Natural variation/ breeding

There is inherently significant natural variation in the composition of milk fat, because it is affected by many factors such as cow breed, stage of lactation, season and diet of the cow (Palmquist et al, 1993 Ashes et al, 1997 Chapters 1 and 2). Although on-farm approaches can be used to alter the composition of milk fat to improve its nutritional composition and physical characteristics (Banks et al, 1980 Banks and Christie, 1980 Hawke and Taylor, 1994 Baer, 2001 Fearon, 2001 Jensen 2002), these will not be covered here (see Chapter 2). 

The normal levels of the major mineral constituents of cow s milk are listed in Table 5-1. These are average values there is a considerable natural variation in the levels of these constituents. A number of factors influence the variations in salt composition, such as feed, season, breed and individuality of the cow, stage of lactation, and udder infections. In all but the last case, the variations in individual mineral constituents do not affect the milk s osmotic pressure. The ash content of milk is relatively constant at about 0.7 percent. An important difference between milk and blood plasma is the rela-... 

Not all characteristics are subject to successful selection pressure. There must be natural variation, differences in reproduction, and inheritability of the trait in order for selection to produce results. Natural selection requires not only that there be genetic variation but also that the genes be expressed (Mulcahy and Mulcahy, 1987). Without the latter, the genetic code present in a BU is simply irrelevant. Those traits that are not genetically determined cannot be selected for. Predation can select for different traits than would otherwise be chosen. An example of this is the color of male guppy fish females breed preferentially with the most colorful ones, but predators can most easily locate and eat the most colorful ones. Thus, male guppies in upland streams where there are few predators are more colorful than guppies downstream where predators abound (Palumbi, 2001). 

Natural selection and classical breeding both employ the same empirical strategy of creating variants and selecting those that perform best— that is the essence of all protein tailoring methods. They differ only in the sources of sequence variation and the methods by which this diversity is tested. 

There are three general approaches used in Jerusalem artichoke breeding (1) controlled crosses conducted under greenhouse conditions, (2) natural open-pollinated crosses using polycross nurseries, and (3) a variation of the latter where isolated pairs are allowed to cross in the field. Each method has its advantages and disadvantages. 

The milk fat composition is the most variable among natural edible fats. It is influenced by feeding and animal factors such as breed, stage of lactation, health, and age of the cow. The feed has the highest influence on the FA composition. Milk fat from pasture feeding has considerably more Cjg FA and less C16 0, C14 0, and C12 0 acids than fat from indoor feeding. Short-chain FA (C4 0 to C8 0) are characterized by the smallest seasonal variation (Table 13.4). 

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