Protein content and composition
In general, milk protein content is relatively unresponsive to feeding factors. However, under most conditions, energy-, but also protein supply, is feed related factors with most pronounced effect on milk protein content (Table 2). Milk protein content may be negatively influenced by high intake of dietary fat by the lactating cow [168]. Thus, there may be a conflict between milk fatty acid composition and protein content. The feeding regime has only small impact on the proportion of the different types of milk proteins and consequently the amino acid composition [101], and will therefore not be further discussed in this review. However, heat treatment of dairy products leads to structural changes of proteins and main whey proteins are modified to lactulosyl residues [174].
Content of minerals
Bovine milk contain a vide range of minerals [4]. Milk concentrations of some minerals that are of special importance in human nutrition are given in Table 2. The calcium concentration in milk is relatively constant, with some variations throughout lactation. Most of the calcium is in the aqueous compartment and it is primarly (65%) associated with casein [175]. The calcium concentration in milk is relatively constant because milk content of casein is unresponsive to feeding factors. Milk content of magnesium and zinc also show only small variations.
The concentration of selenium in bovine milk is related to selenium concentration in the feed, and there are great variations worldwide. In South Dakota the selenium concentration in milk is reported to be between 160 and 1300 ug/l, whereas the concentrations in milk from low-selenium regions may be from 5 to 30 ug/l [176], as in Scandinavia and northern Europe. A Swedish study showed that the average selenium concentration in milk was 14 ug/l and the concentration was more than doubled after supplementation with 3 mg selenium daily from selenium-enriched yeast [177].
As much as about 25 percent of the iodine intake may be excreted in milk [178]. Therefore, milk content of iodine also varies depending on the iodine content and availability in the feeds used. A study on milk and milk products in Norway [179], showed that milk from the summer season had significantly lower iodine concentration (88 ug/l) compared with milk from the winter season (232 ug/l). This is explained by the use of more supplementary feeds enriched with iodine during the winter season. Dairy products supply much of the dietary intake of iodine; in Norway the most [179], and in US the second most [4] of the iodine intake.
Content of vitamins
Vitamins are not synthesized in the udder. Milk content of the fat-soluble vitamins A and E reflects their content in the feed (Table 2). In general, the content of these vitamins in feed plants decrease with maturity and are higher in fresh than conserved material. There are therefore regional and seasonal variations in these vitamins due to feeding regimes [180], with the highest concentrations in fresh grasses at early stage of maturity. For example a study in Finland shows that the concentration of vitamin E in milk is 3–4 times higher during summer than in winter [181]. Enriching the supplementary feeds with proper sources of these vitamins may increase the milk content during the winter season.
All vitamins of the B-complex (riboflavin and vitamin B12, in Table 2) are synthesized by the rumen microbes, normally in sufficient amounts to cover the animal's needs. Milk content of B-vitamins are, however, relatively unrelated to their intake because the amount synthesized by the rumen microbes are unregulated according to the amount ingested [182].