Efficacy of laidlomycin propionate in low-protein diets fed to growing beef steers: Effects on steer performance and ruminal nitrogen metabolism

We conducted two experiments to evaluate the effect of the ionophore laidlo mycin propionate (LP) on steer performance and ruminal N metabolism. Experi ment 1 was a 91-d growth study evaluating the growth and ruminal characteri stics of steer calves consuming supplemental LP. Steers (n = 96; 255 +/- 3 kg; four steers/pen; six pens/treatment) were used in a randomized complete block design with a 2 x 2 factorial arrangement of treatments consisting o f two levels of dietary CP (formulated to be 10.5 and 12.5% of DM) with and without LP (11 mg/kg diet DM). Ruminal fluid was collected via stomach tub e on d 91 from one steer randomly selected from each pen. No CP x LP intera ctions were observed with performance data (P > .64). Final weight and tota l gain were greater (P < .07) for 12.5% CP and LP compared with 10.5% CP an d control steers, respectively. Also, DMI was increased (P = .08) with 12.5 % CP but not with LP supplementation (P = .36). In addition, ADG and gain:f eed ratio were greater (P < .03) for both 12.5% CP and supplemental LP. Rum inal NH3 N concentration was greater (P < .09) with 12.5% CP and LP. Total VFA concentration and molar proportion of acetate were not affected by trea tment (P > .11). However, propionate concentration was increased (P < .09) with 12.5% CP and LP, and acetate:propionate was lower (P =.02) with LP sup plementation. In Exp. 2, six steers were used in a replicated 3 x 3 Latin s quare design to compare ruminal fermentation and protein degradation in ste ers without ionophore feeding or adapted to LP or monensin. In vitro deamin ation of amino acids by adapted ruminal microbes was also assessed. Ionopho re supplementation decreased (P = .07) ruminal NH3 N concentration compared with control steers, and LP increased (P = .02) ruminal NH3 N compared wit h monensin. Molar proportion of acetate was decreased (P = .02) and propion ate increased (P = .01) with ionophore treatment. Consequently, ionophore s upplementation depressed the acetate:propionate ratio (P = .01). In situ de gradation rate of soybean meal (SBM) CP was greater (P = .09) with ionophor e treatment, but estimates of SBM undegradable intake protein were not alte red by treatment (P > .25). Microbial specific activity of net NH3 N releas e and alpha-amino N degradation were decreased (P < .04) with ionophores. B ased on this study, LP and monensin did not affect the extent of ruminal de gradation of SBM CP but decreased amino acid deamination.