Effects of rumen-protected niacin on inflammatory response to repeated intramammary lipopolysaccharide challenges

Nutritional strategies that improve an animal's resilience to various challenges may improve animal health and welfare. One such nutrient is niacin, which has reduced inflammation in mice, humans, and swine; however, niacin's anti-inflammatory effects have not been investigated in cattle. Our objective was to determine whether rumen-protected niacin (RPN) alters lactating dairy cows' inflammatory response to intramammary LPS challenges, whether RPN resulted in any carryover effects, and whether repeated LPS challenges result in signs of immune tolerance or innate immune training. Twenty healthy, late-lactation Holstein cows (232 ± 65 DIM; 39 ± 5.8 kg/d of milk) were enrolled in a randomized complete block experiment that lasted 70 d. Cows received 26 g/d of RPN or no top-dress (CON) for the first 42 d of the experiment. During the final milking of d 27 and 55, cows were challenged in their rear right (RR) mammary gland with 100 µg of LPS suspended in 5 mL of PBS. Milk yield, milk conductivity, and feed intake were measured daily. Milk composition was measured on d 14, 23, 24, 30, 37, 45, and 52. Blood samples were collected at 0, 8, 12, 24, 48, 72, 96, and 120 h after each LPS challenge, whereas RR quarter milk samples were collected at 0, 8, 16, 24, 48, 72, 96, 120, 144, and 168 h after each LPS challenge. Body temperature was measured continuously during each challenge with an intravaginal thermometer. Linear mixed models with repeated measures were used to analyze the results. Before LPS challenge, RPN did not affect feed intake or milk production, but it reduced SCS (1.24 ± 0.41 [SE] vs. 0.05 ± 0.45). After challenge, RPN did not affect feed intake, milk production, milk composition, SCS, body temperature, plasma glucose, or plasma insulin concentrations. Our results suggest RPN reduced peak plasma haptoglobin and lipopolysaccharide binding protein during the first LPS challenge. Plasma haptoglobin tended to be less after the second challenge for cows previously supplemented RPN, and lipopolysaccharide binding protein was similar for each treatment group after the second challenge. The second LPS challenge resulted in decreased plasma haptoglobin compared with the first LPS challenge, suggestive of tolerance, but it also induced a greater peak SCS than the first LPS challenge. Our results suggest that repeated LPS challenges promote a systemic tolerance but heightened local response to LPS-induced mastitis. Feeding RPN reduced SCS before challenge and reduced plasma acute phase proteins after challenge, suggesting that RPN may reduce systemic inflammation without altering the local inflammatory responses.