Three yearling lambs each equipped with a rumen cannula were used to study the effects of supplementation with a urea mineral lick block (ULB) on the kinetics of ruminal fibre digestion, nutrient digestibility and nitrogen (N) utilization of rice straw (RS), ammonia bicarbonate-treated RS (ABRS) and hay prepared from natural pasture. The digestibility of dry matter and organic matter of RS was increased by 13.1 and 12.7 % (P<0.05) and approached to that of ABRS, indicating that the effect of ULB on digestibility of RS is similar to that of treatment with AB. The digestibility of ABRS was improved slightly by ULB feeding. Nitrogen retention was highest in lambs on ABRS alone, followed by hay with ULB, and was lowest in animals on RS with ULB. However both amount of N retention and proportion to intake were increased by ULB supplementation in lambs fed on hay. The proportion of N retained to N digested decreased by ULB supplementation in lambs on RS or ABRS. Block did not greatly influence the rumen degradation of either dry matter or crude protein in any of the all three diets. The RS and hay had similar values for the potential extent of digestion (PED) and digestion rate of PED (kd) of fibrous materials, but the discrete lag time for RS was lower than that for hay. The AB treatment increased the PED (P<0.05) and kd (P.0.05) of RS. Neither the PED nor kd for RS and ABRS was influenced by the ULB feeding, but the kd for hay was increased significantly. The lag time for hay was also shortened by ULB feeding. ULB improved the digestion of fibre in the rumen of lambs on low quality roughage.

It is inferred that while ULB is effective in increasing nutrient digestibility of low quality roughages through the improvement in ruminal fibre digestion, a synchronized supply of N and energy to rumen microbes should be considered to improve the utilization efficiency of N when the basal diet is ammoniated straw.

It has been recognized that when animals are offered a low- nitrogen, high-fibre roughage such as rice straw (RS), one of the critical nutrients is fermentable nitrogen (N) available to rumen microbes. The use of urea/molasses blocks is a convenient way of avoiding an excessive intake of urea, and will ensure an almost continuous supply of ammonia-N (Preston and Leng 1987).

Urea/molasses block feeding has given positive results in many parts of the world (Kunju 1986; Hadjipanayiotou et al 1993b; Chen et al 1993). The blocks which contain molasses are highly palatable, but may not be widely applied in many countries because of unavailability of molasses. Therefore some workers have attempted to manufacture blocks with reduced quantities of molasses (Hadjipanayiotou et al 1993a; Liu et al 1995).

The objective of this study was to evaluate the effect of a urea- mineral lick block (ULB) without molasses on rumen fibre digestion kinetics and on the nutrient digestion and nitrogen utilization of rice straw (RS), ammonia bicarbonate treated RS (ABRS) and hay prepared from natural pasture.

Three yearling lambs each equipped with a rumen cannula and weighing about 30 kg were dosed with antihelminthic and housed individually in metabolism crates. Feeds were offered in two equal meals per day at 900 and 1800 h, and the daily amounts were calculated to exceed that eaten on the previous day by about 10 % to avoid selective feed intake. All animals had free access to drinking water.

The rice straw (RS) (Japonica, cv. "Zhenongda 40") was obtained from the Experimental Farm, Zhejiang Agricultural University. The ABRS was prepared by the stack method: one tonne of RS was treated with 100 kg ammonia bicarbonate and 250 kg water for 30 days at an ambient temperature of 15-200C (Liu et al 1991). Hay was prepared from natural pasture which is the main roughage source for ruminants in our region. The composition of the experimental diets is presented in Table 1.

Table 1: Chemical composition of the experimental feeds
	DM	OM	N	NDF
	(%)	---% of DM---
Rice straw
Untreated	81.8	86.0	1.41	69.9
AB-treated	80.7	85.2	2.0	61.8
Hay	83.1	78.0	1.70	60.0

The ULB was prepared without molasses, and contained 4% N with all minerals. The composition was the same as described in the previous study (Liu et al 1995).

The experimental design was a two 3 x 3 Latin square arrangement, one square for roughage with ULB and one without ULB. Each period consisted of 21 days, the first 10 days of which were for adaptation followed by 11 days of measurements. A digestibility and N balance trial was conducted over 5 days (from day 11 to day 15), while the rate of passage of digesta through the rumen (kp) was determined. The degradation of crude protein (CP) and dry matter (DM), and digestion of neutral detergent fibre (NDF) in the rumen was measured from day 16 to day 21.

The digestibility of nutrients and N balance was determined by total collection of faeces and urine. Feed and faeces were analyzed for DM, organic matter (OM), N (Association of Official Agricultural Chemists, 1990) and NDF (Goering and Van Soest 1970). The N content in urine was analyzed by the Kjeldahl method. The procedures determining the kp were the same as described previously (Liu et al 1995), where the model of Grovum and Williams (1973) was used.

Rumen digestion of NDF and ruminal degradation of CP and DM were determined in sacco (Orskov 1985) and details of procedures are as described previously (Liu et al 1995). The parameters of digestion kinetics of NDF in the rumen were estimated using the model of Mertens and Loften (1980). The nonlinear iterative least square procedure was used to fit the equation:

R=PED x exp(-kd(t-LT))+U

where R is the percentage of NDF recovered at time t (h), PED is the potential extent of digestion at fractional rate kd (kd>0), LT is the discrete lag time of digestion, and U is the indigestible fraction (U=100-PED).

Ruminal degradation of CP and DM was calculated from the disappearance rate from dacron bags incubated in the rumen. The data were fitted to the model of Orskov (1985):

p=a+b(1-exp(-ct))

where p is disappearance rate at time (t), a is the rapidly digestible fraction in the rumen, and b is the fraction slowly digested at rate c (c>0).

The effective degradability (dg) of CP and DM was calculated using the equation presented by Orskov (1985):

dg=a+bc(c+kp)

The results were analyzed as a two-way factorial design (Steel and Torrie 1960), in which square was considered as a factor.

The results of the digestion trial are presented in Table 2. The dry matter intake of all three roughages decreased slightly with ULB supplementation but the differences were not significant. Intake of ULB was estimated to be about 10 g per day, which was similar to that obtained for goats (Liu et al 1995).

Table 2: The effects of using a urea-mineral block lick on the intake and digestibility of experimental diets offered to lambs (NT not treated; AB treated with ammonium bicarbonate)
	Rice straw
	NT		AB		Hay		Significance #
Block	-	+	-	+	-	+	R	B	R x B
Intake (gDM/d)	576	534	683	591	735	705	*	NS	NS
Apparent
digestibility (%)
Dry matter	48.9	55.3	54.4	57.1	49.1	55.0	*	**	NS
Org matter	51.8	58.4	57.6	60.2	53.0	58.5	*	*	NS
N x 6.25	39.5	45.7	60.1	61.0	35.2	48.8	**	*	NS
NDF	62.6	66.8	65.6	68.5	66.2	69.4	*	*	NS

# R roughage effect; B block effect; R x B interaction effect between roughage and block;
* P<0.05; ** P<0.01; NS not significant.

The digestibility of DM and OM of RS was increased by 13.1 and 12.7 % (P<0.05) by ULB and approached that of ABRS, indicating that the effect of ULB on digestibility of RS is similar to that of treatment with AB. The hay used in this study was of low quality as shown by its digestibility, which was the same as that of RS. The digestibility of hay was significantly increased by ULB supplementation (P<0.05). When ABRS was supplemented with ULB, the digestibility of all nutrients was improved.

The results of N balance are shown in Table 3.

Nitrogen intake was lower in lambs given the RS (P<0.05) even with ULB. When RS or hay was given alone the faecal N loss was above 60%. Ammonia treatment and ULB supplementation were able to decrease the faecal N loss on RS diets. The lambs fed on ABRS with or without ULB had the highest urinary N loss, while the lowest urine N losses were from those animals on hay. The ULB increased the N losses from urine on all roughages, regardless of the amount and the proportion to N intake. Without the ULB, N retention (NR) was highest in lambs on ABRS, followed by that on hay, with the lowest in animals on RS. While the feeding of ULB increased the NR in lambs on hay, the NR in animals on ABRS decreased due to the ULB supplementation. No difference was found in the NR from animals on RS with or without ULB.

Table 3: The nitrogen utilization by lambs fed rice straw untreated (NT), or treated with ammonium bicarbonate (AB) or hay with or without a urea mineral lick block
	Rice straw
	NT		AB		Hay		Significance
Block	-	+	-	+	-	+	R	B	RxB
N balance, g/d
Intake	8.1	8.1	13.2	12.3	12.5	12.5	**	NS	NS
Faeces	4.9	4.4	5.4	4.8	8.1	6.4	**	*	NS
Urine	1.6	2.2	2.3	3.1	1.2	1.6	*	*	NS
Retention	1.6	1.5	5.5	4.4	3.2	4.5	**	NS	NS
Percent of intake
Faeces	60.5	54.3	40.9	39.0	64.8	51.2	**	*	NS
Urine	19.8	27.2	17.4	25.2	9.6	12.8	*	*	NS
Retention	19.7	18.5	41.7	35.8	25.6	36.0	*	*	NS
N Retained/N
Digested (%)	50.0	40.5	70.5	58.7	72.7	73.8	**	*	NS

# See footnote in Table 2.

The proportion of N retained to N digested decreased with ULB supplementation in lambs on RS or ABRS, but there was little change in animals on hay. This may be associated with an unbalanced supply of N and energy to the rumen microbes when the straw diet was supplemented only with ULB, resulting in the inefficient use of N.

The results obtained for DM and CP degradation in the rumen are shown in Table 4. Without ULB, the degradability of DM and CP was significantly higher for ABRS than that for RS and hay, with little difference between RS and hay. The ULB had little effect on the rumen degradation of DM and CP in any of the three feeds.

Table 4: Constants of the equation p=a+b(1-exp(-ct)) for the rumen degradation of dry matter and N x 6.25 in lambs fed rice straw untreated (NT), treated with ammonium bicarbonate (AB) or fed hay, with or without a urea- mineral block.
	Rice straw
	NT		AB		Hay		Significance
Block	-	+	-	+	-	+	R	B	RxB
DM degradation
a (%)	16.0	12.4	15.9	16.1	19.5	20.0	**	NS	*
b (%)	52.8	58.0	55.1	53.8	49.2	56.1	NS	NS	*
c (%/h)	3.07	3.45	4.08	5.53	3.25	2.66	**	NS	*
kp (%/h)	2.96	2.88	3.14	2.71	3.54	3.00
dg (%)	42.9	44.0	52.0	52.2	43.7	46.3	*	NS	NS
a+b (%)	68.8	70.4	71.0	69.9	68.7	76.1
N x 6.25 degradation
a (%)	36.1	32.1	35.2	29.3	28.0		NS	NS	NS
b (%)	50.7	50.4	53.1	45.0	46.0	44.6	NS	NS	NS
c (%/h)	0.98	1.72	3.95	5.68	2.06	1.85	*	NS	NS

# See footnote in Table 2.

The parameters of NDF digestion in the rumen are presented in Table 5. When given alone, the RS had a similar value to hay for the potential extent of digestion (PED) and its digestion rate, but the discrete lag time (LT) for RS was lower than that for hay. The AB treatment increased the PED (P<0.05) and kd (P.0.05). Neither the PED nor kd for RS and ABRS was influenced by the feeding of ULB, but the kd for hay was significantly increased. The product of PED x kd (NDF digested per hour) was, however, increased by 9.2 and 30.3 % for RS and hay respectively, though little effect was observed for ABRS. The LT for hay was shortened by ULB feeding.

The effective extent of ruminal fibre digestion (EED) was estimated according to Huang and Xiong (1990) and is shown in Table 5. The RS had an EED value similar to that hay and treatment with AB improved the rumen fibre digestion and increased the EED of RS by 26 %. The ULB improved the EED for all roughages, suggesting that ULB can improve the integrated digestion of low quality roughage fibre in the rumen.

Table 5: Parameters of the ruminal digestion kinetics of dietary fibre in lambs with or without a urea mineral lick block
	Rice straw
	NT		AB		Hay		Significance #
Block	-	+	-	+	-	+	R	B	RxB
PED (%)	59.2	59.6	62.3	63.1	59.6	55.2	*	NS	NS
kd (%/h)	3.29	3.58	5.68	5.19	3.32	4.67	*	NS	*
PEDxkd (%)	1.95	2.13	3.54	3.27	1.98	2.58
LT (h)	5.4	5.4	5.6	5.1	6.3	5.0	NS	NS	NS
kp (%/h)	2.96	2.88	3.14	2.71	3.54	3.00
EED (%)	26.6	28.2	33.6	36.1	23.1	28.9	**	*	NS

# See footnote in Table 2.
EED is calculated as: PED x kd/(kd+kp) x exp(-kp x LT)

The ULB significantly increased the nutrient digestibility of RS and hay, and slightly improved the digestibility of ABRS, possibly as a result of an improved digestion of fibre in the rumen. Both the amount, and the proportion of N retention to intake were increased by ULB supplementation in lambs fed on hay. The proportion of N retained to N digested decreased with the feeding of ULB in animals on RS or ABRS, indicating that the effect of ULB on the efficiency of N utilization varied between different roughages. It was concluded that when low quality roughages high in fibre and low in N are supplemented with ULB containing urea and minerals, a synchronized supply of N and energy to rumen microbes should be considered to improve the utilization efficiency of N.

This study was jointly supported by the Funds for Excellent Youth Teachers from the State Commission of Education, China, and the International Foundation for Science (Grant Agreement No:B/1973- 1). The authors wish to thank X D Xie, Y Zhao, H Y Zhou and X Z Chen for their assistance and care of the animals. Thanks are also due to Mr N A MacLeod for critically reading this manuscript.

(Received 1 October 1995)