**3. Factors affecting energy efficiency in dairy cattle**

Several factors influence energy efficacy in dairy cattle. It is practically influenced by dry matter intake (DMI), production level, body tissue changes, age at first calving (AFC), and environmental factors (Vandehaar, 1998; Linn, 2006). Their approaches to affect the energy efficiency are different.

#### **3.1. Dry matter intake and production**

Dry matter intake (DMI) and production are the direct components of most energy efficiency traits. Dry matter intake (DMI) affects energy efficiency through energy transformation mechanisms from gross to net energy. The basic rule of most common efficiency measures, FCR and GEE, is increased production means increased efficiency, but the relationship between the marginal benefit of increased production and efficiency is not always linear.

DMI establishes the amount of nutrients available to an animal for maintenance and production. Inadequate intake of nutrients negatively affects production, efficient nutrient utilization and health status of the animal. Supplying nutrients in excess also increases feed costs and can result in excretion of nutrients into the environment ( NRC, 2001; Collier et al., 2006). In dairy cows, the average DMI is 22.7 kg/d, and it ranges between 19.8 to 26 kg/d (Ordway et al., 2009; Vallimont et al., 2010). Heritability of DMI was reported from 0.16 to 0.48, and its genetic correlation with energy intake ranged from 0.8 to 0.9 (Veerkamp, 1998; Vallimont et al., 2010). Therefore, DMI and energy intake are genetically almost the same trait. However, the energy transformation mechanisms of DMI, which affect energy efficiency, involve digestion, fermentation and metabolic processes. Gross energy (GE) is the amount of released energy in heat combustion. Net energy (NE) is the energy which is directly used to support maintenance functions, including conversion to milk, conceptus growth and body tissue gain (NRC, 2001). Feedstuffs have energy in the gross form and it is converted to net energy in several steps (figure 2). Some amounts of the GE are indigestible and ultimately appear in feces; the remaining part is called digestible energy (DE). Some part of DE is lost due to gas production (mainly methane) and urinary energy (mainly urea) during the fermentation process. The remaining DE after deduction for gas and urinary production is called metabolizable energy (ME). Finally, converting the ME to net energy (NE) requires metabolic reactions, which produce heat that is termed the heat increment (NRC, 2001; Vandehaar & St-Pierre, 2006). Therefore, the accessible amount of NE from GE depends on the amount of losses in digestion, fermentation and metabolic processes. Practically, it depends on many factors, such as DMI levels, passage rate, and dietary fibre (especially, effective neutral detergent fiber, eNDF) level (NRC, 2001). Some studies have been conducted to determine the relationship between these factors and amount of nutrient losses in the different steps of the transformation mechanisms (Moe, 1981; Van Soest et al., 1992); they concluded there is an optimum point between them. For example, Vandehaar (1998) reviewed the literature and showed that the relationship between level of DMI and DE is not linear. When a dairy cow consumes DMI for its maintenance requirements, almost 80% of GE captured is in the form of DE. Furthermore, there is a reduction in digestibility as DMI increases (Vandehaar, 1998). Overall, NRC (2001) suggested that digestibility is depressed linearly at 4% per multiple of maintenance intake. It assumes that most of the cows consume 3 times their maintenance requirement, which is an optimum point of GEE. In addition, it has been shown that there is an optimum point of NDF level in terms of converting GE to DE, and it is reported to be between 25 to 30%. Amounts of NDF in the diet beyond this range will decrease energy intake. Higher levels of NDF fill the rumen, whereas lower levels may cause some health problems (Eastridge, 2006; Vandehaar & St-Pierre, 2006). Finally, the portion of losses in the different steps of energy transformation shifts and it is dependent on DMI levels. At higher levels of DMI the losses into feces increase while the amounts of losses as heat increment are greater at lower levels of intake (Vandehaar & St-Pierre, 2006).

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

Dry matter intake (DMI) and production are the direct components of most energy efficiency traits. Dry matter intake (DMI) affects energy efficiency through energy transformation mechanisms from gross to net energy. The basic rule of most common efficiency measures, FCR and GEE, is increased production means increased efficiency, but the relationship between the marginal benefit of increased production and efficiency is not

DMI establishes the amount of nutrients available to an animal for maintenance and production. Inadequate intake of nutrients negatively affects production, efficient nutrient utilization and health status of the animal. Supplying nutrients in excess also increases feed costs and can result in excretion of nutrients into the environment ( NRC, 2001; Collier et al., 2006). In dairy cows, the average DMI is 22.7 kg/d, and it ranges between 19.8 to 26 kg/d (Ordway et al., 2009; Vallimont et al., 2010). Heritability of DMI was reported from 0.16 to 0.48, and its genetic correlation with energy intake ranged from 0.8 to 0.9 (Veerkamp, 1998; Vallimont et al., 2010). Therefore, DMI and energy intake are genetically almost the same trait. However, the energy transformation mechanisms of DMI, which affect energy efficiency, involve digestion, fermentation and metabolic processes. Gross energy (GE) is the amount of released energy in heat combustion. Net energy (NE) is the energy which is directly used to support maintenance functions, including conversion to milk, conceptus growth and body tissue gain (NRC, 2001). Feedstuffs have energy in the gross form and it is converted to net energy in several steps (figure 2). Some amounts of the GE are indigestible and ultimately appear in feces; the remaining part is called digestible energy (DE). Some part of DE is lost due to gas production (mainly methane) and urinary energy (mainly urea) during the fermentation process. The remaining DE after deduction for gas and urinary production is called metabolizable energy (ME). Finally, converting the ME to net energy (NE) requires metabolic reactions, which produce heat that is termed the heat increment (NRC, 2001; Vandehaar & St-Pierre, 2006). Therefore, the accessible amount of NE from GE depends on the amount of losses in digestion, fermentation and metabolic processes. Practically, it depends on many factors, such as DMI levels, passage rate, and dietary fibre (especially, effective neutral detergent fiber, eNDF) level (NRC, 2001). Some studies have been conducted to determine the relationship between these factors and amount of nutrient losses in the different steps of the transformation mechanisms (Moe, 1981; Van Soest et al., 1992); they concluded there is an optimum point between them. For example, Vandehaar (1998) reviewed the literature and showed that the relationship between level of DMI and DE is not linear. When a dairy cow consumes DMI for its maintenance requirements, almost 80% of GE captured is in the form of DE. Furthermore, there is a reduction in digestibility as DMI increases (Vandehaar, 1998). Overall, NRC (2001) suggested that digestibility is depressed linearly at 4% per multiple of maintenance intake. It assumes that most of the cows consume 3 times their maintenance requirement, which is an optimum point of GEE. In addition, it has been shown that there is an optimum point of NDF level in terms of converting GE to DE, and it is reported to be between 25 to 30%. Amounts of NDF in the diet beyond this range will decrease energy intake. Higher levels of NDF fill the rumen,

**3.1. Dry matter intake and production**

always linear.

Although production is a fundamental component that determines efficiency, the relationship between marginal benefit of increased production and efficiency is not linear for all the time. During the past 18 years, the average milk production of Canadian Holstein cows has increased about 115 kg/cow/year and currently the average milk yield of a primarily Holstein herd is 9793 kg/cow/yr. The average rate of increase was 1.35% between 1991 and 2009 (DHI, 2009), and it likely will continue to increase. In addition, milk yield heritability is reported as 0.3 (Lee et al., 1992; VanRaden et al., 2009) and ranges between 0.16 to 0.5 (Veerkamp, 1998). This means that still there is still room to increase milk production by exploiting genetic selection. The genetic correlation between GEE and milk production in dairy cattle ranged from 0.88 to 0.95 (Pitchford, 2004). It confirms that selecting dairy cows for milk yield automatically improved GEE (Veerkamp & Emmans, 1995). Consequently, FCR (4% FCM/DM) has increased from 0.91 in 1991 to 1.2 in 2006, and a common goal is 1.5 (Eastridge, 2006). Korver (1991) concluded that the improved GEE and FCR mostly reflects the dilution of maintenance. Dilution of maintenance means that as cows consume more, a relatively small fraction of energy is used for maintenance and a larger portion is captured in milk. Although there is no evidence to suggest that the maintenance requirements depend on milk production and breed, cows with similar body weight and breed may vary for maintenance requirements by about 8 to 10% (NRC, 2001). These assumptions need further investigation. To set a vision for the future, Vandehaar (1998) modelled the optimum point of milk yield. He proposed that above 15000 kg/yr, the marginal increase in efficiency approaches zero. Therefore, the positive correlation between milk production and efficiency that has existed in the past may change in the future, when average milk production surpasses 15000 kg/yr/cow (Vandehaar, 1998).

DMI and milk yield are tightly linked as their genetic correlation is reported to be 0.5 (Vallimont et al., 2010) ranging from 0.46 to 0.84 (Veerkamp, 1998). Consequently, selection decisions which change milk yield and body weight (BW) also change DMI (Veerkamp & Emmans, 1995). Genetic selection mostly focuses on milk yield and it indirectly affects DMI. However, with increased milk production per animal, there is a limit to the increase in DMI because of rumen fill; therefore, the density of NE in dairy rations has been elevated as milk production increased in the last 30 years. For instance, the dietary NE density of dairy cattle rations has increased from 1.23 in 1980 to more than 1.6 Mcal/kg in 2006 (Eastridge, 2006). Thus, it can be inferred that some of the improved efficiency due to increased milk production is withdrawn by increasing the dietary energy concentration in terms of expenses. Furthermore, the linear relationship between milk production and efficiency may change in the future. Therefore, these concerns drive researchers to define net energy efficiency using concepts such as RFI, which is independent from production and maintenance in dairy cattle.

**Figure 2.** Energy transformation processes from gross energy (GE) to net energy (NE). The portion of lost energy in different steps is dependent on DMI level.

## **3.2. Body weight**

Body weight influences energy efficiency through its relationship with milk production and digestive capacity. Heritability of body weight (BW) is reported to be in a range of 0.26 to 0.88 (Verrkamp, 1998). BW is genetically correlated with milk production. Although some researchers (Veerkamp, 1998; Vallimont et al., 2010) reported a negative correlation (range of -0.01 to -0.42), some other researchers reported positive correlations between BW and milk production (range of 0.05 to 0.45) (Veerkamp, 1998). This inconsistency in results could be due to mean differences of BW and milk production between populations under estimation. It can also be suggested that there is an optimum point of relationship between BW and milk production, and consequently energy efficiency. In order to illustrate this optimum relationship, Vandehaar (1998) modelled the relationship between body size, milk production and energy efficiency. He considered two possible relationships in which there were function of BW and digestive capacity (figure 3). In the first, he assumed that the digestive capacity of animals is not a function of BW and it is constant, the solid curve and dashed curve in figure 3. Therefore, increased BW increases the maintenance requirements and consequently decreases energy efficiency. In the second model, the digestive capacity was assumed to be a function of BW, so with increased BW digestive capacity will increase, and consequently a large cow would be efficient, dot-dashed curve and dashed curve in figure 3. In this case if a cow had a 200 kg greater BW (825 kg VS 625 kg), she should produce 60 kg/day more milk to become efficient. He concluded that the relationship between body size and efficiency depends on the relation between digestive capacities with body size (Vandehaar, 1998), and that there is an optimum point of relationship between BW and energy efficiency.

#### **3.3. Body tissue changes**

Body tissue changes increase energy efficiency by supporting milk production and tissue mobilization is a crucial factor in determining energy efficiency of dairy cattle. Although the conversion ratio of lost body reserves to milk production is less than that of regaining the

lost energy in different steps is dependent on DMI level.

**3.2. Body weight** 

BW and energy efficiency.

**3.3. Body tissue changes** 

**Figure 2.** Energy transformation processes from gross energy (GE) to net energy (NE). The portion of

Body weight influences energy efficiency through its relationship with milk production and digestive capacity. Heritability of body weight (BW) is reported to be in a range of 0.26 to 0.88 (Verrkamp, 1998). BW is genetically correlated with milk production. Although some researchers (Veerkamp, 1998; Vallimont et al., 2010) reported a negative correlation (range of -0.01 to -0.42), some other researchers reported positive correlations between BW and milk production (range of 0.05 to 0.45) (Veerkamp, 1998). This inconsistency in results could be due to mean differences of BW and milk production between populations under estimation. It can also be suggested that there is an optimum point of relationship between BW and milk production, and consequently energy efficiency. In order to illustrate this optimum relationship, Vandehaar (1998) modelled the relationship between body size, milk production and energy efficiency. He considered two possible relationships in which there were function of BW and digestive capacity (figure 3). In the first, he assumed that the digestive capacity of animals is not a function of BW and it is constant, the solid curve and dashed curve in figure 3. Therefore, increased BW increases the maintenance requirements and consequently decreases energy efficiency. In the second model, the digestive capacity was assumed to be a function of BW, so with increased BW digestive capacity will increase, and consequently a large cow would be efficient, dot-dashed curve and dashed curve in figure 3. In this case if a cow had a 200 kg greater BW (825 kg VS 625 kg), she should produce 60 kg/day more milk to become efficient. He concluded that the relationship between body size and efficiency depends on the relation between digestive capacities with body size (Vandehaar, 1998), and that there is an optimum point of relationship between

Body tissue changes increase energy efficiency by supporting milk production and tissue mobilization is a crucial factor in determining energy efficiency of dairy cattle. Although the conversion ratio of lost body reserves to milk production is less than that of regaining the

**Figure 3.** Adapted from Vandehaar (1998). He discussed the two possible relationships between digestive capacity and BW. In the first one, digestive capacity was not a function of BW while in the second, the digestive capacity was a function of BW.

reserves from feedstuffs, reasonable levels of losses still increase the energy efficiency. From an evolutionary point of view, mammals use their stored energy reserves to produce milk and support their young when their requirements exceed DMI consumed. In nature as the calf grows older, it gradually relies less on mothers' milk and the mother has an opportunity to regain energy resources for the next lactation (Bewley et al., 2008). Similarly, in the dairy industry, as the feed intake peak occurs later than the milk yield peak, dairy animals have a mechanism to use their body reserves to support milk production in early lactation and regain the body reserves in late lactation (Coffey et al., 2001; Bewley et al., 2008). In early lactation when energy intake is less than that used for milk, maintenance and activity, the cows are in a negative energy balance (NEB). Therefore, they sacrifice their body resources in this period to meet the requirements. Up to one-third of the total milk solids that are produced in early lactation, comes from body tissue reserves (Bewley et al., 2008). Practical measures of changes in energy resources include changes in BW and body condition score (BCS). BCS is a management technique used to appraise the body fat reserves in cattle (Coffey et al., 2001) and it is measured with either a 5 or a 9 point scale. The BCS represents 65, 55 and 66 percent of fat, protein and energy variation in dairy cattle, respectively (NRC, 2001). The ability to manage body reserves varies between animals, and they have a different pattern of BW and BCS changes during lactation and across lactations (Bewley et al., 2008). Heritability of changes in BW reportedly range from 0.1 to 0.27 (Verrkamp, 1998). Heritability estimates for BCS change depend on stage of lactation and range from 0.08 to

0.6 (Bewley et al., 2008). Negative and positive correlations have been reported between milk yield and BW changes (-0.41 to 0.45) (Verrkamp, 1998) and BCS changes (Bewley et al., 2008). One BCS unit (5 point scale) is equivalent to ~400 Mcal of ME and its conversion ratio to milk is estimated at 0.82. It is enough to produce an additional 8 kg milk/day in the first 60 days in milk (Vandehaar, 1998; Bewley et al., 2008). Therefore, losing one unit of BCS supports around 2000 kg of increased milk production over 305 days and it is expected to increase GEE from 25 to 26.5% in cows with a production of 8000 kg milk (Vandehaar, 1998). The lost energy reserves are replaced by cows in late lactation, and its replenishment conversion ratio is less (0.7) than that for loss (0.8) (Moe, 1981), but loss of BCS still increases efficiency (Vandehaar, 1998). Besides the increased efficiency, some researchers point out that side effects of losing energy reserves on other traits like reproduction and health should be considered (Vandehaar, 1998; Bewley et al., 2008). For example, cows restart reproduction activity after they pass the NEB period (Goff, 2006). Some of the metabolic diseases such as ketosis/fatty liver complex are highly correlated with NEB (Collier et al., 2006). Researchers proposed that there is a curvilinear relationship between BCS at calving and milk production; furthermore, maximum milk production is associated with 3.25 to 3.5 BCS at calving (Roche et al., 2007; Bewley et al., 2008). Indeed, during early lactation, a controlled loss of body condition of 0.5 to 1.0 units is associated with optimal milk production, health, and reproductive performance. Moreover, excessive BCS losses at calving predispose the animal to metabolic disorders such as ketosis and fatty liver (Spain, 1996; Bewley et al., 2008).
