Monensin and a blend of castor oil and cashew nut shell liquid used in a high-concentrate diet abruptly fed to Nellore cattle

Monensin and functional oils (FO) were supplemented to a high-concentrate diet abruptly fed to 12 ruminally cannulated Zebu steers to study their effects on rumen fermentation, blood metabolites, and Streptococcus bovis, Megasphaera elsdenii, and Fibrobacter succinogenes relative population. A randomized complete block design with repeated measures over time within 2 experimental periods of 21 d each was used. Treatments were a control (CTR; with no additives), FO (included at 400 mg/kg), and monensin included at 30 mg/kg (M30) or 40 mg/kg (M40). All steers were fed the same high-concentrate basal diet, which consisted of 92.25% concentrate. The first 60 h after transition showed a treatment and hour interaction for ruminal propionate proportion (P = 0.028), and no change in acetate molar proportion (P = 0.633), rumen pH (P = 0.370), and time the rumen pH remained below 5.6 (P = 0.242) were observed. The acetate: propionate ratio decreased (P = 0.020) when monensin was fed in both concentrations (2.30 for the M30 treatment and 2.32 for the M40 treatment) compared with when the CTR was fed (2.85), without being different when the FO (2.71) treatment was fed. Only the M30 treatment did not show pH below 5.2 (P=0.047) over the 60 h after the abrupt transition. Within the entire period, DMI (P = 0.008) and mean ruminal pH (P = 0.040) as well as molar proportions of propionate (P = 0.034) and valerate (P = 0.031) had significant interactions between treatment and day. Total VFA concentration was greater (P = 0.017) for the M30 (117.36 mM) and CTR treatments (115.77 mM) compared with the M40 treatment (105.02 mM), without being different for the FO treatment (111.55 mM). Treatments did not change feed behavior parameters. Blood HCO3-(P = 0.006) and total carbon dioxide (P = 0.003) were greater for the M30 (27.8 and 29.3 mmol/L, respectively) and FO treatments (28.3 and 29.7 mmol/L, respectively) compared with the CTR treatment (25.7 and 26.9 mmol/L, respectively). Fibrobacter succinogenes (P < 0.0001) and Streptococcus bovis (P < 0.0001) decreased their population throughout days, whereas Megasphaera elsdenii (P = 0.026) increased its population. Independent of ciliated protozoa genera, the greatest (P < 0.0001) protozoa counts were observed for the CTR treatment (52.7 x 10(4)/mL), intermediate for the FO treatment (35.3 x10(4)/mL), and least for steers fed monensin in both concentrations (15 x 10(4)/mL for the M30 treatment and 14 x 10(4)/mL for the M40 treatment). Feed additives had different effects to reduce the subacute acidosis. The use of the FO and M40 treatments did not change most of the rumen fermentation variables, especially in the first week after abrupt transition, when the M30 treatment provided higher protection against acidosis.