THE TRUE DIGESTIBILITY AND BIOLOGICAL VALUE FOR RATS OF UNDEGRADED DIETARY NITROGEN IN FEEDS FOR RUMINANTS

In the new UK metabolizable protein system, the estimation of the true digestibility (dUN) of rumen undegraded dietary nitrogen (UDN) may be valid for unprocessed feeds but may be an underestimate in feeds cont aining Maillard products, such as maize gluten and distillers' dark gr ains. The experiments in this study were conducted to obtain direct me asurement of dUN and biological value (BV) of UDN in a range of 10 ing redients containing various amounts of acid-detergent insoluble nitrog en (ADIN) either occurring naturally or 'added' as a result of Maillar d reactions during processing. The UDN was harvested from artificial f ibre bags suspended in the rumen of cows for 18-h, freeze-dried, steri lized and included as the sole source of N in 10 semi-synthetic diets. The true dUN and BV for each ingredient were then determined by feedi ng the UDN-based diets to rats together with a casein-based standard a nd a nitrogen-free diet. The true dUN values ranged from 0.97 for maiz e gluten 60 to 0.22 for distillers' grains. Materials processed under heat and moisture had low dUN values (0.22-0.60). The correlation betw een true dUN and that estimated from ADIN concentration using the expr ession 0.9(UDN-ADIN)/UDN was satisfactory (r2=0.73). Increasing ADIN i ntake increased faecal output of non-ADIN N (r2=0.66), which suggests some antinutrient effects of ADIN, although not to the extent of reduc ing BV. It is concluded that all ADIN in UDN is indigestible and may h ave antinutrient properties. Some 'added' ADIN from Maillard reaction products can be degraded in part in the rumen and so contribute to mic robial protein synthesis.