A 2 x 2 factorial continuous experiment was conducted with 28 Norwegian Red dairy cows in early lactation to compare milk content of phytoestrogens when feeding ad libitum white clover (WCS) or red clover (RCS) grass silages prepared from the second and third cut without and with 10 kg/d supplementation of a standard concentrate. The cows were offered either RCS or WCS for 88 d (period 1) and thereafter a mixed red clover-white clover-grass silage for 48 d (period 2). Total dry matter intake and milk yield were not affected by forage type but increased with concentrate supplementation. Intake of isoflavones was several times greater in RCS than in WCS, whereas intake of lignans was greater in WCS. Concentrate supplementation reduced the intake of most phytoestrogens. Compared with WCS, RCS diets yielded milk with a greater content of flavonoids, whereas milk from WCS diets had greater contents of the mammalian lignans enterodiol and enterolactone. The content of the isoflavan equol was particularly high in RCS diets. There was no apparent carryover effect of clover type on milk phytoestrogen content because there was no difference in content between the silage treatments 3 wk after the cows were transferred to the same silage diet (period 2). Concentrate supplementation reduced the milk contents of the flavonoids equol, biochanin A, and daidzein and increased the content of mammalian lignans. The effects of silage type and concentrate supplementation on milk contents of the individual phytoestrogens were related to the intake of the compound or its precursor, except for the effect of concentrate on mammalian lignans, for which the intake of the known precursors was also reduced. Overall, this study shows that feeding cows with silage containing red clover increases the milk content of flavonoids at both low and high concentrate supplementation levels, and decreases the content of nonflavonoids such as mammalian lignans, when compared with silage containing white clover. The increased content of phytoestrogens in milk may be important when the health benefits of milk are studied.

Dietary phytoestrogens are metabolized or converted in the gastrointestinal tract of ruminants, only limited knowledge exists on the extent and location of this conversion in vivo. The objective of this study was to quantify the gastro-intestinal metabolism of phytoestrogens in lactating dairy cows fed silages with different botanical composition. Four lactating rumen cannulated Norwegian Red cattle were assigned to a 4 × 4 Latin square with 1 cow per treatment period of 3 wk. The 4 treatment silages were prepared from grasslands with different botanical compositions: organically managed short-term timothy (Phleum pratense L.) and red clover (Trifolium pratense L.) ley (2 yr old: ORG-SG); organically managed long-term grassland with a high proportion of unsown species (6 yr old; ORG-LG); conventionally managed perennial ryegrass (Lolium perenne L.) ley (CON-PR); and conventionally managed timothy ley (CON-TI). The herbages were cut, wilted, and preserved with additive in round bales, fed as a mix of the first and third cut at 90% of ad libitum intake, and contributed to 70% of the total dry matter intake. Milk, feed, omasal digesta, urine, and feces were collected at the end of each period and analyzed for the concentrations of phytoestrogens by using a liquid chromatography-tandem mass spectrometry technique. Concentration of total isoflavones was highest in ORG-SG and lowest in CON-TI silage, whereas the content of total lignans was highest in the grass silages. The isoflavones were extensively metabolized in the rumen on all diets, and the recovery of formononetin and daidzein in omasum, mainly as equol, averaged 0.11 mg/mg. The apparent intestinal metabolism was less severe as, on average, 0.29 mg/mg of the omasal flow was recovered in feces. The plant lignans were also strongly degraded in the rumen. However, the flow of lignans to omasum and excretion in feces were, on average, 7.2- and 5.2-fold higher, respectively, than the intake of the plant lignans matairesinol and secoisolariciresinol, known as precursors of mammalian lignans. Thus, excretion to milk could not be directly related to intake, implying that plant lignans other than matairesinol and secoisolariciresinol in forage are precursors for enterolactone production in the rumen and for its content in milk. Equol followed mainly the flow of large particles out of the rumen, whereas the mammalian lignans were distributed between phases proportional to dry matter flow. The main metabolism of phytoestrogens occurred in the rumen and the main route of excretion was through feces and urine, with only a small part being excreted in milk. The concentration of phytoestrogens in milk can be manipulated through intake but the intermediate transfer capacity to milk appears to be limited by saturation.

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