For several decades, essentially no research was published on the effects of metabolism of biotin, a B-complex vitamin, in cattle, according to W.P. Weiss and C.A. Zimmerly of the Ohio Agricultural Research & Development Center, Wooster, Ohio, in a presentation at the 2000 Cornell Nutrition Conference for Feed Manufacturers held Oct. 24-26 in Rochester, N.Y. This lack of interest was probably caused by the inability to produce clinical biotin deficiencies in functioning ruminants and by the consensus that adequate biotin was synthesized in the rumen and intestines to meet the needs of the host, they said.
In the past few years, however, renewed interest in the effects of biotin has developed, they said, adding that most of these recent papers have studied the effect of biotin on hoof health. Biotin is also essential for many metabolic pathways, Weiss and Zimmerly reported, directly involved with milk production. Because of the ever-increasing productivity of dairy cows, the effects of biotin on the metabolism of dairy cows deserve attention, the researchers noted.
Ruminal digestion, metabolism
All of the predominant cellulolytic bacteria found in the rumen require biotin for growth, Weiss and Zimmerly said. In vitro (mixed ruminal culture) cellulose digestibility was increased when biotin was included in the media. Biotin is also required by bacteria to synthesize propionate, and propionate production in vitro (mixed culture) was reduced when biotin was not included in the media and cellulose was the substrate. The effects of supplemental biotin on fiber digestion in vivo have not been studied, they said.
Sources of biotin
Most feedstuffs contain some biotin, they said, with biotin concentrations weakly correlated with concentrations of crude protein in feeds. Most oilseed meals contain 0.25-0.35 mg biotin/kg dry matter. Grains and most grain byproducts generally contain less than 0.15 mg/kg. Brewers grains, distillers grains and especially distillers grains plus solubles are good sources of biotin and can contain up to 0.6 mg/kg of biotin. Forages generally contain between 0.2 and 0.4 mg/kg with higher protein forages tending to have higher biotin concentrations.
According to Weiss and Zimmerly, most basal diets fed to dairy cattle should contain between 0.2 and 0.4 mg of biotin/kg of dry matter. Typical intakes of dietary biotin by unsupplemented lactating cows range from 4 to 10 mg per day, they said.
Bacteria in the rumen and large intestine can synthesize biotin that then can be absorbed by the animal, the two noted. Based on duodenal flow data, ruminal synthesis of biotin ranged from 0 to about 10 mg per day in steer calves (190 kg) consuming 3.5 kg of dry matter of a diet based on alfalfa hay, sudangrass and steam-flaked corn. Most in vitro studies and studies that measure biotin concentration in rumen fluid suggest that some ruminal synthesis of biotin occurs, they said. Adequate, consistent data are not available to allow accurate estimation of ruminal synthesis at this time, Weiss and Zimmerly said.
Consistent data on factors that influence biotin synthesis in the rumen are also lacking, they noted. If higher concentrate diets do reduce biotin synthesis in the rumen, then biotin supply may be reduced when higher concentrate diets are fed to dairy cows, the researchers said. 'This could be a reason why positive responses to supplemental biotin are often observed in recent experiments but were not observed in earlier research when cattle were fed high-forage diets,' they said.
According to these authors, recent research with humans showed that supplemental oral biotin is 100% available, and research with chicks using a relative growth assay found that the biotin contained in grains and grain products was generally less than 50% bioavailable. In one study conducted with cattle, availability of supplemental biotin ranged from 40 to 55%, Weiss and Zimmerly reported, explaining that the lower value was for heifers fed supplemental biotin for 26 days before the experiment and the higher value was for heifers not fed any supplemental biotin until the experiment.
The lower bioavailability of biotin for ruminants compared with humans may be caused by some ruminal destruction of the supplemental biotin. However, they said, significant quantities of supplemental biotin escape the rumen and are absorbed by cattle. In some non-peer reviewed reports, serum biotin concentrations decreased as the number of days supplemental biotin was fed increased, Weiss and Zimmerly said, suggesting that perhaps ruminal destruction of biotin was increasing. Peer reviewed reports, however, do not show decreased plasma biotin concentrations over time, they noted.
In all published experiments that were found, oral biotin supplementation increased the concentrations of biotin in serum and milk. In lactating cows not supplemented with biotin, the researchers reported serum biotin concentrations ranged between 0.5 and 0.7 ng/ml and milk concentrations ranged between 20 and 35 ng/ml. Cows fed 20 mg of supplemental biotin per day have serum biotin concentrations approximately twice as high as controls and milk concentrations are two to three times higher than controls, they said.
Increased secretion of biotin in milk and increased size of the biotin plasma pool only accounts for about 10% of the increased intake of biotin when 20 mg is supplemented, Weiss and Zimmerly said. The fate of the remaining biotin consumed is not known, but urinary excretion probably accounts for a substantial portion, they added.
Unlike many fat-soluble vitamins that reach their plasma lowest point at calving, plasma biotin concentrations spike at calving when supplemental biotin is fed, Weiss and Zimmerly reported; biotin concentrations in plasma of unsupplemented cows do not change greatly between the dry period, parturition and lactation. Concentrations of biotin in colostrum are similar to those in milk when cows are not supplemented with biotin, they said, but can be as much as five times higher than milk when cows are fed 20mg of supplemental biotin during the periparturient period. The biological significance of this is not known, the researchers noted.
Conclusions
Available data, although limited, consistently show increased milk production when high producing cows are fed 20 mg of supplemental biotin, Weiss and Zimmerly said. Cows at lower production (approximately 20 kg per day) did not increase production when supplemental biotin was fed. Some authors attributed the increase in milk production to improved hoof health, Weiss and Zimmerly noted, but newer data strongly suggest that biotin may have an effect on milk production independent of improvements in hoof health and mobility. The actual mode of action is not known, they said, but increased ruminal fiber digestion and increased glucose synthesis may explain the effects.
