Farm Animal Metabolism and Nutrition

Ellis et al., 1999). Therefore, ruminal
residence time can be estimated from
marker profiles in the faeces by fitting
either model that also contains a time
delay parameter to estimate post-ruminal
transit time, . Problems associated with
deviations from the steady-state assump-
tions and representative sampling are less
for faecal as compared with abomasal or
duodenal digesta sampling. Thus fitting
models to marker profiles in the faeces
results in greater precision for resolving
residence time due to non-mixing, post-
ruminal transit time, the lag-rumination
and the mass action turnover pools (Figure
10 of Ellis et al., 1994). A summary of
gastrointestinal residence times resolved
from fitting age-dependent, age-independent
models with a time delay to marker profiles
in the faeces is summarized in Fig. 16.4.
The post-ruminal transit time and the
mean residence time in the lag-rumination
pool are relatively unaffected by dietary

level of indigestible NDF and are relatively

constant at approximately 10 h each in

cattle (Fig. 16.4). In contrast, the mean

residence time in the mass action turnover

pool appears to be related positively and

linearly to dietary concentration of

indigestible fibre.

Models involving assumptions of age-

dependent distribution of residence time,

yield estimates of ruminal residence time

and digesta load that are longer and larger,

respectively, than that modelled by a single

mixing pool such as the models of Orskov

and McDonald (1979), Forbes (1980) and

the Cornell Net Protein Net Carbohydrate

System (Russell et al., 1992). The additional

age-dependent residence time due to the

lag-rumination pool affords an additional

mechanism for maximizing efficiency

of hydrolysing slowly but potentially

hydrolysable structural carbohydrates. The

need for this additional residence time for

reconciling observed versus model estimates

Feed Intake in Ruminants 343

Fig. 16.4.Relationships between mean transit time as estimated for first detectable appearance of marker in
faeces (
and –––), mean residence time in the lag-rumination pool (and – – –) and the mass action
turnover pool (and ---) estimated from marker profiles in the faeces of cattle consuming a variety of
forages.