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Studies from many Australian farms have consistently found that approximately 20% of boars at current slaughter weights are highly tainted.
This level of boar taint is one of the key factors limiting the eating quality of pig meat in Australia.
Internationally more than 95% of all male pigs are castrated before weaning to control boar taint. While castration is effective in controlling boar taint it is at the expense of reduced feed efficiency.
Castrated boars are fatter than normal boars, need more feed and suffer higher losses due to the stress of castration.
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What causes boar taint?
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The two prime causes of taint are:
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- Androstenone, a male sex pheromone, produced in the testes and of similar chemical structure to testosterone; and
- Skatole, a metabolite of the amino acid tryptophan, produced in the hindgut by intestinal bacteria.
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Both androstenone and skatole are highly fat-soluble. Thus, their concentration in some boars can be high. These are the boars that consumers detect as tainted.
As the boar starts to go through puberty, at around 14-15 weeks of age, there is a surge of testosterone and androstenone production. This is quickly followed by an increase in the concentration of androstenone in fat.
The physiology of skatole is somewhat more confusing. While androstenone is only produced in boars, skatole is produced in both male (intact and castrates) and female pigs. However, the fat concentration of skatole is higher in boars than in castrates or gilts. The reasons for this are unclear but are related to male sex hormones.
While gilts and castrates generally have much lower skatole concentrations than intact boars, some individual gilts or castrates can have skatole concentrations high enough to cause the detection of taint. Particularly if kept under dirty conditions.
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Other contributors to taint
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While androstenone and skatole are the primary contributors to boar taint there are several other compounds that may in some carcasses make a significant contribution to the perception of boar taint.
While some of these compounds have only a mild odour compared with androstenone and skatole, they can nevertheless in some circumstances make significant contributions to the overall perception of taint.
Any control options should address all causes of taint, not just one of the key compounds.
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Interaction between androstenone and skatole
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Both androstenone and skatole are important contributors to boar taint in their own right. However, skatole has been shown to enhance the sensory perception of androstenone. Unpleasant odours associated with androstenone can be intensified when high skatole levels are found simultaneously.
Certainly the worst odours and flavours are consistently associated with meat from boars that have high concentrations of both compounds. These are the pigs that must be avoided.
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Effect of weight/age on taint
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Because live weight is closely linked to age and the onset of puberty there is a trend for androstenone to increase with age and weight. This trend is probably a function of increased sexual maturity. Increasing slaughter weight, up to about 120 kg live weight will increase the risk of tainted meat reaching the consumer.
Some retail outlets are attempting to control taint by specifying lighter carcasses. The problem with this is that light carcasses are more expensive to produce and process, and unless extremely light (< ~ 70 kg live) there is no guarantee that they will be taint free.
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Effect of breed on taint
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The concentration of androstenone varies with breed. Duroc boars produce higher levels of androstenone than Large White or Landrace cross boars. Skatole is less affected by breed.
German research, in 1987, showed that androstenone could be successfully reduced by selecting boars with low androstenone concentrations in fat tissue. However, this also resulted in substantial reduction in reproductive performance, and at this stage is not a viable trait to include in commercial selection indices.
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Effect of nutrition on taint
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- Manipulation of the diet can reduce skatole levels in fat. However, androstenone remains unaffected by dietary manipulations.
- High levels of fermentable carbohydrate in the colon favour the growth of fermentative bacteria that use tryptophan for cell growth rather than as an energy source, thus decreasing the amount of tryptophan metabolised into skatole.
- For example, inulin is a complex carbohydrate that is not metabolised by pig gastric enzymes and can reach the hindgut relatively undigested. Feeding inulin for five days prior to slaughter can result in marked reductions in skatole concentrations. Similar results have been obtained by short term feeding of sugar-beet pulp. Neither dietary additive has any affect on androstenone.
- Lupins, which are high in fibre and non-starch polysaccharides and have the capacity to support hindgut fermentation, have been suggested as a dietary additive to reduce skatole. However, lupins often also reduce feed intake and growth rate.
- The inclusion of antibiotics such as zinc bacitracin, avoparcin or virginamycin in the diet over the week prior to slaughter has also been shown to reduce skatole. This is presumably because of a reduction in hindgut microbial biomass.
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Effect of housing on taint
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Housing conditions have been shown to affect both skatole and androstenone, but unfortunately not both together.
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- Cleanliness: Danish research has shown that skatole, from faecal contamination, can be absorbed via the skin and lungs. Because skatole is produced in the hindgut, its concentration in faeces is extremely high. Thus, keeping both boars and gilts in dirty environments, where they are heavily fouled with faeces, can result in dramatic increases in fat skatole compared with pigs kept in a clean environment. Local Australian and New Zealand studies have similarly shown very high skatole concentrations in pigs kept in dirty conditions.
- Straw based housing systems: Australian research has shown that boars reared to about 100 kg live weight in a straw-based group housing system had 3 – 4 times lower skatole concentrations compared with pigs reared on partially slatted concrete floors. In contrast, boars reared under his straw-based group housed system appeared to be more advanced in sexual maturity and consequently had substantially elevated androstenone concentrations.
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Control and management of boar taint
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Basically, there are two strategies that can be used to reduce the risk of tainted carcasses reaching the consumer.
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- The first strategy involves management or treatment of the pig to reduce the risk of tainted compounds. As discussed above these techniques can reduce the concentration of either skatole or androstenone, but rarely both compounds. Thus, they do not eliminate the risk of tainted carcasses reaching the consumer.
- The second strategy involves the complete elimination of taint via the control of all compounds that might contribute to boar taint.
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Strategy 1 - Reduction of taint
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- Lighter slaughter weight: An effective method of reducing taint is to slaughter pigs before sexual maturity, ie. at an early age and quite light live weight. Younger, lighter pigs are generally less advanced in puberty than older, heavier pigs and thus have lower levels of androstenone and skatole. However, the costs of producing and processing light carcasses are significantly higher than for heavier carcasses and result in reduced profitability for producer and processor.
- Screening carcasses: Screening carcasses based on taint has been suggested as a suitable method of reducing the risk of tainted meat from reaching the consumer. For example the Danish, in the belief that skatole was the only cause of boar taint developed a sophisticated on-line system for measuring skatole concentration. This system was installed in all Danish abattoirs in the mid 1990’s with the aim of the entire Danish pig production switching from castrates to intact boars. However, because there are other compounds that contribute significantly to taint this system was found unsuitable as a practical method of reducing the risk of taint to the consumer. Currently there are no suitable practical on-line methods for screening for low taint risk carcasses. Even if such method were routinely available, the real problem with screening is what to do with high taint carcasses.
- Diet and Housing (see above): Various dietary and housing treatments can be applied which can reduce one component of boar taint. However, there are no dietary or housing treatments that can reduce both androstenone and skatole.
- Porcine Somatotropin (pST): Treatment with pST has been shown, sometimes, to partially reduce the concentration of androstenone in fat, presumably by an effect on delaying puberty. However, pST treatment does not affect skatole.
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Strategy 2 - Elimination of taint
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Castration, either surgical or immunological is the only guaranteed method that effectively reduces or eliminates all the compounds that might contribute to boar taint.
Surgical Castration: Internationally, pre-weaning surgical castration of the male pig is the most widely practised method of controlling boar taint. While surgical castration is highly effective, it is at the expense of reduced growth efficiency. Castrates generally have increased carcass fat (4-7mm), decreased lean meat yield (2-4%) and decreased feed conversion efficiency (7-15%). Surgical castration, even pre-weaning, is also associated with significant animal welfare concerns.
Immunological castration: Immunological castration involves vaccination of the boar against its own compounds that control the production of boar taint. The technique is highly effective in controlling ALL the compounds that might contribute to boar taint.
By leaving the boars grow as intact males for most of their production life vaccination offers significant production benefits compared to surgically castrated boars.
In addition, the technique is a welfare friendly alternative method of controlling boar taint without jeopardising growth performance.
For details of Improvac™; the world’s first boar taint vaccine, contact your veterinarian or Pfizer technical services.
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