ABSTRACT
Genetic factors affecting feed efficiency, feeding behaviour and related traits in pigs Duy Ngoc Do, McGill University, Canada; and Haja N. Kadarmideen, Technical University of Denmark, Denmark
1 Introduction
2 Measures of feed efficiency
3 Residual feed intake
4 Genetics of residual feed intake (RFI) in pigs
5 Genome wide association studies (GWAS) of feed efficiency in pigs
6 Component traits of residual feed intake and genetic architecture
7 Selection for feed efficiency
8 Genetic architecture of feeding behaviour traits
9 Genomics of feeding behaviour
10 Towards integrative systems genomics of feed efficiency
11 Conclusion and future trends
12 Where to look for further information
13 Acknowledgements
14 References
A global population already in excess of 7 billion people has led to increased demand for food and water, especially the demand for meat. While the need to increase animal production to meet global needs seems obvious, limited availability of resources is an important challenge in improving meat production. Moreover, the cost of feed used in animal production has also increased dramatically, which requires animal breeders to find alternatives to reduce feed consumption while maintaining animal productivity. One available alternative to alleviate both these problems is to emphasize feed efficiency in animal production. Genetic selection of feed-efficient animals via animal breeding approaches will also provide a sustainable way of improving production while reducing feed intake and hence putting less pressure on the limited amount of resources that are
required for animal production. Since around 34% of global meat consumption is pig meat, increasing pig production is very important to meet the demand for meat. The improvement of feed efficiency in pigs, therefore, is an important goal for livestock industries in many countries. Feed costs account for a significant proportion of the total costs incurred in livestock production, accounting for almost half of the total production cost depending on production systems and livestock species. Therefore, feed efficiency is an important objective in breeding programmes for all breeds. Despite the significance weightage given to feed efficiency in breeding programmes, it is a trait that is difficult to measure. While raw feed intake and utilization traits are easily recorded and are also heritable, selection for reduced feed intake (FI) is not optimal because of its strong negative correlation with production traits such as body weight (BW), composition and average daily gain (ADG). Therefore, alternative measures of feed efficiency such as food conversion ratio are required for use in breeding schemes. Breeding for feed conversion (also called food conversion ratio (FCR)) is routinely done in many pig-breeding companies and the economic weight for this trait accounts for a large proportion of total economic weight in their breeding programmes (Fig. 1)
The ability to measure feed efficiency accurately is important for genetic improvement. Traditionally used FCR does not optimally measure feed efficiency, and hence, there is significant interest in developing alternative measures for feed efficiency. In growing
animals, ratio or residual (regression) traits can be used to describe feed efficiency (Berry and Crowley, 2013). Currently, many breeding companies use FCR or feed:gain ratio as an indicator for feed efficiency. FCR is computed as the total amount of FI divided into the total amount of weight gain. However, FCR is not an ideal indicator (Gunsett, 1984) as a ratio trait for feed efficiency because it is not normally distributed and has no real mean and variance (Atchley and Anderson, 1978). Inclusion of ratio traits also causes problems when using linear selection indices (Werf, 2004) which are normally applied in the livestock industry. Since FCR is influenced by the variability in growth and maintenance requirements, it does not accurately measure efficiency of feed conversion (Aggrey and Rekaya, 2013). Moreover, it has been demonstrated that the genetic improvement of FCR observed in pigs is actually a function of genetic improvement of these constituent traits (Knap and Wang, 2012). In light of these deficiencies, Koch et al. (Koch et al., 1963) introduced the concept of residual feed intake (RFI) and defined it as the difference between an animal’s actual FI and its predicted FI, the prediction usually being estimated on the basis of energy requirements for production, maintenance and body condition change. Therefore, a low RFI animal is more feed efficient than an animal with high RFI. Selection on RFI has been proposed to improve feed efficiency because of its phenotypic independence of growth and maintenance requirement. Besides RFI, Berry and Crowley (Berry and Crowley, 2012) proposed other indicators for feed efficiency including residual BW gain (the residuals from a multiple regression model regressing ADG on both dry matter intake (DMI) and BW). Some indirect measures of feed efficiency such as relative growth rate (growth relative to instantaneous body size) (Fitzhugh and St CS, 1971) and the Kleiber ratio (ADG per unit metabolic BW) (Kleiber, 1961) have also been proposed. Both these measures can be considered as feed efficiency indicators provided that all animals on test are fed the same restricted diet (Berry and Crowley, 2013). Therefore, these measures are rarely used in breeding programmes and the current breeding programme in livestock production mainly uses either FCR or RFI as an indicator for feed efficiency.
