In sacco degradation characteristics as predictors of digestibility and voluntary intake of roughages by mature ewes

Animal Feed Science and Technology 72 Ž1998. 205–219

In sacco degradation characteristics as predictors of digestibility and voluntary intake of roughages by mature ewes A.J.M. Fonseca a , A.A. Dias-da-Silva a

a,)

, E.R. Ørskov

b

UniÕersidade de Tras-os-Montes e Alto Douro, Department of Animal Production, Apartado 202, 5001 Vila ´ Real Codex, Portugal b The Rowett Research Institute, International Feed Resources Centre, Bucksburn, Aberdeen, AB2 9SB, Scotland, UK Received 2 April 1997; accepted 10 December 1997

Abstract The in situ nylon bag technique was evaluated as a method of predicting digestibility and voluntary intake of roughages. Twelve feedstuffs including five untreated cereal straws, two cereal straws treated with urea and five grass hays were each offered to six–eight mature non-pregnant and non-lactating ewes Ž70.4 " 8.4 kg., ad libitum, over a period of eight weeks. Soyabean meal was given as supplement in variable amounts to cover rumen microbes requirements for degradable nitrogen. Two rumen fistulated rams were used to measure the digestibility of each diet and the kinetics of dry matter ŽDM. degradation in the rumen of the roughages. Apparent organic matter digestibility ŽOMD. of the roughages ranged from 0.441 to 0.600 g kgy1 and DM intake from 9.6 to 17.9 g kgy1 of live weight. DM degradation after 96 or 72 h incubation were the best predictors of roughages OMD Ž r 2 s 0.73 and 0.70, respectively. while prediction of DM intake was most accurate from 3 h DM degradation and ADF content of roughages Ž r 2 s 0.68 and 0.67, respectively.. In contrast, digestible DM intake was best predicted by a multiple regression equation including the degradation constants soluble fraction and rate of degradation accounting for 89% of the variation observed. Live weight variation of ewes was also well predicted from a similar equation or from soluble fraction alone Ž r 2 s 0.80. but the predictive capability of ADF was higher Ž r 2 s 0.86. for this purpose. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Roughages; Rumen degradation; Intake; Digestibility; Sheep

)

Corresponding author. Tel.: q351-59-320409; fax: q351-59-320629.

0377-8401r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 7 - 8 4 0 1 Ž 9 8 . 0 0 1 2 1 - 7

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1. Introduction Prediction of voluntary intake of roughages by ruminants has long been a research priority in animal feeding. The level of feed intake is dependent of feed characteristics, animal and environmental factors and their interactions. It is generally recognised that feed characteristics are dominant when low quality roughages are offered. Historically, intake and digestibility have been predicted by simple and multiple regression equations based on chemical composition. Although these equations may allow prediction with acceptable precision, other simple methods of feed evaluation have been proposed. One of these, the nylon bag technique in which samples of feed are directly incubated in the rumen has become widely used. This technique makes possible to study rumen digestion at different periods of time or the kinetics of rumen digestion. Among others Chenost et al. Ž1970. found a significant correlation Ž r s 0.82. between 12 h DM losses from the nylon bags and voluntary intake and data on 48 h incubation time has successfully been used for predicting digestibility ŽAerts et al., 1977; Wanapat et al., 1986.. More recently, researchers have been investigating relationships between voluntary intake and in sacco degradation characteristics ŽHovell et al., 1986; Ørskov et al., 1988; Von Keyserlingk and Mathison, 1989; Carro et al., 1991; Nandra et al., 1993; Khazaal et al., 1993, 1995; Shem et al., 1995.. The information available can be considered limited and the results somewhat inconsistent. Therefore, a study involving twelve different roughages offered to mature non-pregnant and non-lactating female sheep was carried out to evaluate the predictive capabilities of in sacco degradation kinetics compared to chemical composition. 2. Materials and methods 2.1. Animals and housing Ninety Ile de France non-pregnant and non-lactating ewes were used to measure the voluntary intake of the twelve roughages described below. The average initial weight of the animals was 70.4 " 8.4 kg and they were 2 to 4 yr old. The ewes were kept indoors in individual pens under 18 h lighting. The experimental barn was well ventilated. Six to eight animals were assigned to each feed. Only two roughages were evaluated at any one time because of limitation in number of individual pens. Differences between roughages can thus be confounded by the effects of time and animal age. Mature Ile de France rams Ž72.0 " 8.1 kg. fitted with permanent rumen cannulae Ž40 mm internal diameter. and kept in metabolism crates, were used to measure the digestibility and the rumen degradability of the roughages. Prior to the studies all animals were sheared, dewormed and injected subcutaneously with 500 000 i.u. of vitamin A, 75 000 i.u. of vitamin D and 50 i.u. of vitamin E. 2.2. Roughages Twelve roughages obtained from various locations in the north and center of Portugal were used for this experiment Žsee Table 1.. They comprised unknown cultivars of one

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rice straw ŽRIS., two rye straws ŽRS1 and RS2., RS2 treated with urea ŽTRS2., two wheat straws ŽWS1 and WS2., WS2 treated with urea ŽTWS2., two meadow hays ŽMH1 and MH2., two oat hays ŽOAH1 and OAH2. and one Italian Ryegrass hay ŽRH.. Treatment with urea was performed in a stack as described by Dias-da-Silva and Sundstøl Ž1986.. The amount of urea applied was 50 g kgy1 straw. After 60 days the stack was uncovered, the bales were air-dried and stored under cover. At the beginning of the study the average DM content of TRS2 and TWS2 were 82.0% and 72.0% respectively. All the roughages were shredded through a bale grinder equipped with a 40 mm screen prior to feeding. 2.3. Voluntary intake of roughages Voluntary intake was measured over a period of eight weeks after an initial period of two weeks for adaptation of the animals to the diet. All the roughages were supplemented with soyabean meal ŽSBM. in amounts estimated to cover rumen microbes requirements for degradable nitrogen ŽAFRC, 1993.. The level of SBM was adjusted weekly according to total DM intake of each ewe observed in the previous week. The actual intake of SBM ranged from 8.0% to 25.0% of total DM intake Žsee Table 2. which ensured a level of crude protein in diets averaging 13.6% Žrange 11.9 to 16.2%.. All animals received daily 30 g of a mineral–vitamin mixture offered with SBM and had free access to water. The mineral–vitamin mixture provided per kg mixture 120 g Ca, 120 g P, 45 g Na, 25 g Mg, 2600 mg Zn, 3200 mg Mn, 320 mg Cu, 580 mg Fe, 20 mg I, 15 mg Co, 2.5 mg Se, 120 000 i.u. Vitamin A, 60 000 i.u. vitamin D 3 and 135 mg vitamin E as declared by the manufacturer. SBM was given once daily at 8:00 h and was eaten readily in totality. Roughages were offered ad libitum with fresh food being introduced in the morning after SBM and in the afternoon at 16:00 h. The troughs were cleaned out each morning and refusals collected and weighed. They were allowed to represent 15–20% of the roughage offered. Samples of roughages and SBM were taken twice each week for DM determination. Refusals were sampled once each week for the same purpose. The dried material was bulked for later chemical analysis. The ewes were weighed at the beginning of each experimental period and then every two weeks. 2.4. Digestibility in ÕiÕo and degradability in sacco Two rumen fistulated rams were fed the diets described above at levels close the maintenance feeding level ŽAFRC, 1993. to measure rumen degradability of the roughages and the digestibility of the whole ration. Each experimental period lasted for 32 days, comprising 10 days for adaptation to the diet, 15 days for incubations in the rumen and 7 days for faeces collection. The nylon bag technique ŽØrskov et al., 1980. was used to measure the kinetics of DM degradation of the roughages in the rumen. Samples of roughages were ground through a hammermill equipped with a 4 mm screen. Nylon bags containing 2 g of the

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test roughages were incubated in the rumen of each sheep for 0, 3, 9, 18, 24, 36, 48, 60, 72, 96, 120, 144 h. Each incubation was repeated once per sheep. In total, there were 4 replicates for each roughage sample Ž2 sheep = 2 times = 1 bag.. The nylon bags measured 6 = 8.5 cm and had 2515 pores per cm2 . Immediately after removal from the rumen, the bags were washed in cold water and frozen at y158C. At the end of each series of bag collections, they were unfrozen and washed together with the zero time bags Žnot incubated in the rumen. in a washing machine for 40 min at 408C and then dried at 658C for 24 h. Disappearance of DM was measured as the loss in weight of the bag contents. Faeces were collected in totality during the last 7 days. Samples of the roughages and the SBM supplement offered, refusals when present and faeces were taken daily and bulked over the period. As it was observed that SBM was eaten in totality, it was assumed that the refusals only consisted of roughage. Apparent organic matter digestibility ŽOMD. of the roughages was calculated by difference, assuming that OMD of SBM was 0.880 ŽINRA, 1988.. 2.5. Chemical analysis Samples of feedstuffs, refusals and faeces were dried in a forced-air drying oven at 608C for 24 h and the DM content calculated. Ground samples Ž1 mm. were analysed for ash and kjeldahl nitrogen according to the Association of Official Analytical Chemists Ž1990.. Crude protein ŽCP. was calculated as Kjeldahl nitrogen= 6.25. Neutral-detergent fibre ŽNDF., acid-detergent fibre ŽADF. and acid-detergent lignin ŽADL. were determined as described by Robertson and Van Soest Ž1981.. Starch was determined in oat hay 1 by polarimetry ŽAnonymous, 1987.. After wet digestion with perchloric and nitric acids, phosphorus was determined by the colorimetric method of Murphy-Riley and calcium by atomic absorption spectrophotometry ŽJones et al., 1991.. 2.6. Statistical analysis The disappearance of DM from nylon bags was fitted individually by ram to the exponential equation of Ørskov and McDonald Ž1979., modified by McDonald Ž1981. with a simultaneous estimation of lag time as proposed by Dhanoa Ž1988. by Systat nonlinear analysis ŽWilkinson et al., 1992.: P s a for t F L P s a q b Ž 1 y eyc Ž tyL. . for t ) L where P s the DM degradation after time t, a s the immediately soluble fraction, b s the insoluble but potentially degradable fraction, Ž a q b . s the potential degradation, c s the rate constant for the degradation of fraction b and L s the lag time. Data were subjected to standard analysis of variance ŽSteel and Torrie, 1980., using the statistical package Systat ŽWilkinson et al., 1992.. When significant differences occurred, the Tukey’s test was used to compare means. The relationships between different characteristics of roughages and intake or digestibility were obtained by simple linear and multiple regression analysis ŽSteel and Torrie, 1980..

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3. Results 3.1. Chemical composition All feeds were well preserved. Their chemical composition is presented in Table 1. Cell walls ŽNDF., which represent the most important fraction of DM for all roughages, ranged between 638 and 832 g kgy1 DM. Treated straws, as expected, showed higher nitrogen and lower NDF levels than the corresponding untreated straws. The OAH1 outstands from the other roughages since starch represented 172 g kgy1 DM due to the presence of some grain. 3.2. Digestibility and Õoluntary intake Despite the coarse grinding of the roughages, some selective intake did occur since the NDF content of the refusals were always higher and CP always lower than those of the corresponding roughages offered. This effect was particularly noticeable with OAH1: q13.8 and y3.2 percentage units for NDF and CP, respectively. The range of variation for all other roughages was 0.1–3.8 and 0.03–1.0 percentage units for NDF and CP, respectively. Total feed intake, roughage intake, digestible DM intake of whole diets ŽDDMI., OMD of roughages and daily live weight variation of the ewes Ž D LW. are shown in Table 2. There were significant differences Ž P - 0.05. in mean daily DMI, DDMI, D LW and OMD between diets. DM intake of the whole diets ranged from 12.0 to 20.3 g kgy1 LW while the range of values for the roughages was 9.6 and 17.9 g kgy1 LW. Daily DDMI ranged from 6.2 to 12.3 g kgy1 LW. The range of roughages OMD varied from 0.441 to 0.600 g kgy1 . Treatment of straws with urea significantly increased OMD Ž P - 0.05. but had no significant effect on DMI, the values for OMD being comparable or higher than those observed for

Table 1 Chemical composition of the experimental feeds Žg kgy1 DM. Feed

Ash

CP

NDF a

ADF

ADL

P

Ca

Rice straw Rye straw 1 Rye straw 2 Urea-treated rye straw 2 Wheat straw 1 Wheat straw 2 Urea-treated wheat straw 2 Meadow hay 1 Meadow hay 2 Oat hay 1 Oat hay 2 Italian-ryegrass hay Soyabean meal

145 39 34 34 50 54 51 50 46 42 44 77 69

39 27 22 104 42 28 115 89 56 67 52 105 490

675 815 832 799 791 817 791 748 730 638 662 664 169

493 530 501 502 451 491 510 431 427 362 399 376 ND

40 71 60 57 54 60 66 50 52 42 47 52 ND

1.1 0.5 0.4 0.3 0.4 0.6 0.6 1.3 1.1 1.3 1.5 1.7 ND

3.5 1.9 1.9 1.8 2.4 1.5 1.7 2.9 3.5 2.1 2.1 5.6 ND

a

NDF Žash free.. NDs not determined.

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Table 2 Voluntary dry matter intake ŽDMI, g DM kgy1 LW dayy1 . of the whole diets and roughages, digestible DMI of the whole diet ŽDDMI, g kgy1 LW dayy1 ., daily live weight variation of the ewes Ž D LW, g dayy1 . and organic matter digestibility ŽOMD, g kgy1 OM. of the roughages Roughage

DMI Whole diet

Rice straw Rye straw 1 Rye straw 2 Urea-treated rye straw 2 Wheat straw 1 Wheat straw 2 Urea-treated wheat straw 2 Meadow hay 1 Meadow hay 2 Oat hay 1 Oat hay 2 Italian-ryegrass hay SEM Ž ns8 or 2.

ab

15.2 15.5ab 15.8 ab 15.7 ab 14.5ab 12.2 a 12.0 a 18.7 bc 18.7 bc 20.3 c 19.1bc 17.6 bc 0.873

DDMI

D LW

OMD†

8.7 abc 8.2 abc 8.4 abc 8.7 abc 7.8 ab 6.5a 6.9 a 9.7 bc 10.3 cd 12.2 d 12.3 d 10.3 cd 0.480

5.8 abcd y66.1a y24.2 ab y23.8 ab y22.9 abc y30.6 ab y52.4 ab 27.4 bcd 25.5 bcd 55.9 cd 68.6 d 65.0 d 16.5

0.565ef 0.441a 0.464 ab 0.540 de 0.466 ab 0.477 ab 0.572 ef 0.496 bc 0.522 cd 0.590 f 0.600 f 0.563 ef 0.007

Roughages 12.3 abc 11.7 abc 12.7 abcd 14.4 bcde 11.6 abc 9.6 a 10.8 ab 15.5 cde 15.5 cde 17.9 e 15.2 cde 16.2 de 0.778

†

Calculated values—see text. Means in the same columns with different superscripts are significantly different Ž P - 0.05.. SEM, standard error of mean. a,b

meadow hays. Performance of the ewes fed urea-treated straws was not improved over those fed the corresponding untreated straws. 3.3. Degradation of roughages DM losses from the nylon bags incubated in the rumen are graphically represented in Fig. 1a and b. Total washing losses Žzero time bags. represented 9.5 to 29.4% of DM ŽTable 3.. The 144 h incubation time allowed the plateau of degradation to be achieved for all roughages. The visual curve analysis showed the existence of a lag phase. Therefore, the lag time Ž L. was included in the model describing degradation. The constants of the fitted exponential equation are presented in Table 3. The model fitted the data well, since the coefficient of determination Ž r 2 . was in all cases higher than 0.989 and the highest residual s.d. observed was 2.774. A large range of degradation characteristics was obtained: the a, b, c and L values ranged from 9.8 to 29.6%, 48.2 to 58.5%, 0.017 to 0.046 hy1 and 1.0 to 5.7 h, respectively. Significant differences among roughages Ž P - 0.05. in a, b, c values were observed, while no differences Ž P ) 0.05. in L values occurred. 3.4. Prediction of roughages digestibility and Õoluntary intake Organic matter digestibility in vivo was significantly correlated Ž P - 0.05. with all single in sacco degradation values and also with NDF, ADL and degradation constants, alone or in combination ŽTable 4.. However, the best predictors were DM degradation after 96 or 72 h incubation accounting for 73 or 70% of the variation observed, respectively, and giving the lowest residual s.d.

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Fig. 1. Ža. Observed values of dry matter degradability of roughages Žstraws.. Žb. Observed values of dry matter degradability of roughages Žhays..

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Table 3 Dry matter degradation characteristics of roughages in the rumen according to the exponential equation P s aq b Ž1-eyc Ž tyL. . Roughage

Washing losses Ž0 h. c

Rice straw Rye straw 1 Rye straw 2 Urea-treated rye straw 2 Wheat straw 1 Wheat straw 2 Urea-treated wheat straw 2 Meadow hay 1 Meadow hay 2 Oat hay 1 Oat hay 2 Italian-ryegrass hay SEM Ž ns 2.

a

b c

15.0 12.0 b 9.5a 11.1ab 10.7 ab 10.2 ab 14.9 c 19.8 d 19.2 d 22.6 e 29.4 f 24.5e 0.350

16.1 12.4 b 9.8 a 11.8 ab 10.7 ab 10.0 a 15.0 c 19.8 d 21.0 d 24.3 e 29.6 f 24.8 e 0.365

c abcd

52.8 50.7 ab 58.3 de 64.2 e 56.8 cd 54.9 bcd 58.5de 55.5 bcd 50.5ab 49.7 ab 50.9 abc 48.2 a 1.077

L de

0.037 0.020 ab 0.017 a 0.027 abcd 0.022 abc 0.028 abcd 0.034 cd 0.029 bcd 0.029 bcd 0.024 abc 0.025abcd 0.046 e 0.002

4.4 3.2 1.8 5.7 1.4 4.7 5.0 1.0 3.9 2.0 3.1 4.0 1.226

a,b

Means in the same columns with different superscripts are significantly different Ž P - 0.05.. SEM, standard error of mean.

Table 4 Prediction of organic matter digestibility Ž y . of the roughages Žkg per kg OM. from different attributes r2

Residual s.d.

Probability

Chemical components (g kg DM) y s1.01y0.00064 NDF y s 0.981y0.00061 NDFŽash free. y s 0.717y0.0035 ADL

0.61 0.62 0.36

0.036 0.035 0.046

- 0.01 - 0.01 - 0.05

Degradability (Deg) of DM with time (%) y s 0.424q0.0060Ž0 h Deg. y s 0.425q0.0054Ž3 h Deg. y s 0.413q0.0045Ž9 h Deg. y s 0.345q0.0053Ž18 h Deg. y s 0.304q0.0054Ž24 h Deg. y s 0.274q0.0051Ž36 h Deg. y s 0.249q0.0050Ž48 h Deg. y s 0.199q0.0056Ž60 h Deg. y s 0.113q0.0065Ž72 h Deg. y s 0.051q0.0071Ž96 h Deg. y s 0.002q0.0076Ž120 h Deg. y sy0.011q0.0077Ž144 h Deg.

0.52 0.48 0.44 0.57 0.66 0.62 0.62 0.66 0.70 0.73 0.69 0.68

0.040 0.041 0.043 0.038 0.034 0.035 0.035 0.033 0.031 0.030 0.032 0.032

- 0.01 - 0.05 - 0.05 - 0.01 - 0.01 - 0.01 - 0.01 - 0.01 - 0.001 - 0.001 - 0.001 - 0.001

Degradation constants of DM (%) y s 0.422q0.006 a y s 0.382q0.005aq1.94 c a y s 0.364q0.006 aq0.015L y sy0.071q0.008Ž aq b . y sy0.072q0.007Ž aq b .q2.44 c a

0.54 0.61 0.72 0.59 0.70

0.039 0.038 0.032 0.037 0.033

- 0.01 - 0.05 - 0.01 - 0.01 - 0.05

Equation y1

a

The inclusion of a second variable did not improve Ž P ) 0.05. the prediction.

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Table 5 Prediction of voluntary dry matter intake Ž y . of the roughages by ewes Žg DM kgy1 LW dayy1 . from different attributes Equation

r2

Residual s.d.

Probability

Chemical components (g kg y 1 DM) y s 36.1y0.030 NDF y s 32.8y0.026 NDFŽash free. y s 30.2y0.036 ADF

0.61 0.52 0.67

1.64 1.82 1.51

- 0.01 - 0.01 - 0.01

Degradability (Deg) of DM with time (%) y s8.82q0.288Ž0 h Deg. y s8.17q0.294Ž3 h Deg. y s 7.65q0.242Ž9 h Deg.

0.55 0.68 0.59

1.77 1.49 1.69

- 0.01 - 0.001 - 0.01

Degradation constants of DM (%) y s8.60q0.292 a y s9.65q0.315ay51.3c a y s9.82q0.283ay0.314 La y sy9.90q0.329Ž aq b . y sy8.56q0.334Ž aq b .y0.508 La y s9.72q0.301ay8.41bc a

0.60 0.63 0.64 0.43 0.53 0.62

1.66 1.70 1.67 1.98 1.90 1.71

- 0.01 - 0.05 - 0.05 - 0.05 - 0.05 - 0.05

a

The inclusion of a second variable did not improve Ž P ) 0.05. the prediction.

In Table 5, equations predicting DMI from chemical composition, degradability and degradation constants of DM are presented. DM degradation after 3 h incubation in sacco and ADF levels of the roughages were the best predictors of DMI accounting for

Table 6 Prediction of total digestible dry matter intake Ž y . by ewes Žg DM kgy1 LW dayy1 . from different attributes Equation

r2

Residual s.d.

Probability

Chemical components (g kg y 1 DM) y s 28.0y0.025 NDF y s 25.3y0.022 NDFŽash free. y s 21.6y0.027 ADF

0.77 0.67 0.68

0.93 1.13 1.10

- 0.001 - 0.01 - 0.001

Degradability (Deg) of DM with time (%) y s 5.05q0.249Ž0 h Deg. y s 4.67q0.243Ž3 h Deg. y s 4.37q0.195Ž9 h Deg.

0.74 0.84 0.69

0.99 0.77 1.08

- 0.001 - 0.001 - 0.001

Degradation constants of DM (%) y s 4.92q0.248 a y s6.58q0.284 ay81.8c y sy8.38q0.246Ž aq b . y s6.94q0.264 ay1.525bc

0.79 0.89 0.44 0.89

0.90 0.69 1.47 0.69

- 0.001 - 0.001 - 0.05 - 0.001

OM digestibility of roughages (kg kg y 1) y sy1.40q20.1 OMD

0.35

1.58

- 0.05

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Table 7 Prediction of live weight change Ž y . of the ewes Žg dayy1 . from different attributes r2

Residual s.d.

Probability

Chemical components (g kg DM) y s 480.4y0.628 NDF y s 430.1y0.573 NDFŽash free. y s 345.8y0.753 ADF

0.83 0.78 0.86

20.0 22.7 18.0

- 0.001 - 0.001 - 0.001

Degradability (Deg) of DM with time (%) y sy101.7q6.28Ž0 h Deg. y sy106.5q5.90Ž3 h Deg. y sy124.0q5.14Ž9 h Deg. y sy172.4q5.13Ž18 h Deg.

0.79 0.82 0.80 0.76

22.3 20.4 21.7 23.6

- 0.001 - 0.001 - 0.001 - 0.001

Degradation constants of DM (%) y sy102.7q6.14 a y sy108.0q6.02 aq264.6 c a y sy427.9q6.03Ž aq b . y sy428.5q5.51Ž aq b .q1351.4c a y sy106.4q6.11ay2.84 bc a

0.80 0.80 0.44 0.49 0.80

21.5 22.6 36.1 36.3 22.6

- 0.001 - 0.001 - 0.05 - 0.05 - 0.001

OM digestibility of roughages (kg kg y 1) y sy280.4q539.3 OMD

0.41

36.9

- 0.05

Equation y1

a

The inclusion of a second variable did not improve Ž P ) 0.05. the prediction.

68% and 67% of the variation observed, respectively. Degradation constants, alone or in combination, were poor predictors. Using single constants b, c and L as predictors no correlation was found Ž P ) 0.05.. In contrast, a multiple regression equation including a and c constants accounted for 89% of the variation observed in DDMI ŽTable 6., the precision being slightly better than from 3 h DM loss Ž r 2 s 0.84.. Similar results were obtained in predicting LW change ŽTable 7.. However, in this case the precision was improved using cell wall components ŽADF and NDF. as predictors Ž r 2 s 0.86 and 0.83, respectively..

4. Discussion It is commonly accepted with ruminants that maximum intake of roughages is mainly constrained by the physical process of filling and emptying the rumen. The slowly fermented structural carbohydrates, as the main components of these feeds, are thought to play a dominant role in the process of intake regulation by their degradation characteristics and the time they are retained in the rumen. From the levels of SBM used as supplement in this study, it can be assumed that rumen degradable nitrogen was not limiting microbial activity allowing the roughages to be degraded according to their potential. Given the amount and the composition of the mineral–vitamin mixture offered daily to the animals, we also postulate that microbial requirements for minerals were covered.

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Feed intake is also dependent of animal and environmental factors. All roughages were evaluated with ewes of the same breed and from the same flock, 2 to 4 yr old, under the same physiological state. However, as mentioned above, only two roughages were evaluated at one time due to limitations in number of pens. Thus, comparisons across feeds and the accuracy of the prediction equations might have been affected by time and animal age. It is most likely that voluntary intake would be affected to a greater extent than digestibility given the age of the ewes and the barn temperature differences during the study which ranged from a minimum of 88 to a maximum of 288C. This may also explain some inconsistency between DDMI and LW change ŽTable 2.. 4.1. Prediction of digestibility Prediction of OMD by single in sacco DM degradation at times higher than 24 h was slightly better than prediction from cell wall content or degradation parameters. Von Keyserlingk and Mathison Ž1989. also reported that CP and NDF of nine roughages ranging in CP from 40 to 153 and NDF from 442 to 838 g kgy1 DM explained almost as much of the variation in DM digestibility as DM losses from single time incubations at 24 or 36 h. In the study reported by Khazaal et al. Ž1995. where 10 grass hays harvested at different stages of maturity were offered to adult male sheep without nitrogen supplementation, ADL content of the hays was the best predictor of DM digestibility accounting for 66% of the variation observed whereas DM loss in situ at 96 h incubation was a slightly poor predictor Ž r 2 s 0.61.. Parameters of kinetics of DM degradation were still poorer predictors. Van Soest Ž1996. presented experimental data from large number of samples which seriously question the reliability of regression equations using single fibre measurements ŽNDF, ADF. to predict forage digestibility. This is due to the differing environmental factors promoting lignification as opposed to cell wall content. Therefore, the negative association between cell wall and digestibility would be dependent on lignification of cell wall. Although in the present study we have found a strong negative correlation between roughages OMD and NDF, correlation between OMD and the ratio ADLrNDF Ž10 samples, treated straws omitted. did not reach significance Ž r s y0.452.. When treated straws were included correlation was still poor which can be explained by non-fermentable soluble phenolic compounds released by treatment ŽDias-da-Silva and Guedes, 1990; Van Soest, 1996.. A similar lack of correlation can also be drawn from the reports by Khazaal et al. Ž1993, 1995.. When a relatively small number of feeds are studied as was the case in these and in our study, precise sampling and analysis of fibre components, particularly ADL, may become crucial in establishing relationships between digestibility and lignification of cell wall. The finding that degradation constants, alone or in combination, were poor, or, at best, similar predictors of OMD, as DM losses at times higher than 24 h also contrasts Mertens suggestion that simple linear models which includes only feed components could be improved if dynamic parameters of digestion Žand passage. are also considered ŽMertens, 1987.. However, data presented by Khazaal et al. Ž1993. concerning nine legume hays and one grass hay show that the precision in predicting DMD from degradation constants in situ was clearly higher Ž r 2 s 0.84. than from 72 or 96 h incubation or from kinetics of gas production.

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Maximum correlation between in situ degradation and in vivo digestibility seems to occur at about 48 h ŽChenost et al., 1970; Aerts et al., 1977; Wanapat et al., 1986.. Although cell wall levels of the roughages used in the present study were close to those reported by Wanapat et al. Ž1986., we found that the best predictors of OMD were DM degradability after 72 and 96 h incubation accounting for more 8 and 11 percentage units of variation than 48 h incubation time, respectively. Since digestibility in vivo is mainly the product of retention time in the rumen and degradation characteristics as stated by Hovell et al. Ž1986., variation in relating degradability at any fixed time to in vivo digestibility should be expected. 4.2. Prediction of Õoluntary intake The finding that DM degradation after short incubation periods Ž3 h. and the fibre components of the roughages are the best or, at least, good predictors of DMI seems in agreement with most of the published work on this topic of ruminant nutrition. Chenost et al. Ž1970. first demonstrated in a very large number of forages differing in family, species, variety and growth cycle that 12 h incubation data predicted DMI of mature sheep with reasonable accuracy. Similarly, from the data reported by Carro et al. Ž1991. it can be calculated that the best single predictor of DMI of eleven hays fed to mature ewes was DM loss after 6 h incubation Ž r 2 s 0.84.. Residues after short incubation times correspond essentially with neutral detergent extraction of feed samples since cell contents are the main components digesting at these early times. This explains the strong negative correlation we observed between DM losses after 3 and 9 h incubation and roughages NDF and ADF content Ž r s y0.94 and y0.91 for NDF and r s y0.87 and y0.83 for ADF after 3 and 9 h incubation, respectively.. Prediction of DMI and animal performance from rumen degradation characteristics of roughages under non-limiting conditions for fibre fermentation, seems a more sound approach since both the roughage component which increases rumen fill Ž b fraction. and its rate of fermentation are considered. This has been the case in a number of experiments. In the study of Ørskov et al. Ž1988. where five different straws either untreated or ammonia treated were offered to growing steers in a long feeding experiment, differences in fibre and fibre components of the feeds were smaller, as expected, than those observed in the studies above reported and in the present study. However, even in that case, 75% of the variation in DMI was explained by ADF content of straws whereas Ž a q b .DM q cDM explained 79%. The accuracy of predicting DDMI and growth rate of steers was still higher when all the parameters of the kinetics of DM degradation in the rumen were used in a multiple regression equation. This is in agreement with prediction of DDMI in our study and, to a less extent, with prediction of LW change. Also Shem et al. Ž1995. found that degradation constants aDM q bDM q cDM accurately predicted DMI, DDMI and growth rate of growing bulls fed seventeen tropical feeds Ž r 2 s 0.81, 0.86 and 0.86, respectively.. In the report by Khazaal et al. Ž1993. concerning legume hays, NDF accounted for 0.46 of variability in DMI while Ž a q b .DM q cDM accounted for 0.77. Carro et al. Ž1991. improved prediction of DMI over 6 h DM loss by combining in a multiple regression equation aDM q cDM Ž r 2 s 0.90 vs. 0.84..

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In some experiments reported in the literature designed to measure intake, a number of roughages of low nitrogen content were offered without nitrogen supplementation. Under this circumstances cellulolysis might have been impaired due to shortage of rumen degradable nitrogen. Therefore it is not unexpected to find strong correlations between roughages CP and DMI as was the case in the reports by Von Keyserlingk and Mathison Ž1989. and Khazaal et al. Ž1995.. In both studies, CP accounted for 0.71 of the observed variation in DMI. In the study of Khazaal et al. Ž1995. multiple regression equations based on degradation constants gave no better prediction unless CP was included Ž r 2 s 0.84..

5. Conclusion The results of this study demonstrate that, under the conditions it was performed, prediction of OM digestibility, DM intake and live weight change of mature Ile-de-France non-pregnant and non-lactating ewes from the parameters of the equation describing kinetics of DM degradation in the rumen of 12 roughages high in NDF content, did not improve the accuracy over that obtained using fibre components of feeds ŽNDF, ADF. or single incubation times. However, intake of digestible DM was well predicted from rumen degradation characteristics of the roughages. Changes in housing temperature during the study may explain the different predictive capabilities of degradation constants for DMI and DDMI. Since rumen fill is alleviated by digestion and passage of small particles, it seems that the inclusion of a variable measuring the time the roughage is retained in the rumen or more closely related to it than ‘chemical fibre’, may improve the predictive capability of the models. This should be particularly evident under circumstances where nutrients essential for maximal microbial activity in the rumen are not in short supply as supposedly was the case in our study. In this context the development of laboratory methods that simulate the physical resistance of forage to breakdown by chewing as stated by Minson and Wilson Ž1994. should be encouraged.

Acknowledgements This work was partially financed through the National Agency for Science and Technology ŽJNICT, Portugal., which is gratefully acknowledged. The authors also wish to thank J. Potes for insertion of the rumen cannulae. A.J.M. Fonseca was in receipt of a scholarship from JNICT ŽGrant FMRHrBDr901..

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