Abstract #T479

# T479
Investigation of protein digestion kinetics in vitro using 15N-labeled timothy and red clover.
Merko Vaga*1, Kerstin Huss-Danell1, Mårten Hetta1, Pekka Huhtanen1, 1Dept. of Agricultural Research for Northern Sweden, Swedish University of Agricultural Sciences, Umeå, Sweden.

An in vitro method based on 15N-labeled forage N was developed to study ruminal protein degradation of timothy and red clover. Timothy and red clover were grown on the same field with 2 replicate plots, 2 m2 each. One replicate plot was fertilized with 15N-enriched NH4NO3 while the other received non-enriched fertilizer. Labeled timothy and red clover had average enrichment of 2.37 and 1.23 atom % excess 15N, respectively. Forages from the first-cut were preserved either as hay (TH, RCH), untreated (T, RC) or formic acid-treated silage (TF, RCF). Rumen fluid was collected from 2 Swedish red cows fed on grass silage:concentrate diet (60:40 DM basis). Samples of 1 g were incubated in 60 mL of buffered rumen fluid at 39°C for 48 h. For non-labeled forages NH3-N in the inoculum was labeled with 15N enriched (NH4)2SO4. Vessels containing labeled forages received the same amount of non-enriched (NH4)2SO4. Liquid samples were collected every hour at 0 to 10 h, after 12, 16, 24, 30 and 48 h, and later analyzed for NH3-N and 15N. Degraded CP was calculated as a sum of appearance of 15N from labeled forages and disappearance of 15N from labeled NH3-N. Degradation parameters were estimated by the 2 pool exponential model using the SAS NLIN procedure. Timothy forages had higher rapidly degradable fraction (b1). Slowly degradable fraction (b2) was smaller in formic acid treated silages and in red clover silages. Hay had slower degradation rates (kd) than silages. Total degraded CP (a+b) was higher for timothy and for hay. It is concluded that incubating 15N labeled feeds or feed fractions simultaneously with corresponding unlabeled feeds by labeling ammonia pool with 15N can be a useful tool in investigating kinetics of ruminal protein degradation. Table 1. CP concentration and degradation parameters of timothy (T) and red clover (RC)
TFRCFTRCTHRCH
CP, g/kg DM155147166133144137
CP degradation1
 b1, g/kg495418335334473391
 b2, g/kg380362594507490452
 kd1, 1/h0.6961.2501.2631.2000.8150.818
 kd2, 1/h0.1890.1320.1390.1610.0550.091
 a+b, g/kg875780929841963843
1b1 = rapidly degradable, b2 = slowly degradable, kd1 = degradation rate of b1, kd2 = degradation rate of b2.

Key Words: in vitro, 15N, protein