A trial was conducted at the University of Delaware dairy center to compare milk and component responses to supplemental rumen protected B vitamins (BV; Lactation VB, Jefo, 3 g/cow/d) supplying vitamins B5, B6, B7, B9 and B12 in replacement for non-protected vitamin B7 alone (B; 20 mg/cow/d). B was provided for > 30 d (Period 1) before milk testing, switched to BV for the following DHIA test (Period 2), then cows were switched back to B for period 3. Intake was not measured. Milk results were analyzed using individual cows (65) in a paired t-test, adjusted to a common 150 DIM using the following equations: Adjusted milk = milk + milk × (DIM − 150) × 0.0029; Fat % = fat percent − 0.00269 × (DIM − 150); Protein % = protein % − 0.00167 × (DIM − 150). Days adjusted milk and component yields were greater for Period 2 with the use of BV than for Period 1 (P < 0.05; Table 1).There was substantial heat during both periods. High temperatures on test days were 28 and 31°C for Periods 1 and 2, respectively. The highest temperature on the Period 3 test day was 22°C, and results were used to assess production without heat stress. The temperature humidity index was 79, 83, and 70 for Periods 1, 2 and 3, respectively. Comparison of adjusted milk and component yield responses for the 2 B periods indicated that milk losses were substantial when cows were under heat stress conditions (Period 1 Vs Period 3, Table 1). These results showed that BV increased milk and component yields over B during periods of heat stress. When compared with the period without heat stress, BV appeared to alleviate a portion of the loss in production that occurs during heat stress. Table 1.

Item	Heat stress		No heat stress	P value
Period 1	Period 2	Period 3
Control (B)	Test (BV)	Control (B)	1 vs. 2	1 vs. 3
Milk, kg/d	33.7	37.5	39.4	0.001	0.001
ECM, kg/d	34.1	37.9	41.8	0.001	0.001
Fat, kg/d	1.23	1.33	1.50	0.004	0.001
Protein, kg/d	0.93	1.09	1.23	0.001	0.001
Fat, %	3.83	3.60	3.88	0.013	0.628
Protein, %	2.79	2.91	3.13	0.001	0.001