In situ ruminal degradation of phytic acid in formaldehyde-treated rice bran

摘要

Rice bran has a very high content of phytic acid (IP6), which is a nutritional antagonist of Ca. Microbial phytase degrades IP6, but ruminal degradation of nutrients can be reduced by formaldehyde treatment. Milk fever in dairy cows can be prevented by reducing available dietary Ca to stimulate Ca homeostasis. In the present study, effects of formaldehyde treatment on ruminal degradation of IP6 in rice bran were investigated. Two samples of full-fat rice bran were treated with four levels of formaldehyde (i.e., 0, 1000, 2500 and 5000ppm fresh weight) and ruminally incubated in situ for 0, 4, 8, 16, 32, 64 and 336h in three rumen fistulated lactating dairy cows. Dry matter (DM) disappearance was determined, residues were analysed for P and, for one of the products, also for inositol phosphate (IP) forms. Degradation parameters were calculated for DM, P, total IP, and IP6. The in situ washable fraction (W), undegradable fraction (U) and degradable fraction (D) were measured and the rate of degradation (kd) was calculated by exponential regression to the equation: Y(t)= U + D xexp(-kdx t). HPLC analyses confirmed that most P in the original sample and residues was phytate, mostly IP6. DM and P degraded differently in the two rice brans and formaldehyde treatment reduced degradability, lowering W and increasing D and kd. The calculated rumen escape (kp=0.05/h) for P increased from 0.082 at 0ppm to 0.136, 0.284 and 0.398 at 1000, 2500 and 5000ppm of formaldehyde treatment, respectively. Degradation of total IP forms and IP6 corresponded with P disappearance. Formaldehyde treatment reduced W in total IP and IP6, proportionally increasing D, while U was 0 in both. kd decreased with formaldehyde treatment in total IP and IP6, which decreased from 0.309/h at 0ppm to 0.217, 0.116 and 0.071%/h as formaldehyde treatment level increased. The calculated rumen escape (kp=0.05/h) of IP6 were 0.079, 0.126, 0.229 and 0.318 for the increasing formaldehyde treatment levels. Formaldehyde treatment reduced rumen degradability of IP6 in rice bran. One kg of formaldehyde-treated rice bran could bind 7g of dietary Ca post-ruminally, making it a potentially feasible tool to decrease intestinal Ca availability to aid in the prevention of milk fever.