PSVII-25 The influence of trace mineral source on in vitro trace mineral solubility and trace mineral concentrations of rumen microorganisms in canulated steers fed a diet formulated for lactating dairy cows

Abstract Individually fed fistulated steers [n = 8; body weight (BW) = 410 ± 10 kg] were used to investigate the effects of dietary trace mineral (TM) source on in vitro trace mineral (TM) solubility and TM concentration of rumen bacteria and protozoa. Steers were blocked by BW and sorted into two TM treatment groups. Treatments consisted of 10 mg copper (Cu), 40 mg manganese (Mn), and 60 mg zinc (Zn)/kg DM from either sulfate TM (STM) or hydroxy TM (HTM) sources (n = 4/treatment). Steers were individually housed, and dietary TM treatments were mixed with dried distillers grains and top-dressed daily. Steers were fed a diet formulated to meet the nutrient requirements for a high producing lactating dairy cow for 14 d. Strained rumen fluid (SRF) was obtained on d 15 by squeezing the rumen contents of each steer through 2 layers of cheesecloth. One-half of retained digesta was washed with pre-warmed McDougall’s buffer to remove particle-associated microorganisms (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 mixture of SRF-PAO from each steer was used to incubate 0.5 g of the basal diet containing either HTM or STM, respectively. The SRF-PAO fluid from each steer was incubated in triplicate with each dietary treatment at 39°C under anaerobic conditions for 24h. After incubation, abomasal digestion was stimulated by adding 1.0 M HCl and 1.5% pepsin. Tubes were then incubated for 2 h at 39°C and centrifuged at 2,000 x g for 15 minutes. Supernatant (2 mL) was removed for soluble TM determination. The pH of solution was then increased to 6.5 by the addition of 3.5% pancreatin in a 1 M NaOH solution and then incubated at 39°C for 2 h. The mixture was centrifuged at 2000 x g to separate the supernatant and digesta fractions for mineral analysis. Protozoal and bacterial fractions were also collected from SRF and PAO for TM analysis. Data were analyzed for a randomized block design using a linear mixed-effect model. There was no feeding tube interaction for all parameters measured, so only main effects are reported. Manganese and Zn concentrations in the digesta within fermentation tube containing HTM were greater (P < 0.02) than STM. Tubes containing HTM had a greater (P < 0.001) soluble Mn concentration than STM after in vitro abomasal digestion. Trace mineral concentration of bacteria and protozoa were similar across treatments. These data may indicate that the TM source may influence TM solubility in the abomasum and intestine, but has no impact on bacterial or protozoal concentrations of Cu, Mn, or Zn.