In situ degradation of feed purines

Twenty one feeds (roughages: barley straw, meadow hay, lucerne hay and dehydrated lucerne legume forages: lucerne; red clover, white clover, birdsfoot trefoil and sainfoin; cereal forages: oats, rye, wheat and triticale; grasses: brome grass and cocksfoot; concentrates: wheat bran, brewers grain, vetch, soya-bean meal, sunflower meal and fish meal) were evaluated for ruminal purine (P) degradation kinetics and effective degradability (ED) of crude protein (CP) by in sacco incubation. The proportion of ruminally escaped feed P in duodenal flow was indirectly estimated. Meadow hay was had the lowest P content among all feeds. Within the legume forages, lucerne and sainfoin had lower values than red clover, white clover and birdsfoot. Differences also existed among lucerne, lucerne hay and dehydrated lucerne. Annual cereal forages (oats, wheat, rye and triticale) had P contents in the range of 3.0–4.7 mg g−1 dry matter (DM) and the lowest value was obtained for oats. Perennial grasses (brome grass and cocksfoot) did not differ in their P concentrations. In this respect, concentrates varied substantially with the highest value found in fish meal (59.4 mg g−1 DM) followed by soya-bean meal (21.2 mg g−1 DM). Soluble fraction (a) of P in most of the feeds was higher than the slowly degradable fraction (b), with the exception of dehydrated lucerne, brewers grain, soya-bean meal and fish meal. Fraction b varied from 21% (oats) to 53% (soya-bean meal). The maximum potential (a+b) of P degradation was high and did not differ within various legume forages (92–99%) or within cereal forages (94–98%). Lower a+b values were obtained for barley straw, dehydrated lucerne, brewers grain and fish meal. The ED of P, estimated at ruminal outflow rate 0.05% h−1, of the majority of tested feedstuffs was in the range of 80–90%, although a substantially lower value (41%) was obtained for fish meal. Dehydrated lucerne also had very low P degradability (55%). Intermediate ED was found for barley straw (64%) and for brewers grain (71%). There was a correlation between the ED of CP and the ED of P (R2=0.82). The indirectly estimated contribution of undegraded feed P to duodenal flow was negligible (under 2% at outflow rate of 0.05% h−1) in all tested forages, hays and in wheat bran, vetch and sunflower meal. The calculated values for dehydrated lucerne, brewers grain, barley straw and soya-bean meal were in the range of 2–4%. Fish meal had a very high proportion of ruminally escaped P in the duodenal pool.