The true digestibility and biological value for rats of undegraded dietary nitrogen in feeds for ruminants

Abstract

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 containing Maillard products, such as maize gluten and distillers' dark grains. The experiments in this study were conducted to obtain direct measurement of dUN and biological value (BV) of UDN in a range of 10 ingredients containing various amounts of acid-detergent insoluble nitrogen (ADIN) either occurring naturally or ‘added’ as a result of Maillard reactions during processing. The UDN was harvested from artificial fibre bags suspended in the rumen of cows for 18-h, freeze-dried, sterilized 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 feeding the UDN-based diets to rats together with a casein-based standard and a nitrogen-free diet. The true dUN values ranged from 0.97 for maize gluten 60 to 0.22 for distillers' grains. Materials processed under heat and moisture had low dUN values (0.22–0.60). The correlation between true dUN and that estimated from ADIN concentration using the expression 0.9(UDN-ADIN)/UDN was satisfactory ( r 2=0.73). Increasing ADIN intake increased faecal output of non-ADIN N ( r 2=0.66), which suggests some antinutrient effects of ADIN, although not to the extent of reducing BV. It is concluded that all ADIN in UDN is indigestible and may have antinutrient properties. Some ‘added’ ADIN from Maillard reaction products can be degraded in part in the rumen and so contribute to microbial protein synthesis.