1628
Within Laboratory Repeatability of the In Situ Nylon Bag Method
The in situ nylon bag method is the basis of rumen degradation parameters for most feed evaluation models. In our laboratory, over a period of 10 years all in situ rumen incubations contained a standard sample to study within laboratory repeatability. For raw materials (RM) the standard was a mix of corn, soybean meal and grass meal (1:1:1) to represent starch, crude protein (CP) and fiber fermentation, analyzed in 21 runs. For forage standards were 2 sequential grass (GS) and corn silages (CS) analyzed in 12 and 11 runs. Approximately five grams of DM was weighed into 9x18 cm (inner size) nylon bags (40 µm pore size, 30% open surface, Radiometer, The Netherlands). Two or three bags were incubated in the rumen of three lactating fistulated dairy cows at 8 time points (up to 336h). After incubation bags were washed in a washing machine, along with four non-incubated bags per sample (0h). Samples were pooled over cows by incubation time and analyzed for dry matter (DM), CP, starch and NDF. For each component, washable (W) and undegradable (U) fraction were equated to 0h loss and 336h residue. Degradation rate (kd) was estimated by NLIN procedure of SAS. Effective degradability (ED) was calculated as ED = W + (100-W-U)*( kd/(kd+kp)) with a kp (passage rate) of 0.06 for raw material and 0.045 forage standards. Coefficient of variation (CV) was used as measure of repeatability. For the forages CV was based on pooled variation and mean of both standards. Table 1 shows the CV for ED of various components in RM, GS and CS. CV for NDF is numerically highest for all feeds. For RM, CV of DM is lowest, whereas for GS and CS, CV of CP and starch is lowest. For most components within feedstuff, CV of W, U and kd(results not shown) was higher than that of ED. These results show that the lowest values for within laboratory repeatability of the in situ nylon bag method are in the range considered poor for analytical laboratory measurements, whereas repeatability values found for NDF are extremely high.
Table 1: Coefficients of variation (%) for effective degradation of chemical components of feedstuffs.
CV% of ED |
Raw Material |
Grass Silage |
Corn Silage |
DM |
3.9 |
5.0 |
12.2 |
CP |
8.9 |
3.7 |
5.6 |
Starch |
10.5 |
x |
5.2 |
NDF |
13.6 |
9.6 |
16.4 |
Keywords:
In situ, repeatability, fermentation