Intestinal and Total Tract Digestibility of Canola Meal
Several studies have been carried out to estimate intestinal availability and total tract digestibility of CM using the mobile nylon bag technique (de Boer et al. 1987 {1128}; Khorasani et al. 1994 {858}; Ha and Kennelly 1983 {1249}) or cannulated steers (Lardy et al. 1993 {886}; Zinn 1993 {884}). Intestinal DM disappearance was substantially less for CM than SBM, despite similar CP disappearance. These results are consistent with the higher fiber content of CM and its lower overall digestibility. Intestinal disappearance of undegraded dietary protein (UDP) for SBM and CM were 99 and 79%, respectively (de Boer et al. 1987 {1128}). In agreement Zinn (1993 {884}) using the difference technique reported the total tract digestibility of N in CM-40, CM-46 and SBM to be 88.0, 91.3, and 94.8%, respectively. This result is consistent with Matras et al. (1990 as cited by Mustafa 1996), who found that N digestibility of SBM was two to three percentage units higher than that of CM. Following a 16 hr rumen incubation, the proportion of CM protein that disappeared in the rumen, small intestine and the total tract were 74.4, 16.2 and 90.5%, respectively (Moshtaghi Nia and Ingalls 1992 {950}). Conversely, Khorasani et al. (1994 {858}) reported total tract digestibility of protein from CM and SBM to be 73.1 vs. 83.5% following a 24 hr rumen incubation. Soybean meal had the highest whole tract digestibility of DM, NDF and CP at all rumen retention times compared to CM. This was attributed to the large hull fraction (30%; Bell 1984 {1232}) associated with CM, that is poorly digested by all types of animals (Khorasani et al. 1994 {858}). Subuh et al. (1996 {798}) reported a total tract digestibility of N in SBM, high GL-RSM and low GL-RSM to be 95.1, 83.4 and 83.7%. Most reports on CM protein total tract digestibility range from 80% to 90%.
Nine feedstuffs were studied to determine AA concentrations before and after ruminal incubation and after passage through the intestines of dairy cows. Residual AA composition (after rumen incubation) is different than the original feed AA and the changes are more significant for the more rumen degradable feedstuffs (O'Mara et al. 1997 {793}; Varvikko 1986 {1159}). Feed AA are not degraded equally by the rumen microbes. O'Mara et al. (1997 {793}) reported that RSM and SBM AA disappearance (%) for the rumen, intestinal and total tract were 68.8, 78.8, and 93.5% and 62.2, 95.5, and 98%, respectively. Of the nine protein supplements tested CM was the only one that had an intestinal disappearance under 90%. The low CP intestinal digestibility (sample 1 and 2, 57.13 and 80.35%) of RSM was also reported by Frydrych 1992 {953} and a review of the literature gave a intestinal digestibility between 73.3-80%. Conversely, Lardy et al. (1993 {886}) reported RSM supplementation resulted in higher (P < 0.01) duodenal flows of cystine, methionine, arginine and isoleucine than did SBM. Total AA flow to the duodenum was higher for RSM compared to SBM (511 vs. 439 g/d). Zinn (1993 {884}) and O'Mara et al. (1997 {793}) reported that on a isonitrogenous basis, CM may provide more methionine to the small intestine than SBM. In agreement, Lardy et al. (1993 {886}) concluded that RSM supplementation is one way to increase duodenal flow of S-AA for ruminants. A study by Kendall et al. (1991 {1004}) reported total tract AA digestibilities of CM to approach 85% or greater. Most studies agree with this.
Boila and Ingall (1992 {950}) reported
that the amino acid profile of CM protein that bypasses the rumen
to be higher in AA in tyrosine, valine, isoleucine, threonine,
phenylalanine, serine, aspartate and alanine relative to CM that
was unincubated. The increase in AA content ranged from 14 to
27%. Tyrosine was the least degradable AA in CM and glutamate
the most degradable (Boila and Ingall 1992 {950}; Varvikko 1986
{1159}). The effective degradability of DM was lower than that
of AA and CP effective degradability was higher than that of 11
of 17 AA (Boila and Ingalls 1992 {950}). Digestibility values
of individual EAA ranged from 78.2% for arginine to 89.9% of histidine
(Vanhatalo et al. 1995 {829}). Canola meal contributes to the
rumen microbial protein needs with its highly soluble and degradable
fraction and meets the needs of the highly producing ruminant
with its high quality bypass protein that is highly digestible.