Effect of withdrawing an immunomodulatory feed additive from lactating cow diets on peripheral blood mononuclear cell proliferation.

Supplementation of immunomodulatory feed additives, such as OmniGen AF (OG), helps to maintain immune competency; however, it is unknown if immune benefits persist in lactating cows after OG is removed from the diet. The aim of this trial was to evaluate the effect of withdrawing OG from the diet on peripheral blood mononuclear cells (PBMC) proliferation of midlactation dairy cows. Multiparous Holstein cows (N = 32), blocked by parity (2.7 ± 0.8) and days in milk (153 ± 39 d) were randomly assigned to one of the two dietary treatments within each block: diets were top dressed with either OG (56 g/d/cow) or placebo (CTL, 56 g/d/cow). Cows were housed in the same free-stall pen and individually fed once per day through Calan gates. All cows were fed the same diet containing OG for at least 1 yr before the onset of treatments. Cows were milked three time per day and milk yield was recorded at each milking. Milk samples were collected from three consecutive milkings weekly and composition analyzed. Body weight (BW) and condition score were measured weekly. Blood samples were collected at -1, 1, 3, 5, and 7 wk relative to the onset of treatments for the isolation of PBMC. The PBMC were cultured with concanavalin A (ConA) and lipopolysaccharides (LPS) for 72 h in vitro to determine proliferative responses. Prior to the experiment, cows in both treatments had similar disease incidence. During the experiment, cows did not show symptoms of disease. Withdrawing OG from the diet did not affect (P ≥ 0.20) milk yield or composition, intake, or BW. Compared with CTL, feeding OG maintained greater body condition score (2.83 vs. 2.92, P = 0.04). Regardless of time, relative to CTL, PBMC isolated from cows fed with OG had a greater proliferative rate when stimulated with LPS (stimulation index: 1.27 vs. 1.80, P = 0.05) and tended to have greater proliferation when stimulated with ConA (stimulation index: 5.24 vs. 7.80, P = 0.08). In conclusion, withdrawing OG from the diet of midlactation cows reduced proliferative response of PBMC suggesting that the immunomodulatory role of OG is lost as early as 1 wk after its withdrawal from the diet of lactating dairy cows.