The Influence of Trace Mineral Source on Rumen Trace Mineral Solubility and In-Vitro Fiber Digestion in Steers Fed a Lactating Dairy Diet

Abstract Eight individually fed fistulated steers (410 ± 10 kg) were used to investigate the impact of trace mineral source on rumen trace mineral solubility and in vitro fermentation characteristics. Steers were sorted by body weight, into two trace mineral (TM) treatment groups (n = 4/treatment). Treatments consisted of TM supplementation at 10 mg Cu, 40 mg Mn, and 60 mg Zn/kg DM from either sulfate TM (STM) or hydroxy TM (HTM) sources. Steers were fed a diet balanced to meet the nutrient requirements (TMR) for lactation dairy cow. Dietary TM treatments were mixed with dried distillers grains and top dressed daily. Rumen contents were collected at 0, 2, and 4 h post feeding on d 1 and 14 of the experiment. Rumen samples were centrifuged at 1,500 × g for 15 min. The supernatant and the digesta pellet were frozen, separately, at −20°C for TM analysis. On d 15 of the experiment, approximately 500 g (wet weight) of rumen contents were collected from each steer 2 hours post-feeding. Strained ruminal fluid (SRF) was obtained by squeezing the rumen contents through 2 layers of cheese cloth. The retained digesta (150 g wet weight) was washed with 2.2 L pre-warmed McDougall’s buffer to remove particle associated microorganism (PAO). The SRF and the PAO were then filtered through 8 layers of cheesecloth, separately, and mixed at a ratio of 1:2 (SRF:PAO). The SRF:PAO mixture for each steer was incubated with the TMR containing HTM and a separate incubation with the TMR containing STM for 0, 12, and 24 h (3 replicate in vitro tubes/animal/TMR treatment) at 39°C under anaerobic conditions. After each incubation time, samples were centrifuged at 1,500 x g. The supernatant was removed and frozen at -20°C for VFA analysis. The digesta pellet was dried at 60°C and DM, NDF, and ADF digestibility determined. Data were analyzed as a completely randomized block design with individual animal as the experimental unit. There was a treatment x day x sample type interaction for rumen Zn (P < 0.04) and Mn (P < 0.03) concentrations. Zinc and Mn concentrations were greater in digesta and lesser in supernatant in STM compared with HTM supplemented animals. Rumen distribution of Cu was not impacted by TM source. In vitro molar proportions isobutyric acid were greater (P < 0.01) in HTM compared with STM supplemented animals, while the molar proportion of propionic acid were greater (P < 0.01) in STM supplemented animals. Total VFA concentration, molar proportions of acetic and propionic acid, and DM, NDF, and ADF digestibility were not impacted by dietary trace mineral source or rumen fluid source. These data may indicate that the source of trace mineral can influence trace mineral solubility in the rumen.