Given their related metabolic roles, the aim of this study was to determine if a low biotin (B8) supply reduced the effects of a combined supplement of folic acid (B9) and vitamin B₁₂ (B12) during the transition period on cow productivity and metabolism. Cows (n = 32) were assigned to either one of the following treatments: 1) No supplement (CON); 2) 20 mg/d of top-dressed B8 (B8); 3) 2.6 g/d of top-dressed B9 and 10 mg/wk of intramuscular injection of B12 (B9B12); 4) B8 with B9B12 (B8B9B12) from 3 wk before the expected calving date until 3 wk postpartum. According to a 2 × 2 factorial arrangement, Proc MIXED of SAS with repeated measures was used to analyze data (log-transformed when needed). Supplementary B8, B9 and B12 increased their respective plasma concentrations throughout the trial (P ≤ 0.006). No supplementation effect was noted on prepartum plasma concentrations of glucose, insulin, free fatty acids (FFA) and β-hydroxybutyrate (BHB; P ≥ 0.11). With similar dry matter intake and plasma concentrations of glucose and BHB from calving to 3 wk postpartum (P ≥ 0.29), supplementary B9B12 increased body weight (BW) loss, milk and total solid (TS) yields and plasma concentration of FFA and decreased plasma concentration of insulin for cows not receiving B8 (P ≤ 0.09; Table 1) whereas it had no effect when cows received B8 (P ≥ 0.1; B8xB9B12 interaction, P ≤ 0.07). These results suggested that B9B12 supplements increased productivity and altered energy metabolism in early lactation but, under the current conditions, when combined with B8, these effects were not observed. Table 1 (Abstr. W95). Weekly average of milk and TS yields and postpartum plasma concentrations of metabolites and BW loss following B8, B9B12 supplements, or both (mean and 95% CI)

Item	Treatment					P-value
CON	B8	B9B12	B8B9B12	B8	B9B12	B8 × B9B12
Milk, kg/d	40.1(36.2–44.0)	43.3(39.0–47.5)	45.5(41.8–49.2)	41.0(37.1–44.9)		NS	NS	0.06
TS, kg/d	5.1(4.6–5.6)	5.2(4.7–5.7)	5.8(5.4–6.2)	5.0(4.6–5.5)		NS	NS	0.07
Insulin, µU/mL	5.3(4.0–7.1)	4.1(3.0–5.7)	3.4(2.6–4.5)	5.6(4.2–7.4)		NS	NS	0.02
FFA, µM	218(178–266)	314(252–391)	276(228–334)	251(205–307)		NS	NS	0.03
BW loss, kg	15(1–28)	26(11–40)	38(26–51)	23(9–36)		NS	NS	0.05