Effects of various starch feeding regimens on responses of dairy cows to intramammary lipopolysaccharide infusion.

Endotoxin tolerance (ET) can develop in mammals that have been challenged repeatedly with sublethal amounts of lipopolysaccharide (LPS). Previous research has shown that subclinical ruminal acidosis can increase circulating concentrations of LPS. We investigated whether ET would develop in Holstein cows that were subjected to chronic subacute ruminal acidosis (SARA) or acute SARA followed by intramammary infusion of LPS. Twenty-four cows, both primiparous and multiparous, were assigned to 8 blocks of 3 cows. Cows within blocks were randomly assigned to 1 of 3 treatments: (1) control (diet DM was 24% starch and 35% NDF), (2) high starch (formulated to induce chronic milk fat depression with 29% starch and 32% NDF), and (3) acidosis (designed to cause acute bouts of milk fat depression by short-term feeding of a diet with 32% starch, some of which came from wheat grain, and 30% NDF). Cows on the control and high-starch treatments were fed their respective diets throughout the 24-d trial. The acidosis cows were fed the control diet during most of the experiment, except during two 2-d bouts (d 10 and 11 and 17 and 18 of the experiment) in which a high-starch diet was fed. Cows on the high-starch and acidosis treatments produced milk fat with an altered fatty acid profile indicative of SARA (e.g., increased concentrations of specific trans, and odd-, and branched-chain fatty acids), but only cows on the high-starch treatment had milk fat depression. Concentrations of serum amyloid A were elevated in cows on the acidosis treatment, but did not differ between control and high-starch cows. On d 20 of the experiment, all cows were given an intramammary infusion of 10 µg of LPS into 1 mammary quarter 3h after morning milking. Milk yield and DMI decreased the day of the infusion, but the response was not affected by dietary treatment. No systemic indicators of ET were observed among treatments, but evidence of an ET response at the local level of the mammary gland was observed. Cows fed the control diet had higher concentrations of serum amyloid A in milk 12 and 24h postinfusion than did cows fed the high-starch diet and higher concentrations than cows on the acidosis treatment at 12h postinfusion. Our data suggest cows that experienced varying degrees of SARA (based on altered milk fatty acid profile) and subsequent experimental endotoxin mastitis experienced a blunted inflammatory response at the level of the mammary gland, but not a systemic reduction in some inflammatory mediators.