Livestock Research for Rural Development 9 (5) 1997

Citation of this paper

The effect of forage and supplement on the intake, digestibility and balance of nitrogen in sheep fed with diets high in molasses/urea


H Losada*, E Aranda** and R Alderete***

*Animal Production Systems Area, Departament of Biology of Reproduction. Division of Biological and Health Sciences, Universidad Autónoma Metropolitana-Iztapalapa. Av. Michoacán y la Purisíma. Col. Vicentina. Iztapalpa. México DF, CP 09340
** Postgraduate College, Campus Tabasco, México
*** Faculty of Zootechny, Universidad Autónoma de Chihuahua. México.


Eight pelibuey male sheep with a mean live weight of 40 kg and with fistulas in the rumen were used in a random design of treatments as follows: free access to molasses (with 2.5% urea); molasses/urea and cassava meal; molasses/urea and restricted access to forage; and molasses/urea, cassava meal and restricted forage. The animals were housed in metabolic cages in order to measure the digestibility and balance of nitrogen (N). All the sheep had free access to the molasses/urea, mineral salts and water. The grass provided was poor quality Napier (Pennisetum purpureum) at a rate of 1.5 kg (fresh basis) for each 100 kg of animal live weight, and dehydrated cassava meal was used as a supplement at a level of 150 g/animal/day. The experiment had a duration of 50 days, of which samples were collected on the last 5 days only, with the objective of extending the period of adaptation to the diet to a maximum and reduce the animal variability.

There was no evidence of toxicity by molasses in the treatments where forage was omitted. The daily intakes of molasses/urea in the treatments that included a source of forage were 0.615 and 0.562 kg per animal, in contrast to 0.427 and 0.330 kg per animal (P = 0.01) in the treatments where forage was not provided. This was reflected in the values obtained for the total intake of dry matter (DM) of: 0.427, 0.452, 0.945 and 1.0 kg per animal/day (P=0.001) in treatments of: molasses/urea only; molasses/urea and cassava meal; molasses/urea and forage; and molasses/urea, cassava meal and forage, respectively. The values for apparent digestibility of DM were 84.3, 84.9, 80.8 and 80.5% in that order, while those reported for the retention of N (as % of intake) were 1.4, 2.5, 25.1 and 24.4%, respectively. The intakes of water were 4.28, 4.60, 4.90 and 4.82 litres/animal/day (P= 0.05) which represented a ratio of water to DM of 10.0, 10.1, 5.2 and 4.8 litres/kg respectively.

The role of forage in diets of molasses/urea is discussed within the context of its relationship to voluntary consumption of the feed.

Key words: Intake, molasses/urea, sheep, digestibility, nitrogen retention, molasses toxicity.



In meat production systems based on the utilisation of molasses/urea as the energy source (Preston et al 1967), the need to supply restricted quantities of green fodder has been emphasised in order to promote optimal animal performance and to avoid metabolic problems such as toxicity by molasses (Losada et al 1971; Losada and Preston 1973a). Although the quantity of forage in the diet in this feeding system is minimal (1.5 kg fresh forage/ 100 kg LW), its action in the animal is not clearly understood. It is considered by some authors as a source of nutrients (protein and vitamins according to Elias et al 1968) and by others as having a stimulatory function on the rumen wall (Losada and Preston 1973b). The occurrence of molasses toxicity, associated with the omission of forage from the diet, has not been studied in detail, although preliminary observations by one of the authors (Losada 1973) suggest that the absence of forage in adult sheep did not necessarily induce illness. To this end, the study of the effects of forage and supplements in a high molasses-urea diet appeared to be relevant.

The objective of the present experiment was to study the effect of forage and supplements on the intake, digestibility and N balance of sheep fed on diets of molasses/urea offered ad libitum.

Materials and Methods

Animals and treatments

A total of eight male pelibuey sheep with an average LW of 40 kg, and with fistulas in the rumen, were allocated at random to the following four treatments:

Work plan

The animals were housed in eight metabolic cages, in a shed open on all sides with partial shade and a concrete floor. Throughout the period of the experiment, the animals had unrestricted access to a mixture of molasses and urea (2.5%), water and mineral salts (48% common salt, 48% rock phosphate and 4% trace minerals). The forage was poor quality napier grass (Pennisetum purpureum) offered at a rate of 1.5 kg (fresh basis)/100 kg LW. The supplement was dehydrated cassava root meal supplied at a rate of 150 g/animal/ day. The experiment had a duration of 50 days, with the first 45 days being considered to be the adaptation period - to avoid the residual effect of the forage in the digestive tract - and the remaining 5 days regarded as the experimental phase. Samples of offered and rejected (where applicable) feed, along with the total quantity of faeces and urine produced, were refrigerated (at a temperature of less than 3C). Sulphuric acid (0.1 N) was used to preserve the urine. The determination of the DM of the molasses was carried out using the Toluene method, and the remaining analyses were done following conventional procedures (AOAC 1990).



There was no evidence of molasses toxicity in the treatments where forage was not supplied. Nevertheless, the animals showed obvious symptoms of stress, in the form of constant rubbing and biting of the cages, that indicated a need for roughage. The rumen of these animals, at the level of the fistula, demonstrated a putrid and acidic odour.

Feeds and animals

The composition of the feed resources is presented in Table 1. The contents of DM and crude fibre in the grass were high while that of N was low. The cassava meal was low in both N and crude fibre.

The data for the intake and digestibility of DM of the different treatment diets are presented in Table 2.

The intake of molasses/urea was higher in the animals that received restricted quantities of forage (P<0.001) than for those treatments where forage was absent. The addition of dehydrated cassava had the effect of acting as a substitute for the molasses. The total intake of DM showed the same tendency reported for the intake of molasses (P<0.001). There were no significant differences in the values of the apparent digestibility of DM, although there was a tendency for it to be less with the addition of a source of forage.

The intake of N was higher for the animals that received restricted forage (Table 3), in response to the higher intake of DM. The major losses of N for all the treatments was via the urine. In those animals that did not have a source of fibre, the retention of N was minimal, although the values of apparent digestibility were high for all treatments.

The measurements obtained for the intake of molasses/urea and water, and the output of urine are presented in Table 4.

The intake of water was greatest (P <0.05) in the animals that received restricted amounts of forage as well as the supplement, in comparison with those that received the molasses/urea only. The output of urine showed a similar tendency to that observed for the intake of water (P<0.05), while the estimated ratio of water to DM was twice as high for the sheep that did not receive forage in their diet.


The random design used in the present study could be considered as a limiting factor in the interpretation of results. It contrasts with the conventional method used for metabolic studies which is usually some form of latin square design, which allows the cancelling out of the possible effects of the individual animal. However, under the particular conditions of the current experiment the use of an adaptation period of 45 days in order to avoid confusion in animal response to the treatments was preferred. The extended adaptation period of 45 days contrasts with the conventional seven days considered viable with latin squares (Broderick and Wallace 1988).The justification of the design was ratified by the fact that significant differences between treatments were obtained, directly demonstrating the reduced variability within the treatments.

The aspects most relevant in the present experiment were those related to the effect of the forage on intake, digestion and the absence of symptoms of molasses toxicity. The forage supplied to the animals was selected on the criterion of offering a poor quality source of fibre which would suppress to the utmost the possible contribution of nutrients. In the same way an energy supplement of high rumen solubility (the cassava root meal) was also used (Losada and Alderete 1979; Santana and Hovell 1979).

The objective was to confine the site of digestion of both components (forage and supplement) to the reticulum-rumen. With the addition of forage to the diet, the consumption of the molasses/urea mixture was increased on average by 36% (P<0.01), and of the total diet dry matter by 118% (P<0.001). In contrast, the addition of cassava meal reduced the consumption of the molasses/urea mixture by 17%, although the total intake of DM was higher.

In accordance with these results, it is possible to suggest that when ruminant animals have free access to a mixture of molasses with 2.5% urea, the presence of a source of fibre represents an important factor in the primary control of voluntary intake, with a primordial action initiated through the movements of the reticulum-rumen, induced by the forage and regulated by the effects of the increase in osmotic pressure caused by the fermentation of the soluble components (Benavides and Preston 1971).

The earlier observations, that the absence of forage is not the direct cause of toxicity by molasses in sheep, has been clearly confirmed by this experiment. With the usual time for the manifestation of such illnesses being seven days (Losada et al 1971), the 50 day duration of the present study represents a sufficiently long period for the occurrence of toxicity. In the same way , the sheep that did not receive forage maintained an adequate intake of DM and water, and demonstrated a positive balance of N, which would confirm their state of health, as opposed to the altered behaviour and cessation of intake in animals affected by molasses toxicity (Losada et al 1971).


The authors wish to thank the authorities of the Universidad Autónoma Metropolitana (Autonomous Metropolitan University) for the facilities and to Ms R Pealing, a researcher from UK sponsored by the interchange CONACYT-British Council, for help with correction of the manuscript after translation from Spanish to English.


AOAC 1990 Association of Official Analitical Chemists: Official Methods of Analyses. Third Edition. AOAC. Washington, D C

Benavides M C and Preston T R 1971 Synthetic plastic roughage in molasses-based diets for cattle. Revista cubana Ciencia Agricola 5:319-329.

Broderick G A and Wallace R J 1988 Effects of dietary nitrogen source on concentrations of ammonia, free aminoacids and fluorescamine-reactive peptides in the sheep rumen. Journal Animal Science 66 (9): 2233-2238.

Elías A, Preston T R and Willis M B 1968 Intensive beef production from sugar cane. 8: Effect of rumen inoculatíon and different levels of forage on the performance of bulls fattened on high levels of molasses/urea. Revista cubana Ciencia Agrícola (English Edition) 3:19

Losada H, Dixon F and Preston T R 1971 Thiamine and molasses toxicity. 1. Effect with roughage-free diets. Revista. Cubana Ciencia Agricola (English Edition). 5:369.

Losada H 1973 Alteraciones metabólicas del ganado de carne alimentado con mieles como fuente principal de energía. Intoxicación por mieles. Tésis de grado Msc Agricola ICA Universidad de la Habana, Cuba.

Losada H and Preston T R 1973a Molasses toxicity and cerebro cortical necrosis (NCC). Revista. cubana Ciencia Agricola (English Edition) 7:169.

Losada H and Preston T R 1973b Effect of forage on performance, content of the reticulo rumen and VFA in rumen and caecum of calves fed diets based on molasses/urea. Revista. cubana Ciencia Agricola (English Edition) 7:179.

Losada H and Alderete R 1979 Effect of cassava root meal and urea level on the performance of steers grazed on poor quality pasture with free access to molasses. Tropical Animal Production. 4:47.

Preston T R, Elías A, Willis M B and Sutherland T M 1967 Intensive beef production from molasses and urea. Nature 216:721

Santana A and Hovell F O DeB 1979 Degradation of various sources of starch in the rumen of Zebu bulls fed sugar cane. Tropical Animal Production 4:107-108

Received 15 November 1997

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