Effect of pregnancy on intake, digestibility and digesta kinetics in heifers fed urea treated rice straws*

Five rumen fi stulated heifers weighing 298±4 kg, fed ad libitum urea treated rice straw were used during trimesters 1, 2, and 3 of pregnancy. Voluntary dry matter intake (VDMI), dry matter (DM) and neutral detergent fi bre (NDF) digestibility, rumen pool sizes and passage rates were measured. Indigestible NDF was measured by long incubation in situ and used to derive digesta kinetics. The VDMI, digestion and passage rates for DM and NDF fractions, increased (P<0.05) between the 1st and 3rd trimester. In conclusion, pregnant heifers fed high NDF based diets, maintains high forage intake by increasing digestion and passage rates.


INTRODUCTION
Forages fed ad libitum in ruminants appear to be constrained by physical fi ll in the rumen, particularly during pregnancy, causing reduction in feed intake (Forbes, 1970).In addition, pregnancy increase total nutrients requirements.Hence, it is most likely that pregnant cows in the tropics, where poor quality roughage forms the basal diet, have a mechanism of coping with the physical limitation to intake during pregnancy.This study was carried out to measure the effects of different stages of pregnancy on the rumen pool sizes and digestion kinetics and their subsequent effects on intake by heifers fed ad libitum on high fi brous diets.

MATERIAL AND METHODS
Five rumen fi stulated (Boran × Friesian) heifers with mean body weight of 298±4 kg were synchronised for oestrus and bred using artifi cial insemination.Three experimental periods, based on pregnancy trimesters were between 7 th and 11 th weeks of pregnancy (Trimester 1), 19 th to 23 rd weeks (Trimester 2) and 32 nd to 36 th weeks (Trimester 3).In each trimester, the experimental period lasted for 28 days.
Animals were housed in pens and fed individually.Experimental diet consisted of rice straws treated with 50 g urea/kg DM in 600 ml of water per kg DM straw, covered with polythene sheet and ensiled for 14 days.Animals were fed ad libitum twice a day at 9.00 and 15.00 h and each supplemented with fi sh meal (200 g), minerals (100 g) and urea 20 g/100 kg body weight per day.Clean water was provided freely.Feed intake and in vivo digestibility were measured.Feeds were sampled at random.Rumen pool sizes of dry matter (DM) and neutral detergent fi bre (NDF) were measured by rumen evacuation technique, whereas indigestible NDF (INDF) was determined by long incubation in situ, both as described by Mgheni et al. (2004).Rumen fractional rates of intake (k i ), passage (k p ) and digestion (k d ) for DM, NDF and indigestible NDF were derived as described by Robinson et al. (1987).General Linear Model procedure of Statistical Analysis System (SAS) (1990) was used to analyse the data.

RESULTS
The mean values of voluntary dry matter intake (VDMI), rumen pool sizes, DM and NDF digestibility as infl uenced by stage of pregnancy are presented in Table 1.

EFFEFCT OF PREGNANCY ON INTAKE
There was an increase in DM and NDF intake between 1 st , 2 nd and 3 rd trimesters.Apparent digestibility coeffi cients increased between 1 st and 2 nd trimester and decreased slightly between 2 nd and 3 rd trimester for DM, with no difference for NDF (Table 1).The rumen pool sizes of DM and NDF increased from 1 st to 2 nd trimester followed by a decrease from 2 nd to 3 rd trimester.The pool size of indigestible NDF increased progressively between 1 st , 2 nd and 3 rd trimesters.
Derived rates of intake, digestion, passage and mean rumen retention time (MRRT) are presented in Table 2. Rate of intake for DM (k i DM), NDF (k i NDF) and INDF (k i INDF) increased as pregnancy progressed from 1 st to 3 rd trimester.Rate of digestion for DM (k d DM), NDF (k d NDF) and digestible NDF (DNDF), (k d DNDF) increased during the same period.Passage rates of DM (k p DM), NDF (k p NDF), k d DNDF and INDF (k p INDF) response were also similar during the same period.The MRRT increased between 1 st and 2 nd trimester and decreased between 2 nd and 3 rd trimester for DM, NDF and INDF, whereas MRRT for DNDF decreased with advanced pregnancy.

DISCUSSION
The observed progressive increase in intake and rumen pool sizes with more or less similar digestibility coeffi cients, as pregnancy advances is contrary to what was expected.This mechanism was possibly contributed to the increased passage and digestion rates with reduced MRRT during the last trimester.The MASHA E.K. ET AL. current fi ndings are in agreement with the work of Stanley et al. (1993) who reported increased VDMI and increased passage rate of 87% for indigestible acid detergent fi bre (IADF) from 61 to 6 days before calving for Hereford × Angus cows consuming chopped lucerne hay.The results, however, are contrary to other fi ndings of Journet and Remond (1976) that rumen contents and thus intake decreased proportionally with growth of foetus.The observed increase in rate of digestion of DM, NDF and DNDF between 1 st and 3 rd trimester can partly account for lack of well-known negative effects of increased passage rate on digestibility.In the present study, there is also a possibility of existence of rumen elasticity for fi ll that allowed space for increased intake.

CONCLUSIONS
It is concluded that pregnant animals fed high NDF based forages adapt to these diets and maintain high feed intake by increasing digestion and passage rates as pregnancy progresses.It is therefore recommended that pregnant animals should be fed readily digestible forages to enable them extract suffi cient nutrients as pregnancy advances between 2 nd and 3 rd trimester.

Table 1 .
The effect of stage of pregnancy on feed DM and NDF intake (kgd -1 ), digestibility (%) and rumen pool sizes, kg in this and subsequent tables a,b,c means within rows with different superscripts are different (P<0.05) 1

Table 2 .
Digestion kinetics for DM and NDF fractions at different stages of pregnancy