Passage of 13C-labelled feed components through the digestive tract of dairy cows

Keywords: fractional passage rate, grass, silage, ¹³ C-isotope, internal markers, dairy cowsThe main aim of this thesis was to acquire knowledge on the feed specific fractional passage rates of feedstuffs and feed components through different compartments of the gastro-intestinal tract of dairy cows. Fractional passage rates form, next to fractional degradation rates, the determining factors to quantify the escape part of feed components from the rumen. Current feed evaluation systems use feed specific fractional degradation rates, but adopt fixed fractional passage rates. It is increasingly realised that also passage rates vary, amongst others with the level of feed intake, diet quality and diet composition. Besides, international developments in feed evaluation move towards so-called "nutrient based" feed evaluation systems that are more dynamic in nature. Knowledge on feed specific fractional passage rates is essential to further develop such dynamic systems. To meet our main aim, several animal trials were conducted in which the passage behaviour of feed particles originating from grass and grass silages were studied and compared with the traditionally used external markers Cr-NDF and Co-EDTA, using rumen and ileum cannulated animals. The roughages were labelled with the stable isotope of carbon ( ¹³ C), thus allowing to follow the passage behaviour of the dry matter, the cell wall and the non cell wall fractions. In all cases, ¹³ C gave slower ruminal passage rates compared to the external markers, and with respect to the labelled fractions the ¹³ C-labelled cell wall fractions gave the slowest fractional passage rates. A reduction in the level of feed intake of 40% gave decreased marker passage rates with the highest impact on the rumen residence times for the cell wall fractions. The effect of roughage quality on ruminal marker passage was more pronounced for the internal markers compared to the external markers. In combination with in situ degradation characteristics our data indicates that ¹³ C seems to be a useful tool to quantify the relations between passage and degradation. Based on our findings it was concluded that the fixed fractional passage rates currently adopted in the Dutch protein evaluation system overestimate the in vivo situation. Besides, our data suggests the possibilities to introduce feed component specific fractional passage rates as a function of animal and dietary characteristics.