Introduction

Interest in dairy cow milk fatty acid (FA) profile has increased in recent years due primarily to a growing consumer interest in the nutritional quality of dairy products. Also, recent developments in the ability of milk mid-infrared spectrometry to accurately predict some milk fatty acids facilitated the routine collection of individual cow milk FA profile from routine milk recording schemes and milk payment systems. Milk fatty acid profile can be altered via three routes:

• cow nutrition and management,
• cow genetics, and
• dairy manufacturing technologies (Waker et al., 2004).

This paper focuses on the potential of genetic selection to alter milk FA profile in the dairy cow thereby improving the quality of the dairy products but also the robustness of dairy cows. Genetic selection for milk FA will be discussed in the context of the four criteria outlined by Shook (1989) for including a trait in a breeding program:

• the trait must be easily measurable, ideally at a low cost,
• the trait should exhibit heritable genetic variation,
• the trait must be of economic, social or environmental value or
• the trait should be genetically correlated with a trait of economic importance that is more difficult or expensive to measure or can only be measured later in life.

Milk fatty acid contents are measurable

The reference analysis for milk FA (i.e., gas chromatography) requires skilled staff and is expensive and time-consuming to undertake. However, previous studies (Soyeurt et al., 2006, 2011; Rutten et al., 2009) documented the potential of using mid-infrared spectrometry to predict FA contents in bovine milk. Because of its use by regular milk recording and milk payment systems to quantify major milk components (i.e., fat, protein, urea, and lactose) and its proven robustness, the MIR technology offers the opportunity to routinely measure the major FA content in milk at low cost.

Milk fatty acids exhibit heritable genetic variation

Several studies from a range of populations have clearly documented considerable genetic differences among breeds and animals in FA contents in milk and in milk fat (e.g., Soyeurt et al., 2007; Bobe et al., 2008; Stoop et al., 2008; Mele et al., 2009; Gion et al., 2011). Short and medium chain FA that are synthesized de novo in the mammary gland (C4:0 to C14:0, and almost half or C16:0) are generally under stronger genetic control than the FA excreted from the blood to the udder (the remaining C16:0 and almost all of the longer chain FA). Blood lipids may be derived from the digestion and absorption of dietary fat or from mobilization from adipose tissue (Grummer et al., 1991). Bastin et al. (2012a) documented heritability estimates of the major FA contents in milk predicted by mid-infrared spectrometry; heritability estimates were about 0.20–0.25 for FA originating from the diet and body fat mobilization and were about 0.35–0.40 for de novo synthesized FA. Moreover, the heritability of the FA was generally greater in mid and late lactation.

Milk fatty acids are of economic importance

Milk is a consumer product and its composition influences its value as well as its nutritional, technological, and sensory qualities. There are a plethora of studies that have investigated the effects of dairy products on human health (e.g., Haug et al., 2007; Ebringer et al., 2008; Dror and Allen, 2013). Some FA are known to have potential beneficial effects (e.g., the anticarcinogenic properties of C18:2 cis-9, trans-11; Moate et al., 2007) or potentially deleterious effects (e.g., the hypercholesterolemic effects of C16:0; Grummer, 1991). Other than the individual effect of milk FA on human health, the effect of the consumption of milk and dairy products as a whole must also be considered. Several studies concluded that milk and dairy products, which are the main sources of calcium and other essential nutrients, help to reduce the risk of majority of chronic diseases (Ebringer et al., 2008). Furthermore, as an example of the influence of milk FA on the technological and sensory qualities of dairy products, Palmquist et al. (1993) indicated that a greater concentration of C18:2 resulted in softer butter, however milk with more than 20% of C18:2 was not acceptable based on sensory quality assessment. In such milks, off-flavors were predominantly of an oxidized type, whereas significant oxidized flavor was absent in freshly drawn milk. Nonetheless, the direct economic value of milk FA remains unclear since most of the milk producers, to-date, do not receive bonuses or penalties for different milk FA profiles.

Milk fatty acids as indicator traits to improve robustness

Cow robustness may be defined as the ability of an animal to minimize the extent and duration of energy balance, where energy balance describes the difference between energy intake and energy expenditure (Berry et al., 2013). Generally, dairy cows experience negative energy balance for about 2 to 4 months following calving. In response to the energy deficit, cows mobilize tissue reserves.

Milk fatty acids and energy balance

Milk fat composition has been associated with negative energy balance and body fat mobilization. Bastin et al. (2011) reported an increase of C18:0 and C18:1 cis-9 in milk in early lactation indicating the release of long-chain FA from the mobilization of body fat reserves. Concomitantly, the content in milk of de novo synthesized FA tended to be lower in early lactation because the high uptake of long-chain FA in the udder inhibits de novo synthesis of FA by mammary gland tissue. Also, this inhibition intensifies with increasing chain lengths (Palmquist et al., 1993). Stoop et al. (2009) further indicated that negative energy balance was associated with an increase in C16:0 and C18:0. Van Haelst (2008) indicated that high proportions of C18:1 cis-9 in milk fat were associated with subclinical ketosis resulting from excessive fat mobilization. Gross et al. (2011) reported that the proportions in fat of short and medium-chain FA up to C16:0 increased with decreasing negative energy balance postpartum, while the proportion of long-chain FA, especially C18:1 cis-9 decreased as mobilization of body fat reserves declines. They concluded that, within the scope of their study, the close relationship with energy balance makes changes in C18:1 cis-9 as well as in groups of FA (SFA, MUFA,de novo synthetized and preformed FA) suitable indicators of the energy balance in dairy cows.

Milk fatty acids and fertility

Since energy balance is well known as the major contributing factor influencing fertility of dairy cows, it was hypothesized that milk FA profiles were related to the fertility of dairy cows. Demeter et al. (2009) reported that greater concentrations of trans FA in total milk fat were related to poorer reproductive performance. Although they considered the level of unsaturated FA as a proxy for the dietary profile in the herd, Hostens et al. (2011) related the fertility performances from 90 dairy herds to the level of unsaturated FA in milk bulk tank samples. They found that higher bulk tank unsaturated FA level was associated with decreased conception rate to first insemination, greater days post-calving to first insemination, and greater days to conception. Finally, Bastin et al. (2012b) reported genetic correlations between the contents in milk of major FA predicted by mid-infrared spectrometry and the number of days from calving to conception (or days open). Correlations between days open and content in milk of 17 group and individual FA ranged from -0.37 to 0.39. Estimates varied greatly according to the trait and the stage of lactation in which the milk sample was taken. Genetic correlations with days open for the content in milk of unsaturated FA, monounsaturated FA, long-chain FA, C18:0, and C18:1 cis-9 were positive in early lactation but negative after 100 days in milk. For the other FA, genetic correlations with days open were negative across the entire lactation. The strongest correlation existed for C18:1 cis-9 content in milk at 5 days in milk, indicating that greater concentration of this FA in milk during the early postpartum period is related to a longer interval from calving to conception. Therefore, Bastin et al. (2012b) concluded that the content in milk of C18:1 cis-9 in early lactation might indeed serve as a good genetic indicator trait for fertility.

Milk fatty acids as indicator trait for fertility

In order to investigate the benefit of using the content in milk of C18:1 cis-9 at 5 days in milk as an indicator trait of fertility, Bastin et al. (2013) calculated the accuracy of a fertility index including either days open and/or C18:1 cis-9 for sires with varying numbers of daughters. With consideration to the parameters given in Table 1, accuracy of selection based on the different schemes evaluated was calculated (Table 2). As expected, direct selection for days open provided the greatest accuracy of selection for the fertility index compared to indirect selection on C18:1 cis-9 only.

However access to routine information on fertility (and especially days open) might not always be early available on large progeny groups; however large progeny group sizes are needed for fertility traits to achieve a high accuracy of selection because fertility traits tend to be of low heritability. Therefore, incorporating information on milk FA profile into national genetic evaluations could improve the accuracy of selection for fertility in especially young bulls and cows thereby increasing the rate of genetic gain.

Consequences of selection

On one hand, higher content of C18:1 cis-9 at early post-calving seems to be associated with poor fertility performances while on the other hand, consumption of C18:1 cis-9 is considered to be favorable for human health (Haug et al., 2007; Ebringer et al., 2008). Thus, higher content in milk of C18:1 cis-9 would be desirable for the nutritional properties of milk fat while lower content of C18:1 cis-9 in early lactation would be desirable for improved fertility. However, results inferred from the study of Bastin et al. (2011) indicated that the genetic correlation between content in milk of C18:1 cis-9 in early lactation and its content at 50, 100, 200, and 300 post-calving were 0.96, 0.75, 0.27, and 0.31, respectively. It may therefore be possible to lower the content in milk of C18:1 cis-9 at 5 days in milk could without impacting considerably on the average MUFA content in milk across the entire lactation.

Getting more out of milk

This paper focused on milk FA profile and its relationship with energy balance status and fertility of dairy cows. To a larger extent, there is an array of milk components that could be useful in terms of improving milk quality and dairy cow health. Among them, lactoferrin is interesting for human health as a biologically active food component but it also has multiple roles involved in immune reactions of the dairy cows and it could be an indicator of udder health (Soyeurt et al., 2012).

Lactoferrin could be also predicted by mid-infrared spectroscopy. Based on the primary assumption that the spectrum provided by the mid-infrared analysis of milk is a fingerprint of the whole milk composition, the OptiMIR project (www.optimir.eu) aims to provide innovative farm management web applications (i.e. indicators of fertility, feeding, health, rejection of pollutants, and milk quality) that will use the spectral analysis of the milk recording samples.

Summary

Milk FA profile influences the nutritional as well as the technological and sensory qualities of milk. The opportunity now exists to produce dairy products containing fat with a greater proportion of unsaturated FA, which is thought to be advantageous to human health. Moreover, changes in milk FA profile have been associated with energy balance status of dairy cows in early lactation, which is recognized as one of the most important factors impacting fertility and health. Furthermore, the milk FA profile exhibits heritable genetic variation and the tools now exist (i.e., mid-infrared spectrometry analysis of milk) to routinely generate milk FA profile on individual cows and bulk milk samples on a routine basis at little or no marginal cost. Therefore, a breeding program that seeks to alter the milk FA profile should balance improvement of the nutritional properties of milk and fertility of dairy cows, potentially exploiting differences in FA profiles and their association to animal robustness across the lactation. Nonetheless, the inclusion of milk FA contents in breeding schemes has also to be considered with respect to the overall breeding goal, their respective economic values, their relationships with all economically important traits as well as the desirable direction of change.

Acknowledgements

Research presented here was conducted through OptiVal & OptiVal+ projects (Service Public de Wallonie – DGARNE). This research also received financial support from the European Commission, Directorate-General for Agriculture and Rural Development, under Grand Agreement 211708 and from the Commission of the European Communities, FP7, KBBE-2007-1 (RobustMilk project, www.robustmilk.eu). This paper does not necessarily reflect the view of these institutions and in no way anticipates the Commission’s future policy in this area. Additional financial support was provided by the National Fund for Scientific Research (FNRS, Belgium). The partners of research (Walloon Breeding Association; Walloon Research Center; Comité du Lait de Battice; Teagasc Moorepark; and the partners of the OptiMIR project, www.optimir.eu) are also acknowledged.

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