Livestock Research for Rural Development 11 (2) 1999

Citation of this paper

Urea supplementation of rice straw for Sindhi x Yellow cattle; sprayed in solution, as a soft cake or hard block

Ho Quang Do, Vo Van Son, Do Vo Anh Khoa 
and Nguyen Thi Kim Khang

College of Agriculture, Cantho University, Cantho, Vietnam


Twelve growing cross-bred heifers (Sindhi X Yellow cattle) of 124 kg (SD=13), receiving a basal diet of ad libitum rice straw and restricted fresh grass (about 4 kg/head/day), were allocated to four treatments: urea sprayed on the straw (50 g in 1.5 litres water, daily); as a hard block (10% urea) offered ad libitum; as a soft cake (10% urea) offered at 500 g/head/day; and unsupplemented control. Intakes of rice straw and of total dry matter were higher (P=0.001) on all supplement treatments compared with the control.  The cattle ate all the soft cake that was offered (500 g/day) but those offered the hard block ate only 209±3 g/day.  Growth rates were highest (P=0.001) with the soft cake supplement (280 g/day) followed by the hard block (160 g/day), the spray solution (125 g/day) and the unsupplemented control (63 g/day). Growth rate was positively  correlated (RČ=0.90)  with the estimated intake of urea.

Key words: Cattle, urea, blocks, cakes, feed intake, rice straw, growth


Vietnam grows over 6 millions ha of rice annually and it is estimated that this yields around 18 million tons of rice straw. This residue is the most abundant feed for ruminant animals in Vietnam, especially during the dry season. Many methods have been proposed for improving the nutritive value of rice straw, usually by some treatment involving the use of urea or ammonia (see review by Nguyen Xuan Trach 1998).  Incorporating urea in a hard block based on molasses and rice bran, with lime or cement as hardening agent, was proposed by Leng (1984) and popularised in many countries by the Feed Resources Group of FAO (Sansoucy 1986). Positive effects of this technology to improve use of rice straw by growing heifers were reported by Bui Xuan An et al (1992).  Attempts to feed the hard block to swamp buffaloes in the Mekong delta were not successful as the animals would not consume them (Nguyen Van Thu et al 1993). Nguyen Phuc Tien and Preston (1998) observed the same phenomena with swamp buffaloes in SE Vietnam, exposed to one hour of work daily driving a sugar cane crusher; by contrast, the same block was consumed by native Yellow cattle on the same work schedule.  The solution to the intake problem, reported by Nguyen Van Thu et al (1993),  was to prepare the urea-rice bran- molasses mixture as a "soft cake" and, where necessary, to force-feed the mixture using a "bamboo pipe". 

There appear to be no reports comparing the hard "block" versus the soft "cake" as sources of urea in rice straw diets (see review by Nguyen Xuan Trach 1998).  The following experiment was designed to compare three methods of supplying the urea: sprayed on the straw as an aqueous solution; in a hard block; and as a soft cake.

Material and methods

Location, animals and treatments

The trial was carried out on the research farm of the Farming Systems R&D Institute of  Cantho University, situated in Hoaan village, Phunghiep district, Cantho Province. Twelve growing cross-bred (Sindhi x Yellow) heifers (initial weight 124; SD 13 kg), receiving a basal diet of ad libitum rice straw and restricted grass, were allocated in a completely random design to four treatments:

Feeding system

The rice straw was purchased in Hoaan village in three separate batches at 30 day intervals. Fresh grass was purchased every day. The rice straw was offered ad libitum, fresh quantities being given morning and afternoon to ensure there was a residue of at least 0.5 kg.  The grass was a mixture of local varieties (Sacciolepis myuros and Sacciolepis interupta) and was given fresh in a single feed in the morning at a level of approximately 0.5 kg (dry matter equivalent) for each 100 kg liveweight. It was planned that the offer level of block and cake should be the same (0.5 kg/day); however, it was observed that the cattle on the block treatment did not consume all that was offered whereas the soft cake was consumed completely. It was therefore decided to feed the block ad libitum, replacing each block with a new one as soon as the residue was seen to be less than 100 g. The offered block and the residue were weighed when the change was made.  The soft cake was offered in the morning immediately after the grass was consumed. The order of feeding was thus: grass, followed by cake followed by straw in the morning beginning at 7.00am and straw again in the afternoon at 4.00pm).

Table 1: Composition (air-dry basis) of the hard block and the soft cake
Soft cake Hard block
Molasses 35 0
Rice bran 40 30
Coconut meal 10 0
Urea 10 10
Salt 5 5
Lime 5
Cement 10
Bone meal 10
Clay 30

There were major differences in the composition of the cake and the block (Table 1). The cake contained 35% molasses,10% coconut meal and 5% of mineral matter (salt). The block had 60%  of the total in mineral form and thus had a lower content of digestible nutrients. Also the hard block contained no molasses.

The soft cake was made by mixing the urea, salt, coconut meal and rice bran. The molasses was then added, mixed thoroughly.and 1 kg of the mixture put into a box and pressed. For the hard block, the bone meal and cement were added to the rice bran and mixed together. A slurry was made of the lime, clay, salt and  water and this was then mixed with the dry ingredients.  The final mixture (lots of 2 kg) was pressed into a container and allowed to set in the shade for one week before being fed.


The cattle were weighed on two consecutive days at 30 day intervals.  Intakes of rice straw, grass and the soft cake were recorded every day. The hard block was weighed when first offered to the animals and again prio to being replaced with a new block.  The rice straw and grasses were sampled every month and analysed for dry matter by drying in a forced air oven at 100 °C.

Statistical analysis

Liveweight gain for individual animals was estimated from the linear regression of liveweight on days in the trial. These data and those for feed intake were analysed by the ANOVA option of the Minitab software package (Version 11). The sources of variation were treatments and error. The linear regression between liveweight gain and estimated urea intake was calculated using the same Minitab software package.

Results and discussion

Feed intake

There were marked differences in intake of the cake and the block (Table 2).  All the cake offered (500 g/head/day) was consumed while of the 500 g of block that were offered, less than one half (208 g/day) was consumed.  Apparently the problem was not one of adaptation as average intakes in consecutive months were the same (207, 209 and 209 g/head/day; SEM±6.5). Intake of rice straw was increased (P=0.001), and so was intake of total diet dry matter (P=0.037), through urea  supplementation with the most marked increase being observed with the soft cake supplement.  The regression of liveweight gain on dry matter intake showed that 62% (RČ = 0.62) of the variation in gain could be explained by the intake.  This is indirect evidence that the supplementation had brought about improvements in the efficiency and / or extent of rumen fermentation, (which would be expected to support greater microbial protein production) and hence a better rate of growth. 

Table 2: Mean values of liveweight, feed intake and conversion of Sindhi*Yellow cattle fed rice straw and urea supplements
Unsupp. Sprayed Hard block Soft cake SEM Prob.
Liveweight, kg
Initial 123 124 123 124 4.31
Final 129 135 137 149 4.11 0.40
Daily gain 0.063 0.125 0.160 0.280 0.009 0.001
Feed intake, kg/day
Rice straw# 2.79 2.97 2.99 3.15 0.053 0.001
Grass# 3.67 3.67 3.67 3.67
Block 0.209±0.0032 0.500
Urea 0 0.025(?) 0.021 0.050
Dry matter 3.03 3.18 3.20 3.34 0.059 0.037
Feed conversion 48.1 25.5 20.0 12.0 1.43 0.001
# Dry matter (%):  Rice straw 86.9±0.41; Sacciolepis myuros 19.8±0.50; Sacciolepis interupta 15.2±0.60

The main findings (Figure 1) show that supplementing rice straw and a small amount of grass with a soft cake containing 10% urea increased the growth rate from 63 to 280 g/day. For crossbred Yellow cattle with a mature liveweight of the order of 300 kg, a gain of 280 g/day from a basal diet of unsupplemented rice straw is indicative of the potential of this approach using urea-rich supplements rather than "treating" the straw with urea. The amount of supplementary dry matter as cake was 400 g/day and the increase in liveweight was 217 g/day, giving a conversion rate for the supplement of 2:1.

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The original hypothesis - that a hard block that is consumed in small amounts through the day would be better than a soft cake which is consumed in one single meal - could not be tested as there was confounding effect between the hardness of the block / cake and the  composition. There was 30% molasses in the cake and none in the block. Thus due to either the hardness, or the absence of molasses, the block was consumed (209 g/day) at less than half the rate of the soft cake (500 g/day). Thus the urea intake (sources of ammonia being the main limiting nutrient in rice straw) was only 21 g/day on the block versus 50 g/day with the cake. If it is assumed that approximately 50% of the urea nitrogen sprayed on straw is lost through volatilization of ammonia (Bui Van Chinh 1991), then there is a close relationship (RČ = 0.90) between the estimated intake of urea and the liveweight gain  (Figure 2).

Of economic importance is the fact that spraying an aqueous solution of urea on the straw resulted in poorer performance compared with the cake or block supplements. Volatilization of ammonia is also undesirable from the environmental standpoint.


Sindhi*Yellow cattle consuming rice straw and a small amount of grass increased their growth rate from 63 to 280 g/day, when supplemented with 500 g/day of a soft cake containing 10% urea and 35% molasses. Results with a hard block supplement, with the same  urea content but no molasses, were poorer probably because block (and hence urea) intakes were less than half what was observed with the soft cake.


Bui Van Chinh 1991 Scientifical information of AHRI, Number 5, Hanoi ( Vietnamese).

Bui Xuan An, Ngo Van Man and Luu Trong Hieu 1992 Molasses-urea block (MUB) and Acacia   mangium as supplements for crossbred heifers fed poor quality forages. Livestock Research for Rural Development. Volume 4, Number 2:

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Received 30 April 1999

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