Victoria, Australia) was administered intramuscularly at the base of the neck for six weeks prior to slaughter. A commercial grower diet (Diet 1, Table 1) containing 180 g crude protein (CP), 11g lysine, 14 MJ/kg digestible energy (DE)/kg and oxytetracycline (10%) (included at 2 kg/ton) was fed for the first 14 d after arrival. Thereafter Diet 2 was fed until six weeks into the trail, when the animals attained an average live weight of ca. 65 kg. Diet 2 had the same composition as Diet 1, but contained no medication. Diet 3, containing 160 g CP, 9 g lysine and 13.5 MJ DE/kg was then fed to all the animals from 6 - 12 weeks into the trial (average live weight ca. 95 kg).

It is well documented that the voluntary feed intake of pST-treated pigs decreases significantly (Kanis et al., 1990; Johnston et al., 1993). To ensure that the control groups and the pST-treated groups had similar total protein (lysine) intakes, a diet with a higher concentration protein was fed to the pST-treated groups. Thus, when administration of pST commenced at 12 weeks, the pST-treated animals were fed a 180 g CP/kg diet (Diet 2) until the end of the trial. The control animals remained on Diet 3 (160 g CP). All animals had ad libitum access to feed and water.

Feed intake and animal weights were measured weekly. The data was then used to calculate a feed conversion ratio (FCR) by dividing the total feed intake by the total body mass gain of the animals during the trial period. Average daily gain was analysed by fitting a linear model.

After 15 weeks on these treatments, animals were slaughtered at an average live weight of 135 kg in a commercial abattoir. The transport and handling of the animals ante mortem was conducted in such a manner so as to minimise stress. Due to the vehicle design, animals had to be mixed during transport and in the holding pens at the abattoir.

Animals were led into a stunning cage where they were stunned with an electrical stunner set at 220V and 1.8A, with a current flow of no longer than six seconds. Electrodes were placed at the base of the ear. Within 10 seconds exsanguination followed, and within 50 seconds shackling and hoisting of the carcass was completed. Scalding commenced within five min after stunning. Thereafter the carcasses were dressed using the standard commercial procedures.

The carcasses were hung in a cold storage room (4 ºC) for 12 h before they were dissected. After the head, tail, kidneys, peritoneal and intestinal fat had been removed from each carcass (Figure 1) it was sawn into two halves from top to bottom. The right side was dissected to determine the yield of skin, bone, fat and lean meat.

 

 

The fillet was removed by cutting it away on the inside of the carcass directly below the hip bone by cutting along the hip bone and the lumbar vertebrae. Membranes and connective tissue were removed from the fillet. The neck was removed by cutting at a 90º angle to the ventral line between the last cervical and first thoracic vertebrae, after which the rind, bones and fat were dissected. The shoulder was removed by cutting along the inside of the front leg and around the shoulder blade up to the spinal cord and along the 5th and 6th thoracic vertebrae. The trotters were removed by cutting through the metacarpal region (joint between the carpal bones and the radius and ulna). The rind, fat and bones were dissected from the remaining part of the right shoulder.

The hind leg was removed between the 2nd and 3rd sacral vertebrae perpendicular to the stretched leg. The trotter was removed from the ham at the distal end of the tibia and fibula parallel to the cut made to remove the leg from the carcass. The trotters were removed by cutting through the crural region (across the middle of the tibia and fibula). The rind, fat and bones were dissected from the remaining part of the right leg. The belly was removed from the back by cutting parallel to the spinal cord, next to the eye muscle, approximately 18 cm from the spinal column, i.e . a straight line from the posterior ventral point of the M. psoas major to the cranio-ventral edge of the 4th thoracic vertebra at the anterior end.

The back was split into the loin back and middle back by cutting through the back between the vertebrae at the caudal position of the last rib. The last three ribs were removed from the middle back by cutting through the spinal cord above the 3rd last rib (3-rib cut). The belly was split into the middle belly and loin belly by cutting at the position of the last rib. The rind, bones and fat were dissected from each of the parts.

Data obtained in this study were analysed using the GenStat (2000) statistical program. A randomised trial design utilising 18 pens (animals) was used in a 2 (treatment) X 3 (sex) factorial design. Each sex group consisted of six pens, three control animals and three treated with pST. One of the animals died during the trial (cause of death was a heart-attack at the abattoir) and no data were obtained from it. Analysis of variance (ANOVA) was used to test for differences between the groups. Treatment means were separated (Snedecor & Cochran, 1980) by using Fisher's F-test with a protected least significant difference (LSD) at a 5% level of significance (P <0.05). Percentage variance accounted for was calculated as the percentage ratio of the sum of squares of the parameter in relation to the total sum of squares. Values below 10% indicated a low residual and were therefore deemed coincidental.

Average daily gain (ADG) was analysed by fitting a linear model to the weekly live weight measurements of the animals. The slope of the model would then represent the average daily gain.

 

Results and Discussion

Feed intake, calculated cumulatively on a weekly basis, was not significantly influenced by pST or sex type in this study (Tables 2 and 3). This is in contrast with the findings of Goodband et al. (1993) and Klindt et al. (1995) who found a decrease in feed intake for animals treated with pST and fed diets with increased levels of lysine. Klindt et al. (1995) also found a dose-dependent decrease in feed intake of boars and gilts slaughtered between 81.1 and 94.4 kg. However, lysine levels were not adjusted in diets fed in their study and the eventual slaughter weight was lower than in the current study (94.4 kg vs. 135 kg) and pST treatment only started at a more advanced age in the current study which could be the cause for the lack of response noted.

It can be seen in Tables 4 and 5 that pST had no significant effect on the live weight increase of animals grown up to 135 kg, this finding confirms the results of Klindt et al. (1995) who found gains to be unchanged or greater in the pST treated group and that the effect of pST treatment was dose dependant. Given the fact that the trial of Klindt et al. (1995) grew the animals up to 94.4 kg and the current study only started treating with pST at ca. 95 kg live weight the lack of response could further be attributable to a lower than needed dose for response. Etherton et al. (1986) found a significant increase in carcass length and live weight of pST-treated pigs, slaughtered between 76 and 80 kg live weight, in contrast with what was observed in the current study, where animals were slaughtered at a higher weight. Chung et al. (1985) also found an increase in live weight, but found no significant influence on carcass length of pigs slaughtered at 60 kg live weight. Klindt et al. (1995) found a dose-dependent increase in gain of boars and gilts slaughtered between 81.1 and 94.4 kg. In this study no significant differences in live weight (P >0.05) and carcass weight were detected between pST animals and control animals (P >0.05). Animals were slaughtered at an average live weight of 135 kg (ranging from 113 - 160 kg) and average carcass weight was 113.4 kg. Since all documented studies tested the use of pST up to a substantially lower slaughter weight it is believed that the dose of pST expressed as mg per kg of live weight was much lower in the current study and probably insufficient to obtain a proper response.

When the FCR of the animals was calculated, a significant effect pertaining to sex type and pST was detected (Tables 6 and 7). Contrary to the results obtained by Etherton et al. (1986) and Chung et al. (1985), all animals treated with pST in this study had an increased FCR. It is interesting to note that from week 10 onwards very low F-probabilities (weeks 10 and 13 were significant) were detected for the interactions between sex type and pST treatment. Since these probabilities described more than 10% of the variation, these observations cannot be seen as merely co-incidental. It can thus be seen in Table 8 that boars had a tendency to have an increased FCR when treated with pST. For all treatments boars converted their feed to live weight better than barrows and gilts from week 10 onwards. However, pST had a negative effect on the FCR of boars although they had the best FCR of all sex types. It would appear that the animals did not have the ability to utilise the higher quality feed for growth and actually deposited this oversupply as carcass fat, this inability could also be explained by the suboptimal levels, on a mg per kg live weight basis, believed to have been administered in the trial. The animals are therefore physiologically older than what the researchers expected and the supply of the trial diets actually represented a substantial oversupply of nutrient in relation to level of maturity.

Average daily gain was analysed by fitting a linear model R2 >0.98. Although sows (0.869 kg/day) and barrows (0.848 kg/day) had a slower rate of gain than boars (0.934 kg/day), pST had no significant effect on the rate of gain attained by any group of animals in this experiment. This is in contrast with results documented by earlier investigators who generally reported an increase in ADG (Campbell et al., 1989; McLaughlin et al., 1989). This response was, however, further noted to be dose dependent (Fabry et al., 1991; Klindt et al., 1995) with the former author noting some responses only at 6 mg per day. The lack of response thus noted in this study is then attributed to the probable below optimum level of administration of pST. Animals were slaughtered and dissected as described earlier. Probabilities for differences are tabulated in Table 9.

As can be seen from Table 9, pST had very little effect on the commercial cuts. Animals treated with pST had a significantly lower percentage (11.2%) loin back than control animals (12.1%). The percentage middle back showed a treatment X sex interaction where pST caused a significantly lighter middle back in the boars and barrows (see Table 10), but not in the gilts. No significant differences could be detected with pST treatment for any of the other carcass cuts.

Although the treatment effect was not significant (P >0.05) for the entire middle back, a trend (P <0.1) was detected for percentage bone in the middle back, with the pST-treated (14.2% vs. 13.2%) animals having more bone than the control animals. This is supported by the data analysed for the 3-rib cut: pST- treated animals contained a significantly higher percentage bone (13.4% vs. 11.5%) than the control animals. When the data was combined for all the cuts (Table 11) it was found that animals treated with pST had a higher percentage (P = 0.024) skin (5.04%) than the control animals (4.28%). Although boars had a significantly higher percentage skin than barrows and gilts, no sex X treatment interaction was detected (Table 6).

 

Table 12

 

These results did not support reports by numerous authors (Klindt et al., 1992; 1995) who studied lighter animals and found a reduction in body fat as well as an increase in lean muscle. It could be speculated that the lack of response detected in this study (especially on % fat) could probably be attributed to the low number of replicates combined with the high slaughter weights which could indicate a suboptimal level of administration of pST, where the percentage fat reduction with pST could be so small that the variation between treatments was higher than the possible effect of the treatment.

The increase in average daily gain and decrease in FCR, as referred to by Etherton et al. (1986), Chung et al. (1985), Fabry et al. (1991) and Klindt et al. (1995) were not seen in this study. However, it appears that pST treatment caused boars to have similar (though slightly higher) FCRs than gilts and barrows. Since the animals slaughtered in this study were slaughtered at a higher body mass, this could have caused the animals to respond in a different manner than less mature animals used in the referenced studies. Another facet that may have influenced this result was the fact that pST was introduced later in life and at a higher live weight than in the cited literature and therefore may have had a limited effect. This warrants further investigation, especially as pertaining to increased doses for higher live weight animals.

 

Acknowledgements

The following are thanked for their financial contribution and/or the use of their facilities: Alpharma (Instavet), the Technology and Human Resources for Industry Programme (THRIP; #1312), the Agricultural Research Council - Livestock Division, Stellenbosch University, South African Pork Producers Organisation and the RTV abattoir. Special thanks to E.L. Gloy and B. Hambrock for their technical assistance.

 

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# Corresponding author. E-mail: elsjep@sun.ac.za