We conducted three experiments to determine the effects of dietary thiamin
supplementation on milk production by dairy cows. In trial 1, 28 Holstein c
ows were blocked by parity and assigned randomly to either placebo or thiam
in top-dress for the 8-wk experiment to provide a supplemental thiamin inta
ke of 0 or 150 mg/d per cow. Within each of these groups, cows were further
assigned randomly to two total mixed rations (TMR) for 4 wk, with the TMR
treatments then reversed for a second 4-wk experimental period. Milk yield
was 2.7 kg/d higher for thiamin-supplemented cows. Yields of milk fat and p
rotein were increased 0.13 and 0.10 kg/d, respectively, by dietary thiamin
supplementation. In trial 2, 20 multiparous Holstein cows were used in a cr
ossover design with 4-wk periods. Placebo or thiamin premixes were added to
TMR to provide an approximate daily supplemental thiamin intake of 0 or 30
0 mg/cow. Milk and protein yields tended to be 0.7 and 0.04 kg/d higher, re
spectively, for thiamin-supplemented cows. In trial 3, 16 multiparous Holst
ein cows were used in a replicated 4 x 4 Latin square with 21-d periods. Pl
acebo or thiamin premixes were added to TMR to provide an approximate daily
supplemental thiamin intake of 0 or 300 mg/cow. Dry matter intake tended t
o be 0.8 kg/d lower for thiamin-supplemented cows. Milk fat percentage tend
ed to be 0.18 percentage units lower and fat yield was 0.08 kg/d lower for
thiamin-supplemented cows. Thiamin supplementation tended to increase milk
and component production when dietary concentrations of neutral and acid de
tergent fiber were lower and nonfiber carbohydrate was higher than recommen
ded.