The objective of this study was to determine the effect of including soy co-products (soybean meal and soy oil) in the diet on the hemocytology of cattle after an endotoxin challenge. For the growing phase, crossbred Angus steers (n = 36; initial body weight 289 ± 31 kg) were stratified by body weight and sire and randomly assigned to pastures (n = 9; 0.45 ha mixed grass). Pastures were assigned randomly to 1 of 3 dietary treatments: 1) a control supplement containing no soy co-products (CON); 2) a supplement containing soybean meal (SBM); or 3) a supplement containing soy oil (SBO). All supplements were isonitrogenous and isoenergetic. Cattle were fed supplements (2.45 kg DM/day) for 56 days. At the conclusion of the growing phase, cattle were assigned randomly to 1 of 2 groups for a lipopolysaccharide (LPS) challenge (i.v. infusion of 0.5 µg LPS/kg of body weight). A minimum of 18 hours before sampling, cattle were fitted with jugular vein catheters and placed into stanchions. Blood was collected starting 2 hours before LPS infusion (-2 hr), immediately prior to LPS infusion (0 hr), and at 2, 4, 6, and 8 hr. Statistical analyses were performed using the MIXED procedure of SAS 9.4® as repeated measures with treatment, time, and treatment × time interaction as fixed effects, challenge group as random effect, and calf specified as the subject. Statistical significance was declared at P ≤ 0.05 and tendencies declared at 0.05 < P ≤ 0.1. There were treatment × time interactions for neutrophils (P = 0.003), percentage of neutrophils (P = 0.002), percentage of lymphocytes (P = 0.006), and neutrophil/lymphocyte ratio (NE:LY ratio; P = 0.001). Briefly, neutrophils, percentage of neutrophils, and NE:LY ratio were lower for cattle on CON diets compared to SBM at -2 hr and 0 hr (P < 0.01). Also, the percentage of neutrophils were greater at 8 hr for cattle on SBO diets than SBM diets (P = 0.018). The percentage of lymphocytes were greater for cattle on the CON diets compared to the SBM diets at 0 and -2 hr (P < 0.015). There was a treatment effect for percentage of monocytes (P = 0.035), as the CON diet had greater monocyte percentage compared to the SBM diet, and the SBO diet tended to have greater monocyte percentage compared to SBM diet. There was a time effect for all other hematology variables (P < 0.0001), as leukocytes were typically higher pre-administration of LPS, sharply decreased at 2 and 4 hrs post-administration of LPS and began to increase at 6 and 8 hrs.
Preliminary results indicated that the inclusion of soy co-products in growing cattle diets influenced hemocytology.