G.I. Alexander*
Nonprotein nitrogen (NPN) supplements, particularly urea, have been the subject of field trials in a wide range of grazing situations in Australia. Most of the early experiments were in southern Australia with sheep where the mature dry pastures had a moderate level of protein and energy. The results in live weight and wool growth were variable. In northern Australia, where there is a relatively short summer growing season, the pastures had crude protein levels of 7–10 percent at the peak of their growth, depending on the pasture, and dry season levels of 2 percent. These pastures, without any substantial growth of a native legume, have at maturity very low digestible protein and total digestible nutrient levels, a high fibre content and low palatability. Intake of these plants by animals is reduced with maturity. It is on these pastures that appreciable responses to NPN supplements have been demonstrated.
Where the dry season is prolonged and there are no showers of rain which would produce a limited growth in the pasture, NPN supplements have proved to be most effective. Even very light rainfall producing a slight growth of green feed can drastically limit the value of NPN supplements because the animals cease to eat the supplement and congregate on what green pasture there is. If the limited green pasture dries up from lack of further rain and from heat, the animals have to be reeducated to the Supplements after a substantial weight loss.
* Department of Primary Industries, Brisbane, Queensland, Australia.
The most commonly used NPN supplement is urea which is toxic if ingested rapidly by the animal in more than very small amounts.
Methods of feeding NPN supplements
Methods of feeding urea have been primarily concerned with providing it in sufficient quantities to be of value to the animal and reducing the risk of mortality from urea toxicity. The rationale behind these methods has been based on findings that sheep and cattle could ingest quite large amounts of urea provided it was consumed slowly so that the intake was spread over several hours.
Pasture spraying
This method is normally applied only where there are good stands of mature grass. The spray usually consists of 39 percent molasses, 6.25 percent urea, and water, and is applied at the rate of 360 kilograms per hectare. While the method has value in certain areas, it has generally been abandoned because:
As an addition to hay
The hay is either sprayed or dipped into a mixture of the same composition as the pasture spray. Under northern Australian conditions, this method has had limited application since it involves the conserving of low quality hay- an uneconomic way of using financial resources.
As a liquid in troughs
An alternative to pasture spraying is feeding urea and molasses in troughs placed in the paddock. This requires less labour but involves greater risk from poisoning as each animal has free choice. A mixture of 0.2 kilogram of urea to 1 kilogram of molasses fed in troughs was eaten at only a slow rate but there is always the need for close supervision to prevent excessive intake. Biuretmolasses mixtures can be fed by this method.
It was not until a way of controlling intake in the troughs was developed that the system became generally accepted (Mutch, 1966). Based on United States wheel regulators, the roller-drum lick feeder proved a cheap and simple method of feeding urea, removing many of the problems and risks associated with feeding urea to cattle. Intake is regulated by variation in the concentrations in the components of the lick. A basic mixture is:
| Parts by weight | |
| Urea | 1 |
| Water | 10 |
| Molasses | 10 |
By providing this mixture at the rate of 0.45 kilogram of molasses per head per day, the consumption rate can be approximated by doubling the number of kilograms of molasses eaten in a given period. Where molasses is expensive it is possible to halve the intake of molasses without any apparent difficulties.
With sheep, a type of wooden float has been developed for use with troughs, the sheep being unable to rotate the large drums. Some farmers have developed a very light small drum for sheep with apparently successful results.
Figure 1. Cattle licking the roller drum
As a block lick
Originally developed in South Africa, salt-urea-molasses blocks have been tested under tropical Australian conditions. These are proprietary blocks consisting of urea (40 percent), molasses (10 percent), trisodium phosphate (2.5 percent) and salt (47.5 percent). Intake was usually between 50 to 80 grams of block per head per day (Alexander et al., 1970).
A home-made urea block was developed by extension officers of the Queensland Department of Primary Industries. It contained approximately 34 percent crude protein equivalent and was made up of the following range of components:
| Palatability | ||
| High | Low | |
| Parts by weight | ||
| Crushed grain | 40 | 32 |
| Coarse salt | 20 | 32 |
| Molasses | 20 | 15 |
| Urea | 10 | 10 |
| Bone meal | 7 | 7 |
| Meat meal | 5 | 5 |
A proprietary premix is also available for use with home-grown grain to make up a block of the following composition: grain, 30 percent: molasses/yeast fermentation distillers solubles, 30 percent; and common salt, 30 percent.
Inclusion in drinking water
Administration of urea via drinking water for sheep and cattle on dry feed carries such a great risk of toxicity that it is not used.
With cereal grains in dry licks
Urea or biuret can be fed with cereal grain and salt, the consumption being regulated by the salt to provide 0.23 kilogram of urea or biuret per head per day. As there is a fairly high risk of toxicity by using urea in this method, biuret is preferred. The amount of grain fed can vary from 1:1 biuret: grain to 1:4. This mixture is added to the salt in ratios of from 1:1 to 1:3 salt: grain/ biuret.
Figure 2. Left: A roller drum lick feeder consisting of a petrol drum mounted on a spindle in a trough. Right: The roller drum lick feeder with the drum removed from the trough
 |  |
Figure 3. A lick feeder for sheep. This has a slatted wooden float on the surfaces of the liquid in a trough
Results from feeding NPN supplements
Cattle
The initial work on feeding urea to cattle was developed as part of pen studies into drought feeding and was extended into a field investigation of supplements for grazing beef cows in time of drought. The use of urea in a molasses, urea and milled pasture hay supplement reduced mortalities in lactating cows and produced a substantial body weight gain in both cows and calves (Alexander et al., 1970).
In a series of field experiments on the use of urea as a supplement to grazing cattle, two experiments with yearlings under average seasonal conditions (dry period of 100–120 days) showed no lasting benefit from the supplement although a response was observed during the feeding period. In another experiment with yearlings, however, drought conditions were experienced for 195 days, and the use of the molasses-urea supplement produced a lasting improvement in performance (Alexander et al., 1970).
Two experiments with weaner cattle (less than 15 months old at the end of the feeding period) showed improved growth rates as the result of feeding a urea block, first in a normal year (86 days feeding) and then in a drought year (148 days feeding).
A further series of experiments with growing cattle have been undertaken at the Swan's Lagoon Cattle Research Station. Millaroo, in northeastern Australia. These experiments indicate that animals fed on urea-molasses gained during the feeding period and this weight advantage was generally maintained after a postfeeding period of approximately six months (Winks et al., 1970; Winks and Laing. 1972). In an experiment to assess the level of urea to be fed to yearling cattle (Winks, Laing and Stokoe, 1972) no significant difference was found in the response of Brahman-cross steers to 26, 49 and 69 grams urea per head per day in an 83-day feeding period, while Shorthorn steers showed a significantly greater response to the high rate of urea with no difference between the other two rates. It is of interest that the Brahman-cross steers gained significantly faster than even the best group of Shorthorns during the feeding period, but postfeeding gains were comparable in both breed types.
Sheep
Most research on supplementing sheep's feed with urea has been carried out on temperate pastures. Entwistle (1972) showed that nitrogen supplementation with urea at the rate of 4.05 grams N per head per day during October to December, when pasture quantity and quality were a limiting factor, significantly increased clean wool production and linear rate of wool growth. However, there was no response in body weight during this period.
Figure 4. Cattle using blocks licks
Figure 5. Cattle eating a home-made lick in a half drum
Figure 6. Cattle eating a home-made lick in a hollowed-out tree trunk
Conclusions
The investigations with sheep indicate that feeding urea does not always produce an increase in wool production, and the low levels of the increases where they do occur do not make the practice economically worth while.
Urea supplements may partly arrest decreases in body weight of sheep grazing dry, poor quality pasture.
Where the dry season extends into drought and poor quality pasture remains available, urea supplements may enhance the sheep's chances of survival on such pastures. This increased survival rate may prove the feeding of urea to be economical. Unfortunately the majority of grazing experiments have been conducted with temperate species and there needs to be more information before these conclusions can be accepted as applying to the tropical grazing lands.
Experiments with cattle on tropical grazing lands have shown that feeding cattle on dry, poor quality pasture with urea and molasses will, in some instances, result in a body weight gain which persists over the next pasture growing season.
In a limited study with lactating cows, feeding urea and molasses increased body weight of the cows and their calves and enhanced their chances of survival.
These conclusions would suggest that urea-molasses may have beneficial effects when combined with dry season feeding on tropical grazing lands where the dry season is long and approaches drought conditions. Since it is difficult to predict this and because of the economic circumstances prevailing, the practice of supplementing regularly during the dry season is developing on cattle grazing lands. The same cannot be said for sheep farms. In both cases where the stock have the opportunity to select even a small quantity of high quality pasture such as dry herbage or green shoots after light falls of rain, urea supplements may be of little benefit.
References
Alexander, G.I., Daly, J.J. & Burns, M.A. 1970. Nitrogen and energy supplements for grazing beef cattle. Proc. 11th Int. Grassld Congr., Surfers Paradise, p. 793– 796.
Entwistle, K.W. 1972. Performance of merino ewes grazing Mitchell grass pastures when supplemented with nitrogen, phosphorous and vitamins A, D, E. Proc. Aust. Soc. Anim. Prod., 9. (In press)
Mutch, C.B. 1966. Handy urea-molasses feeder. Qd agric. J., 92: 630–632.
Winks, L. & Laing, A.R. 1972. Urea, phosphorus and molasses supplements for grazing beef weaners. Proc. Aust. Soc. Anim. Prod., 9. (In press)
Winks, L., Alexander, G.I. & Lynch, D. 1970. Urea supplements for grazing beef weaners. Proc. Aust. Soc. Anim. Prod., 8: 34–38.
Winks, L., Laing, A.R. & Stokoe, Janet. 1972. Level of urea for grazing yearling cattle during the dry season in tropical Queensland. Proc. Aust. Soc. Anim. Prod., 9. (In press)
