03 May The Impact of Feeding Lambs with High Grain Diets for Long Periods on Consumer Eating Quality
US Trip Report by Emily Webb Ware
In light of strong domestic and export demand for lamb, along with unpredictable climatic conditions and volatile rainfall patterns, the Australian lamb industry is turning to lot feeding as an alternative to pasture-based lamb production systems (Agriculture Victoria, 2017). Australia may look to the US lamb industry, where lot feeding lambs is more common, as well as industry research, to consider the impact an increase in lot feeding production systems may have on the industry. This report will investigate the impact of long-term lot feeding, with high grain diets, on lamb eating quality, as well as a number of changes occurring within the Australian and American lamb industries relating to eating quality.
The US and Australian lamb industry
An important feature of the US lamb industry is the high percentage of lambs, estimated at up to 90%, which are finished in feedlots (Male, 2012). This is partially due to the way producers are paid for lambs; premium price is paid up to 68 kg liveweight, and producers are not discounted severely for heavier lambs (Livestock Marketing Information Centre, 2018). Margins can still be made on growing lambs out past these premium weights, so lambs are commonly fed on grain-based diets for long periods, with the aim of producing the biggest, heaviest (and consequently fattest) lambs possible (Male, 2012). There is little incentive to produce lighter, leaner animals. This has seen an increase in the average size of lambs produced across the industry; as of February 2018, the average live weight of lambs sold for market in the US was 65kg, and the average carcase weight was 33kg (National Livestock Information Centre, 2018).
Heavier carcases often means additional fat, and create issues relating to increasing cut sizes, which is undesirable to consumers and presents challenges for processors, retailers and exporters (MLA, 2018a). Furthermore, to achieve these heavier carcase weights, animals are usually on concentrate-based diets for a long period of time, therefore age and nutrition are more likely to have an impact on eating quality (Tatum et al, 2014).
The Australian lamb industry is based more on pasture production and finishing systems, with lambs being more heavily discounted if they fall outside Hot Carcase Weight (HCWT) and Fat score on abattoir payment grid specifications. Consequently, average carcase weights are considerably smaller than those in the US (Male, 2012). However, improved genetics, better livestock management and changing feeding regimes (with grain finishing systems becoming more common) have seen lamb slaughter weights push higher over the past decade (MLA, 2017a). The average carcase weight of an Australian lamb is now 22.9kg, and this number is forecast to rise to 23.3kg by 2022 (MLA, 2018a). Furthermore, while lambs continue to be sold based on HCWT (as opposed to lean meat yield), producers will focus more on achieving heavier carcase weights, as opposed to producing lean, high quality animals (Craig, 2017).
Australia’s own feedlot industry has the potential for expansion in the future, as a means of producing a consistent supply of quality lambs to meet growing demand, regardless of changing climatic conditions (Agriculture Victoria, 2017). Growth in this sector could contribute to the increase in the age and weight of lambs coming into the supply chain.
There is increasing evidence to suggest that long-term lot feeding with high grain diets as an influence on lamb eating quality, particularly in terms of flavour. This becomes important as lamb remains an unfamiliar protein to a number of consumers, particularly in overseas markets such as the US (Gibbs, 2017), and the distinct taste of lamb can be a barrier to increasing demand in unhabituated consumers (Shiflett, 2018). It is therefore important for the industry to understand any impacts long term grain feeding may have on the flavour and overall eating quality of lamb meat.
The impact of nutrition and age on lamb eating quality
Feeding lambs a high grain diet for a long period of time, as in a feedlot operation, changes the compounds present in the animal’s fat, which in turn may affect the flavour profiles of the meat (Beriain et al, 2000).
The distinct flavour of lamb is partially attributed to two aromas linked to the odour of cooked sheepmeat. These aromas, known as ‘pastoral’ and ‘mutton’ flavours, are influenced by a number of compounds present in ovine fat (MLA, 2017b). During cooking, oxidation reactions in the fat tissue result in odour active volatile compounds that influence the flavour of the meat (Watkins et al, 2013).
‘Pastoral’ flavour in lamb is associated with animals grazed on pasture, and is described to include “sheepy”, “gamey”, “barnyard”, “animal”, “fecal”, “grassy” and “milky” flavour notes (Tatum, 2014). The main compounds found responsible for these flavours have been identified as 3-methylindole (known as skatole and attributed to boar taint in pork) and associated metabolite indole, as well as 4-methylphenol (commonly known as p-cresol), which together are most evident in the cooked meat of pasture-fed sheep (Watkins and Warner, 2011). These compounds form in the rumen as a result of degradation of pasture proteins to amino acids and the formation of metabolites. These products are normally further metabolised in the liver, but if enough is produced, some is released into the bloodstream for storage in fat tissue. A diet higher in protein, for example, if lambs are grazed on lucerne or clover, results in a greater accumulation of these compounds in the adipose tissue (Warner et al, 2013).
These ‘pastoral’ flavours are often familiar as typical ‘lamb’ flavours (in the case of Australian and New Zealand consumers), but may be unacceptable in other consumer markets where these pasture-associated flavours are unfamiliar (Watkins et al, 2013). However, very high concentrations of these main ‘pastoral’ compounds are disliked by all consumers (Prescott et al, 2001).
Other studies suggest that ‘mutton’ flavour, which is associated with the age of an animal, may also be influenced by the diet. Branched chain fatty acids (BCFAs) are formed as a result of propionate metabolism. Ruminants fed grain-based diets produce higher levels of propionate, meaning that lambs fed grain for a period of time (as is typical in US lamb production systems) may have a higher concentration of volatile BCFAs in their fat (Tatum et al, 2014). Concentrations of these BCFAs also gradually increase with an animal’s age (Watkins et al, 2010). These BCFA molecules, particularly that of 4-methyloctanoic acid and, to a lesser extent, 4-ethyloctanioc acid and 4-methylnonanoic acid are largely responsible for the ‘mutton’ aroma present in cooked sheepmeat. Stronger ‘mutton’ flavour is associated with lower consumer acceptance, and is often cited as a reason that sheepmeat consumption is low in a number of markets, including the US, Canada, and Japan (Watkins et al, 2014, Locke and O’Connor, 2017).
Previous work by Watkins et al 2014 demonstrated that nutrition can have an influence on concentrations of BCFA in sheep fat, and by extension, can also have an impact on the aroma of the associated cooked lamb product. Fat samples were taken from the lamb carcasses that had been fed on either a concentrate-based diet or a pasture-based diet. These samples were then tested for the concentrations of a number of compounds relating to sheepmeat eating quality, including BCFAs affecting ‘mutton’ flavour. The results found that consumers rated sensory scores lower in meat samples that had fat with higher concentrations of these BCFA molecules (Watkins et al, 2014). Furthermore, higher concentrations of these BCFAs were seen in the lambs fed on concentrate-based diets. This suggests that long-term grain feeding systems could have negative consequences on flavour, and consumers could become less likely to continue to consume lamb products.
Other indirect effects on flavour should be considered. Lambs that have been fed on grain over a time period are likely to see a greater deposition of intramuscular fat. While intramuscular fat content of the meat has a positive influence on meat juiciness, texture and flavour during eating (Pannier et al 2014, Wood et al, 2008), increased fat as a proportion of meat may increase the negative impacts of volatile compounds as a result of long term grain feeding.
The vitamin E status of muscle is another example of the impact feed can have on meat quality. Vitamin E, which decreases lipid oxidation, can help prevent the undesirable deterioration in meat flavour after slaughter (CSIRO, 2012). As green feed and pasture is the best natural source of vitamin E, grass-fed lambs see less rapid fat oxidation and better flavours than lambs fed a long-term high-grain diet (CSIRO, 2012). However, work by Ripoll et al 2011 has demonstrated that supplementing vitamin E into a grain-based diet can avoid the effects of lipid oxidation, and in this way, increase the shelf life of the meat.
Future direction for the US and Australian lamb industries
The American Lamb Board, having identified the importance of lamb meat flavour, is investing in research aimed at the potential of producing in plant technologies to evaluate lamb flavour based on volatile compounds and fatty acid composition (Hoffman, 2018a). This technology would operate at production speed, and be capable of identifying and characterising compounds linked to specific flavours in lamb associated with both positive and negative flavour notes. From this, processors would have the ability to sort carcases into expected flavour groups, enhancing the consistency of the product delivered to retailers and consumers (Maneotis et al, 2017).
The American Sheep Industry Association (ASI) is also working to improve the USDA definition of ‘lamb’. The ASI has noted that the industry would benefit from more clarification about the specific definition of lamb to improve consistency in labelling and marketing of lamb. This will also help to better reflect changes in eating quality that occur with age (Hoffman, 2018b).
As the Australian lamb industry is presented with consistently increasing carcase weights, the challenge is finding and increasing access to markets that will accept heavier products (MLA, 2018a). The industry must also adapt to additional fat coming through the supply chain, control portion sizes, and become more innovative with available cuts and packaging of lambs to remain appealing to consumers (Male, 2012).
Part of the solution will also be through changing the supply chain to better reflect demand, by moving away from the current system that focuses on HCWT and Fat score. The Australian industry is now moving towards value based marketing, which will see market specifications and pricing based on lean meat yield and eating quality of individual carcases. This will give real incentives to producers to finish animals in a way that better meets value chain requirements, through price signals aligned with the needs of processors, retailers, exporters, and importantly, consumers (Pearce, 2016). These changes would favour production of leaner carcases at more desirable weights, while long term grain feeding systems that focus solely on putting more weight on carcases would likely not be as profitable.
New technologies, including Dual Energy X-Ray Absorptiometry (DEXA) and Hyperspectral Imaging, will allow objective measurement of carcase characteristics, and provide producers with accurate feedback on both lean meat yield and eating quality factors (including intramuscular fat). In particular, the industry can strive to improve lean meat yield while still maintaining intramuscular fat that contributes to juiciness and flavour within the meat (Craig, 2017).
The topic of lamb eating quality is complex, and a large number of factors come together to influence the final product seen and experienced by the consumer. If the Australian feedlot industry continues to grow, the nutritional background and age of the animal will become increasingly important when considering the consumer eating quality experience. Most importantly, although a number of changes are occurring in the Australian lamb industry, eating quality should continue to be a priority for the industry into the future.
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