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Prediction of dry matter intake based on rumen evacuation
Small Ruminant Research ELSEVIER
Small Ruminant Research 18 ( 1995) 133-136
Prediction of dry matter intake based on rumen evacuation G.D. Mendoza M.“,* , R. Ricalde V.b, T. Arroyo M.b ‘Centro de Ganaderia, Colegio de Postgraduados, Montecillo, Estado de Mexico, Mexico C.P. 56130, Mexico hUniver.yidadAuthnoma Metropolitana, Xochimilco, D.P.A.A. Calz. &I liueso No. 1100, Mexico, D.F. C.P. 04960, Mexico Accepted 20 January
1995
Abstract This experiment was conducted to estimate dry matter intake (DMI) from indigestible fiber ruminal contents. Eight sheep with ruminal cannulae were fed two levels of alfalfa (ad libitum and 80%) in a completely randomized design. Animals were adapted for 20 days followed by 5 days of total fecal output collection. At the end of the digestibility study, ruminal contents were removed, mixed, sampled, and analyzed for indigestible acid detergent fiber (I-ADF). Intake was predicted with a model which assumed that intake and passage of indigestible fraction digesta were in steady state. DMI values were 1.643 and 1.294 kg day-’ and predicted values with the model were 0.119 and 0.107 kg day-’ for the ad libitum and restricted treatment respectively. Regression analyses showed that the I-ADF ruminal pool can account for 14% of the observed intake (Y= 0.798 +6.002X, ? =0.14). Results of this study indicate that DMI can not be predicted from the I-ADF ruminal pool. Keywords: Intake; Rumin evacuation;
Modeling;
Digestibility
1. Introduction Accurate
estimation
of forage
consumed
by grazing
sheep is critical in order to know the nutritional status of the animal and to establish supplementation programs (Cochran et al., 1987). The most common procedures to estimate intake are the combination of in vitro digestibility with measured fecal output (Cordova et al., 1978) and the use of external markers (Galyean et al., 1987; Pond et al., 1987). Another approach to estimate intake is with regression equation analyses (NRC, 1987) or simulation models (Mertens, 1987; Williams et al., 1989). Dry matter (DM) and fiber kinetics can be determined in ruminally cannulated sheep when intake and rumen contents are known (Poppi et al., 1980,198 1a,b; Froetschel and Amos, 1990). A method to estimate * Corresponding
author.
0921-4488/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved .S.S010921-4488(95)00716-4
intake without use of external markers and fecal collection would be desirable for grazing studies. This study was conducted to study the relationship between indigestible acid detergent fiber (I-ADF) in the rumen and DM intake (DMI) .
2. Materials
and methods
Eight sheep with ruminal cannulae were individually fed with alfalfa hay (Me&ago satiua) at two levels: ad libitum and 80% of ad libitum. A 20 day adaptation period was allowed followed by 5 days of total fecal output collection. Feed was offered in two meals daily. On the last day of the trial, ruminal contents were evacuated, mixed, and sampled (Goetz et al., 1990). After sampling, the remainder of the rumen contents was returned immediately to the rumen. Samples of both
134
G.D. Mendozn et al. /Small Ruminant Research I8 (1995) 133-136
rumen digesta and feces were stored at - 5°C until fiber analyses were carried out (Goering and Van Soest, 1970). I-ADF concentration was obtained from dry samples ( 10 g) incubated in polyester bags ( 11 cm X 5 cm) for 100 h in the rumen of a Holstein steer fed with alfalfa. Retention time (T,.) in the rumen and rate of passage of the indigestible fraction (K,) were calculated as described by Poppi et al. (1981a) as follows:
DMD was not statistically different; however, there was a tendency to be higher in animals fed ad libitum (P=O.17). In vivo DMD was correlated (R= -0.71, P < 0.04) with DM mean retention time and with rate of passage (R=0.76, PcO.04). From the model described by Allen and Mertens (1988) a theoretical increment of 4 units in DMD is expected in the restricted animals, assuming the same rate of digestion ( Kd)and rate of passage (K,)with the relationship kdl
T, = Total fraction
(Kd + K,).
in rumen/Fraction
eaten h- ’ = l/K,
Fiber in forage was divided into two fractions (Allen and Mertens, 1988): digestible (Fd) and indigestible (Fi) where: Fd+Fi=
1
Intake of indigestible fiber was predicted assuming that intake and passage of indigestible fraction digesta were in steady state with the following model: Fi=P(dCldt)
Mean retention time of I-ADF was negatively correlated with DMI (R= -0.72, P
+PC
where P is rumen pool size (kg), C is concentration of the indigestible marker in the rumen (kg kg-‘), Fiis indigestible fraction intake (kg day-‘). If the concentration of indigestible fraction in the rumen remains constant, then dCl dt = 0, and Fican be estimated:
Fi=PC DMI was estimated by the concentration of the indigestible fiber fraction in the forage. Rate of passage was not included in the model because it was determined from intake previously. Mean values from response variables were compared with a T-test (Steel and Torrie, 1980). Regression analysis was used to study the relationship between DMI and I-ADF ruminal pool (Draper and Smith, 198 1) .
3. Results and discussion Results for intake, ruminal contents and DM digestibility (DMD) are shown in Table 1. Voluntary intake of alfalfa by sheep fed ad libitum was higher (P < 0.05) than in the restricted group. However, differences were not found (P> 0.05)in DM and I-ADF intake and ruminal contents in the rumen.
Table 1 Effect of alfalfa restriction characteristics Item
(80% of ad libitum) on ruminal digesta
Ad libitum
Restricted
SE
DMI (kg day-‘) Indigestible ADF (kg day-‘) Ruminal contents
1.6” 0.11”
1.3b O.OP
0.07 0.004
DM (kg) I-ADF (kg) I-ADF Rate of passage (h-l) Mean retention time (h) In vivo DMD (%)
0.58 0.12
0.50 0.11
0.03 0.007
0.04 26.1 70.1
0.03 30.8 72.1
0.002 2.0 1.7
SE, standard error (II = 4). a.bMeans in a row with different superscripts
differ (P < 0.05).
Table 2 Dry matter intake (DMI) observed and predicted by the models Item
Ad libitum
Restricted
Observed DMI (kg day-‘) DMI predicted I-ADF pool” (kg) Regressionb (kg)
1.6
1.3
0.2 1.5
0.11 1.5
aBased on rumen pool size. %sing I-ADF ruminal pool as independent 6.002X, 2 = 0.14.
variable Y = 0.798 +
G.D. Mendoza et al. /Small Ruminant Research 18 (1995) 133-136
pool model underestimated intake, giving results without biological meaning. One of the reasons of the failure of the model was that intake and passage of I-ADF were not in steady state. The term steady state is applied to a marker when it reaches the equilibrium in a closed system with continuous infusion (Shipley and Clark, 1972). If the condition of the indigestible marker is in non-steady state the amount of material in the pool increases or decreases because rates of input and output are unequal, resulting in an under- or overestimation of intake. The correlation between I-ADF and DMI was low ( r = 0.37) and the coefficient of determination did not justify its use to predict DMI. Estimation of intake in sheep from cell wall concentration in forage has been proposed by the NRC ( 1987) ; however, this equation can predict only one value per forage and does not take into account the differences in physical process or forage availability. Use of a continuous dose of a single marker or a controlled release chromic oxide capsule to estimate intake under grazing conditions has been used to estimate fecal output and intake (Parker et al., 1989); however, those procedures require several days to achieve steady state conditions and, if the marker is dosed every day, a practical field method to administer the marker is needed. Cost and losses of regurgitated capsules are a concern when chromium capsules are used. In both cases, fecal samples must be collected over a period of several days; therefore, a faster method to estimate DMI would be desirable. The model described in this experiment can be modified to provide a better method for intake predictions including an estimation of rate of passage with an external marker. In order to estimate intake under grazing conditions, digestibility needs to be estimated with the ratio technique, with an internal marker. Different internal markers such as chromogens, lignin, acid insoluble ash, indigestible neutral detergent fibre or I-ADF, and silica, have been used to estimate digestibility (Kotb and Luckey, 1972; Nelson et al., 1990; Sunvold and Cochran, 1991). If an external marker such as chromic oxide is used in a single dose, Kp and P can be determined in a short period of time. P can be estimated by rumen evacuation or from the intercept and Kp from the slope of the relationship between time and natural logarithm concentrations of marker after dosing. Indigestible fiber
135
intake then could be estimated with the equation Fi=K$ (Poppi et al., 1981a,b; Czerkawski, 1986).
4. Conclusion Results of this experiment indicate that DMI can not be predicted from the I-ADF ruminal pool using the model based on ruminal content evacuation.
Acknowledgements We wish to thank Andres Lee-Hernandez for his help with the analyses at the Nutrition Laboratory of the Centro de Ganaderia.
References Allen, M.S. and M&ens, D.R., 1988. Evaluatingconstraints on fiber digestion by rumen microbes. J. Nutr.. 2: 216-270. Cochran, R.C., Vanzant,E.S., Jacques, K.A.,Galyean, M.L., Adams, D.C. and Wallace, J.D., 1987. Internal markers. Proceedings, Grazing Livestock Nutrition Conference, 23-24 July 1987, Jackson, Wyoming, pp. 3948. Cordova, F.J., Wallace, D. and Pieper, R.D., 1978. Forage intake by grazing livestock: a review. J. Range Manage., 31: 430-435. Czerkawski, J.W., 1986. An introduction to Rumen Studies. Pergamon Press, New York, 236 pp. Draper, N. and Smith, H., 1981. Applied Regression Analysis, 2nd edn. Wiley, New York, 709 pp. Froetschel, M.A. and Amos, H.E., 1990. Effects of dietary fiber and feeding frequency on ruminal fermentation, hydration capacity and fractional turnover of contents using ruminal evacuation. J. Anim. Sci., 68 (Suppl. 1): 51. Galyean, M.L., Krysl, L.J. and Estell, R.E., 1987. Marker-based approaches for estimation of fecal output and digestibility in ruminants. In: Symposium Proceedings: Feed Intake by Beef Cattle, MP 121, Agricultural Experiment Station, Oklahoma State University, pp. 96-l 13. C&ring, H.K. and van Soest, P.J., 1970. Forage fiber analysis (apparatus, reagents, procedures and some applications). USDA Agriculture Handbook No. 379, Washington, DC, 90 pp. Goetz, M.J., Wagner, J.J. and Johnson, P.A., 1990. Effects of rumen evacuation on intake and ruminal recovery of dry matter. J. Anim. Sci., 68 (Suppl.1): 563. Kotb, A.R. and Luckey, T.D., 1972. Markers in nutrition. Nutr. Abstr. Rev., 42: 813-845. Mertens, D.R., 1987. Predicting intake and digestibility using mathematical models of ruminal function. J. Anim. Sci., 64: 15481558.
136
G.D. Mendoza et al. /Small RuminantResearch I8 (1995) 133-136
Nelson. M.L., Motlope, L., Finley, J.W. and Parish, SM., 1990. Ashfree indigestible acid detergent fiber as an internal marker to estimate digestibility with grazing ruminants. J. Range Manage., 43: 224-233. NRC, 1987. Subcommittee on Feed Intake, Committee on Animal Nutrition, Board of Agriculture. Predicting Feed Intake of Foodproducing Animals. National Academy Press, Washington, DC, pp. 75-82. Parker, W.J., McCutcheon, S.N. and Carr, D.H., 1989. Effect of herbage type and level of intake on the release of chromic oxide from intraruminal controlled release capsule in sheep. N. 2. J. Agric. Res., 32: 537-546. Pond, K.R., Bums, J.C. and Fisher, D.S., 1987. External markers use and methodology in grazing studies. Proceedings, Grazing Livestock Nutrition Conference, 23-24 July 1987, Jackson, Wyoming, pp. 3948. Poppi, D.P., Minson, D.F. and Temouth, J.H., 1980. Studies of cattle and sheep eating leaf and stem fractions of grasses. I. The voluntary intake, digestibility and retention time in the reticulorumen. Aust. J. Agric. Res., 32: 99-108. Poppi, D.P., Minson, D.F. and Temouth, J.H., 198la. Studies of cattle and sheep eating leaf and stem fractions of grasses. II.
Factors controlling the retention of feed in the reticulaAust. J. Agric. Res., 32: 109-121.
rumen.
Poppi, D.P., Minson, D.F. and Temouth, J.H., 198lb. Studies of cattle and sheep eating leaf and stem fractions of grasses. III. The retention time in the rumen of large feed particles. Aust. J. Agric. Res., 32: 123-137. Shipley, R.A. and Clark, R.E., 1972. Tracer Methods for In Vivo Kinetics. Theory and Applications. Academic Press, New York, 239 pp. Steel, R.G. and Tonie, J.H., 1980. Principles and Procedures of Statistics: A Biometrical Approach, 2nd edn. McGraw-Hill, New York, 633 pp. Sunvold, G.D. and Cochran, R.C., 1991. Technical note: evaluation of acid detergent lignin, alkaline peroxide lignin, acid insoluble ash, and indigestible acid detergent fiber as internal markers for prediction of alfalfa, bromegrass, and paririe hay digestibility by beef steers. J. Anim. Sci., 69: 49514955. Williams, C.B., Oltenacu, P.A. and Sniffen, C.J., 1989. Application of neutral detergent fiber in modeling feed intake, lactation response, and body weight changes in dairy cattle. J. Dairy Sci., 72: 652-663.
Small Ruminant Research 18 ( 1995) 133-136
Prediction of dry matter intake based on rumen evacuation G.D. Mendoza M.“,* , R. Ricalde V.b, T. Arroyo M.b ‘Centro de Ganaderia, Colegio de Postgraduados, Montecillo, Estado de Mexico, Mexico C.P. 56130, Mexico hUniver.yidadAuthnoma Metropolitana, Xochimilco, D.P.A.A. Calz. &I liueso No. 1100, Mexico, D.F. C.P. 04960, Mexico Accepted 20 January
1995
Abstract This experiment was conducted to estimate dry matter intake (DMI) from indigestible fiber ruminal contents. Eight sheep with ruminal cannulae were fed two levels of alfalfa (ad libitum and 80%) in a completely randomized design. Animals were adapted for 20 days followed by 5 days of total fecal output collection. At the end of the digestibility study, ruminal contents were removed, mixed, sampled, and analyzed for indigestible acid detergent fiber (I-ADF). Intake was predicted with a model which assumed that intake and passage of indigestible fraction digesta were in steady state. DMI values were 1.643 and 1.294 kg day-’ and predicted values with the model were 0.119 and 0.107 kg day-’ for the ad libitum and restricted treatment respectively. Regression analyses showed that the I-ADF ruminal pool can account for 14% of the observed intake (Y= 0.798 +6.002X, ? =0.14). Results of this study indicate that DMI can not be predicted from the I-ADF ruminal pool. Keywords: Intake; Rumin evacuation;
Modeling;
Digestibility
1. Introduction Accurate
estimation
of forage
consumed
by grazing
sheep is critical in order to know the nutritional status of the animal and to establish supplementation programs (Cochran et al., 1987). The most common procedures to estimate intake are the combination of in vitro digestibility with measured fecal output (Cordova et al., 1978) and the use of external markers (Galyean et al., 1987; Pond et al., 1987). Another approach to estimate intake is with regression equation analyses (NRC, 1987) or simulation models (Mertens, 1987; Williams et al., 1989). Dry matter (DM) and fiber kinetics can be determined in ruminally cannulated sheep when intake and rumen contents are known (Poppi et al., 1980,198 1a,b; Froetschel and Amos, 1990). A method to estimate * Corresponding
author.
0921-4488/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved .S.S010921-4488(95)00716-4
intake without use of external markers and fecal collection would be desirable for grazing studies. This study was conducted to study the relationship between indigestible acid detergent fiber (I-ADF) in the rumen and DM intake (DMI) .
2. Materials
and methods
Eight sheep with ruminal cannulae were individually fed with alfalfa hay (Me&ago satiua) at two levels: ad libitum and 80% of ad libitum. A 20 day adaptation period was allowed followed by 5 days of total fecal output collection. Feed was offered in two meals daily. On the last day of the trial, ruminal contents were evacuated, mixed, and sampled (Goetz et al., 1990). After sampling, the remainder of the rumen contents was returned immediately to the rumen. Samples of both
134
G.D. Mendozn et al. /Small Ruminant Research I8 (1995) 133-136
rumen digesta and feces were stored at - 5°C until fiber analyses were carried out (Goering and Van Soest, 1970). I-ADF concentration was obtained from dry samples ( 10 g) incubated in polyester bags ( 11 cm X 5 cm) for 100 h in the rumen of a Holstein steer fed with alfalfa. Retention time (T,.) in the rumen and rate of passage of the indigestible fraction (K,) were calculated as described by Poppi et al. (1981a) as follows:
DMD was not statistically different; however, there was a tendency to be higher in animals fed ad libitum (P=O.17). In vivo DMD was correlated (R= -0.71, P < 0.04) with DM mean retention time and with rate of passage (R=0.76, PcO.04). From the model described by Allen and Mertens (1988) a theoretical increment of 4 units in DMD is expected in the restricted animals, assuming the same rate of digestion ( Kd)and rate of passage (K,)with the relationship kdl
T, = Total fraction
(Kd + K,).
in rumen/Fraction
eaten h- ’ = l/K,
Fiber in forage was divided into two fractions (Allen and Mertens, 1988): digestible (Fd) and indigestible (Fi) where: Fd+Fi=
1
Intake of indigestible fiber was predicted assuming that intake and passage of indigestible fraction digesta were in steady state with the following model: Fi=P(dCldt)
Mean retention time of I-ADF was negatively correlated with DMI (R= -0.72, P
+PC
where P is rumen pool size (kg), C is concentration of the indigestible marker in the rumen (kg kg-‘), Fiis indigestible fraction intake (kg day-‘). If the concentration of indigestible fraction in the rumen remains constant, then dCl dt = 0, and Fican be estimated:
Fi=PC DMI was estimated by the concentration of the indigestible fiber fraction in the forage. Rate of passage was not included in the model because it was determined from intake previously. Mean values from response variables were compared with a T-test (Steel and Torrie, 1980). Regression analysis was used to study the relationship between DMI and I-ADF ruminal pool (Draper and Smith, 198 1) .
3. Results and discussion Results for intake, ruminal contents and DM digestibility (DMD) are shown in Table 1. Voluntary intake of alfalfa by sheep fed ad libitum was higher (P < 0.05) than in the restricted group. However, differences were not found (P> 0.05)in DM and I-ADF intake and ruminal contents in the rumen.
Table 1 Effect of alfalfa restriction characteristics Item
(80% of ad libitum) on ruminal digesta
Ad libitum
Restricted
SE
DMI (kg day-‘) Indigestible ADF (kg day-‘) Ruminal contents
1.6” 0.11”
1.3b O.OP
0.07 0.004
DM (kg) I-ADF (kg) I-ADF Rate of passage (h-l) Mean retention time (h) In vivo DMD (%)
0.58 0.12
0.50 0.11
0.03 0.007
0.04 26.1 70.1
0.03 30.8 72.1
0.002 2.0 1.7
SE, standard error (II = 4). a.bMeans in a row with different superscripts
differ (P < 0.05).
Table 2 Dry matter intake (DMI) observed and predicted by the models Item
Ad libitum
Restricted
Observed DMI (kg day-‘) DMI predicted I-ADF pool” (kg) Regressionb (kg)
1.6
1.3
0.2 1.5
0.11 1.5
aBased on rumen pool size. %sing I-ADF ruminal pool as independent 6.002X, 2 = 0.14.
variable Y = 0.798 +
G.D. Mendoza et al. /Small Ruminant Research 18 (1995) 133-136
pool model underestimated intake, giving results without biological meaning. One of the reasons of the failure of the model was that intake and passage of I-ADF were not in steady state. The term steady state is applied to a marker when it reaches the equilibrium in a closed system with continuous infusion (Shipley and Clark, 1972). If the condition of the indigestible marker is in non-steady state the amount of material in the pool increases or decreases because rates of input and output are unequal, resulting in an under- or overestimation of intake. The correlation between I-ADF and DMI was low ( r = 0.37) and the coefficient of determination did not justify its use to predict DMI. Estimation of intake in sheep from cell wall concentration in forage has been proposed by the NRC ( 1987) ; however, this equation can predict only one value per forage and does not take into account the differences in physical process or forage availability. Use of a continuous dose of a single marker or a controlled release chromic oxide capsule to estimate intake under grazing conditions has been used to estimate fecal output and intake (Parker et al., 1989); however, those procedures require several days to achieve steady state conditions and, if the marker is dosed every day, a practical field method to administer the marker is needed. Cost and losses of regurgitated capsules are a concern when chromium capsules are used. In both cases, fecal samples must be collected over a period of several days; therefore, a faster method to estimate DMI would be desirable. The model described in this experiment can be modified to provide a better method for intake predictions including an estimation of rate of passage with an external marker. In order to estimate intake under grazing conditions, digestibility needs to be estimated with the ratio technique, with an internal marker. Different internal markers such as chromogens, lignin, acid insoluble ash, indigestible neutral detergent fibre or I-ADF, and silica, have been used to estimate digestibility (Kotb and Luckey, 1972; Nelson et al., 1990; Sunvold and Cochran, 1991). If an external marker such as chromic oxide is used in a single dose, Kp and P can be determined in a short period of time. P can be estimated by rumen evacuation or from the intercept and Kp from the slope of the relationship between time and natural logarithm concentrations of marker after dosing. Indigestible fiber
135
intake then could be estimated with the equation Fi=K$ (Poppi et al., 1981a,b; Czerkawski, 1986).
4. Conclusion Results of this experiment indicate that DMI can not be predicted from the I-ADF ruminal pool using the model based on ruminal content evacuation.
Acknowledgements We wish to thank Andres Lee-Hernandez for his help with the analyses at the Nutrition Laboratory of the Centro de Ganaderia.
References Allen, M.S. and M&ens, D.R., 1988. Evaluatingconstraints on fiber digestion by rumen microbes. J. Nutr.. 2: 216-270. Cochran, R.C., Vanzant,E.S., Jacques, K.A.,Galyean, M.L., Adams, D.C. and Wallace, J.D., 1987. Internal markers. Proceedings, Grazing Livestock Nutrition Conference, 23-24 July 1987, Jackson, Wyoming, pp. 3948. Cordova, F.J., Wallace, D. and Pieper, R.D., 1978. Forage intake by grazing livestock: a review. J. Range Manage., 31: 430-435. Czerkawski, J.W., 1986. An introduction to Rumen Studies. Pergamon Press, New York, 236 pp. Draper, N. and Smith, H., 1981. Applied Regression Analysis, 2nd edn. Wiley, New York, 709 pp. Froetschel, M.A. and Amos, H.E., 1990. Effects of dietary fiber and feeding frequency on ruminal fermentation, hydration capacity and fractional turnover of contents using ruminal evacuation. J. Anim. Sci., 68 (Suppl. 1): 51. Galyean, M.L., Krysl, L.J. and Estell, R.E., 1987. Marker-based approaches for estimation of fecal output and digestibility in ruminants. In: Symposium Proceedings: Feed Intake by Beef Cattle, MP 121, Agricultural Experiment Station, Oklahoma State University, pp. 96-l 13. C&ring, H.K. and van Soest, P.J., 1970. Forage fiber analysis (apparatus, reagents, procedures and some applications). USDA Agriculture Handbook No. 379, Washington, DC, 90 pp. Goetz, M.J., Wagner, J.J. and Johnson, P.A., 1990. Effects of rumen evacuation on intake and ruminal recovery of dry matter. J. Anim. Sci., 68 (Suppl.1): 563. Kotb, A.R. and Luckey, T.D., 1972. Markers in nutrition. Nutr. Abstr. Rev., 42: 813-845. Mertens, D.R., 1987. Predicting intake and digestibility using mathematical models of ruminal function. J. Anim. Sci., 64: 15481558.
136
G.D. Mendoza et al. /Small RuminantResearch I8 (1995) 133-136
Nelson. M.L., Motlope, L., Finley, J.W. and Parish, SM., 1990. Ashfree indigestible acid detergent fiber as an internal marker to estimate digestibility with grazing ruminants. J. Range Manage., 43: 224-233. NRC, 1987. Subcommittee on Feed Intake, Committee on Animal Nutrition, Board of Agriculture. Predicting Feed Intake of Foodproducing Animals. National Academy Press, Washington, DC, pp. 75-82. Parker, W.J., McCutcheon, S.N. and Carr, D.H., 1989. Effect of herbage type and level of intake on the release of chromic oxide from intraruminal controlled release capsule in sheep. N. 2. J. Agric. Res., 32: 537-546. Pond, K.R., Bums, J.C. and Fisher, D.S., 1987. External markers use and methodology in grazing studies. Proceedings, Grazing Livestock Nutrition Conference, 23-24 July 1987, Jackson, Wyoming, pp. 3948. Poppi, D.P., Minson, D.F. and Temouth, J.H., 1980. Studies of cattle and sheep eating leaf and stem fractions of grasses. I. The voluntary intake, digestibility and retention time in the reticulorumen. Aust. J. Agric. Res., 32: 99-108. Poppi, D.P., Minson, D.F. and Temouth, J.H., 198la. Studies of cattle and sheep eating leaf and stem fractions of grasses. II.
Factors controlling the retention of feed in the reticulaAust. J. Agric. Res., 32: 109-121.
rumen.
Poppi, D.P., Minson, D.F. and Temouth, J.H., 198lb. Studies of cattle and sheep eating leaf and stem fractions of grasses. III. The retention time in the rumen of large feed particles. Aust. J. Agric. Res., 32: 123-137. Shipley, R.A. and Clark, R.E., 1972. Tracer Methods for In Vivo Kinetics. Theory and Applications. Academic Press, New York, 239 pp. Steel, R.G. and Tonie, J.H., 1980. Principles and Procedures of Statistics: A Biometrical Approach, 2nd edn. McGraw-Hill, New York, 633 pp. Sunvold, G.D. and Cochran, R.C., 1991. Technical note: evaluation of acid detergent lignin, alkaline peroxide lignin, acid insoluble ash, and indigestible acid detergent fiber as internal markers for prediction of alfalfa, bromegrass, and paririe hay digestibility by beef steers. J. Anim. Sci., 69: 49514955. Williams, C.B., Oltenacu, P.A. and Sniffen, C.J., 1989. Application of neutral detergent fiber in modeling feed intake, lactation response, and body weight changes in dairy cattle. J. Dairy Sci., 72: 652-663.