**2. Energy efficiency traits**

Researchers have proposed many measures of energy efficiency such as feed conversion ratio (FCR), gross energy efficiency (GEE), residual feed intake (RFI) (Koch et al., 1963; Archer et al., 1999; Crews, 2005) and life time efficiency (LTE) (Vandehaar, 1998; Vandehaar & St-Pierre, 2006). Their definition, applications and benefits are different.

FCR and GEE are the most common measures of efficiency in the literature. FCR is the ratio of input (e.g. feed) to output (e.g. weight gain or milk production) (Crews, 2005). In the dairy cow, the GEE is defined as the energy in the milk divided by the total energy intake (Veerkamp & Emmans, 1995). These approaches lead to only limited insight into efficiency of the entire production system (Crews, 2005). The problems of GEE and FCR have been discussed in numerous studies (Korver et al., 1991; Veerkamp & Emmans, 1995; Crews, 2005) and are mainly categorized in three groups. First , the energy intake has different partial efficiencies for maintenance, lactation, pregnancy and body tissue gain or loss, but the GEE and FCR do not distinguish between them (Veerkamp & Emmans, 1995). Secondly, FCR and GEE are well known to be phenotypically and genetically correlated with measures of growth, production and mature size. Therefore, selection of animals based on these measures may increase the maintenance requirements. Finally, changes in GEE and FCR can be the result of changes in either intake (numerator), yield (denominator) or both (Gunsett, 1984; Veerkamp & Emmans, 1995) and selection direction cannot be predicted very well. Then, selection for improvement of FCR (i.e. decreased FCR) and GEE would result in increased growth rate, mature size, and consequently mature maintenance requirements (Korver et al., 1991). It can be concluded that improving FCR and GEE by selection for increased growth rate do not necessarily improve net feed efficiency, because of drawbacks associated with increased maintenance requirements ( Van der Werf, 2004; Crews, 2005).

Lifetime efficiency (LTE), another measure of energy efficiency, is defined as "the capture of feed energy in milk, conceptus, and body tissue divided by gross energy intake during the life of cow, starting at birth" (Vandehaar, 1998; Vandehaar & St-Pierre, 2006). This index attempts to summarize an animal's entire life efficiency and is a good criterion to set up a long term vision. In order to compare the LTE in dairy cows, total milk production should be standardized for all factors such as housing, feeding, age at first calving and calving interval. The LTE mostly depends on the precalving interval and intercalving intervals. The Precalving Interval is defined as the period from birth to first parturition and Intercalving Intervals are the intervals between successive calvings (King, 2006). The main concerns related to LTE are: lots of information is required to calculate the LTE, it is applicable for the entire life, and it is influenced more by precalving and intercalving intervals.

To overcome the problems associated with FCR, GEE, LTE and other measures of energy efficiency, an alternative measure can be expressed as residual feed intake (RFI). RFI is a measure of feed utilization corrected for live weight and production, and it is often referred to as net feed or energy efficiency ( Koch et al., 1963; Korver et al., 1991; Luiting et al.,1992). The concept of RFI can be described as "the difference between the actual feed intake and that predicted on the basis of mean requirements for body weight maintenance and levels of production" (Koch et al., 1963); it is explained schematically in figure 1. RFI relies simply on partitioning feed energy intake into portions required for body maintenance, stage and levels of production, and a residual portion. This residual portion is related to the true metabolic efficiency of an animal and would be comparable across individuals (Crews, 2005). Variation in RFI probably reflects underlying biological efficiency after adjustment for energy deposition (Crews, 2005; Herd & Arthur, 2009 ). In a population, the mean RFI index over all individuals is zero and approximately half of all individuals have RFI values below or above the mean. The efficient animals have low RFI values; it implies they consume less feed without compromising their production (Crews, 2005). Indeed, RFI is a net feed efficiency measurement and it can be calculated at any time of an animal's life.

122 Milk Production – An Up-to-Date Overview of Animal Nutrition, Management and Health

& St-Pierre, 2006). Their definition, applications and benefits are different.

Researchers have proposed many measures of energy efficiency such as feed conversion ratio (FCR), gross energy efficiency (GEE), residual feed intake (RFI) (Koch et al., 1963; Archer et al., 1999; Crews, 2005) and life time efficiency (LTE) (Vandehaar, 1998; Vandehaar

FCR and GEE are the most common measures of efficiency in the literature. FCR is the ratio of input (e.g. feed) to output (e.g. weight gain or milk production) (Crews, 2005). In the dairy cow, the GEE is defined as the energy in the milk divided by the total energy intake (Veerkamp & Emmans, 1995). These approaches lead to only limited insight into efficiency of the entire production system (Crews, 2005). The problems of GEE and FCR have been discussed in numerous studies (Korver et al., 1991; Veerkamp & Emmans, 1995; Crews, 2005) and are mainly categorized in three groups. First , the energy intake has different partial efficiencies for maintenance, lactation, pregnancy and body tissue gain or loss, but the GEE and FCR do not distinguish between them (Veerkamp & Emmans, 1995). Secondly, FCR and GEE are well known to be phenotypically and genetically correlated with measures of growth, production and mature size. Therefore, selection of animals based on these measures may increase the maintenance requirements. Finally, changes in GEE and FCR can be the result of changes in either intake (numerator), yield (denominator) or both (Gunsett, 1984; Veerkamp & Emmans, 1995) and selection direction cannot be predicted very well. Then, selection for improvement of FCR (i.e. decreased FCR) and GEE would result in increased growth rate, mature size, and consequently mature maintenance requirements (Korver et al., 1991). It can be concluded that improving FCR and GEE by selection for increased growth rate do not necessarily improve net feed efficiency, because of drawbacks associated with increased maintenance requirements ( Van der Werf, 2004;

Lifetime efficiency (LTE), another measure of energy efficiency, is defined as "the capture of feed energy in milk, conceptus, and body tissue divided by gross energy intake during the life of cow, starting at birth" (Vandehaar, 1998; Vandehaar & St-Pierre, 2006). This index attempts to summarize an animal's entire life efficiency and is a good criterion to set up a long term vision. In order to compare the LTE in dairy cows, total milk production should be standardized for all factors such as housing, feeding, age at first calving and calving interval. The LTE mostly depends on the precalving interval and intercalving intervals. The Precalving Interval is defined as the period from birth to first parturition and Intercalving Intervals are the intervals between successive calvings (King, 2006). The main concerns related to LTE are: lots of information is required to calculate the LTE, it is applicable for the

To overcome the problems associated with FCR, GEE, LTE and other measures of energy efficiency, an alternative measure can be expressed as residual feed intake (RFI). RFI is a measure of feed utilization corrected for live weight and production, and it is often referred

entire life, and it is influenced more by precalving and intercalving intervals.

**2. Energy efficiency traits** 

Crews, 2005).

**Figure 1.** Schematic concept of residual feed intake (RFI). Two animals which have the same BW and ADG, are expected to consume the same amount of feed but in reality cow A consumes more than expected while cow B consumes less, so cow B is more efficient than A.
