Fractional rate of degradation (kd) of starch in the rumen and its relation to in vivo rumen and total digestibility

Fractional rate of degradation (kd) of fermentable nutrients in the rumen is an important parameter in modern feed evaluation systems based on mechanistic models. Estimates of kd for starch was obtained on 19 starch sources originating from barley, wheat, oat, maize and peas and treated in different ways both chemically and physically. The starch sources were fed in mixed diets together with grass silage and soya bean meal and allocated ad libitum to fistulated dairy cows. The starch content varied between 13 and 35% in ration dry matter for the different starch sources. The design was a series of cross-over experiments with two cows and two periods. Ruminal starch pool was estimated from rumen evacuation and starch flow was estimated by duodenal and faeces sampling. Fractional rate of rumen degradation was estimated from the equation [kd = rumen degraded/rumen pool] and rumen and total digestibility of starch from flow measurements. The relation between kd and rumen digestibility (RD) was estimated to [kd = 1.139 -3.580*RD + 3.078*RD2; R2 = 0.45; valid for 0.580.81]. It is concluded that the relations between kd and starch digestibility can be used to estimate kd for feed table use from experiments where starch digestibility is known and within the range valid for the equations estimated.