ARCHIVE FILE
This article was published in 1961
See the original document
INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
Biological and Economic Aspects of Supplementary Feeding
G. L. McClymont, B.V.Sc., Ph.D., School of Rural Science, University of New England, Armidale, N.S.W.
SUMMARY
Some of the biological and economic factors involved in a rational approach to supplementary feeding are discussed, with particular reference to the definition of the major limiting factors in the nutrition of the grazing animal, the depressing effect of supplementary feeding on pasture intake, compensatory growth, the effect of cutting for conservation on pasture growth and subsequent animal performance, and opportunity costs.
INTRODUCTION
Definition of Supplementary Feeding: Provision of feed in addition to that available from low-density or low protein and/or low digestibility pasturage to prevent or alleviate effects of such pasturage on growth or reproduction. (Drought feeding, defined as provision of feed with the primary object of preventing deaths from inanition obviously overlaps with supplementary feeding as defined, but as the biological and economic problems involved differ to some degree it is useful to consider them as separate problems).
Violent fluctuation in the density and quality of pastures is normal to most of Australia. Supplementary feeding offers one means of alleviating the effects of such fluctuations on livestock production. Nevertheless, despite a great deal of advocacy of supplementary feeding by extension authorities and individuals (including the writer in the past!) the actual amount undertaken in Australia is in fact quite limited.
Is this due to backwardness of producers, or because supplementary feeding is a more complex issue, biologically and economically, than a first approach would suggest? After some considerable study of the problem (prompted in the first place by the failure of a "demonstration experiment" on supplementary feeding to achieve any significant extra production and realisation that there was virtually no experimental data to support the dogma), I strongly incline to the second explanation!
Some of the basic factors involved in supplementary feeding which make it a complex issue, and which must be taken into account in arriving at any rational decision on such feeding are as follows:—
Definition of the major limiting factor in the nutrition of the grazing animal:
If protein is the major limiting factor, providing a low protein supplement such as grain or gramineous roughage will give only a very small response, while a similar weight of a high protein con
Submaximal performance of stock on pasture may be primarily due to insufficient intake of energy and/or protein, (excluding mineral deficiencies). The response of supplementary feeding thus depends on the composition of the supplement.
If protein is the major limiting factor, providing a low protein supplement such as grain or gramineous roughage will give only a very small response, while a similar weight of high protein concentrate (including urea and grain or molasses mixtures), or leguminous roughage may give a many-fold greater response (e.g. Franklin, Briggs & McClymont, 1955). This is of course due to the fact that protein deficiency depresses food intake, digestibility and efficiency of utilisation of absorbed energy. Norman (1960 and later unpublished work) has recorded particularly striking responses of beef cattle on northern Australian pastures to small amounts of protein supplement.
Where energy deficiency is the major limiting factor, high and low protein supplements will give a response proportionate to their energy content, but, for the reasons indicated below, the response per unit weight of input is far less than the response to a unit weight of protein supplementation in protein deficiency.
Thus maximal response per input of supplementary feed depends on assessment of the relative deficiency of energy and protein, and rational choice of supplements.
Determination of the relative deficiency depends largely on assessment of the effect of the density and uniformity of the pasture on intake, and of the ingestibility and protein content of the pasturage consumed (far from simple matters!) or experimental comparison of responses to supplements providing equal amounts of energy but high and low amounts of protein.
The depressing effect of supplementary energy on pasture intake
In a particular instance cattle may be maintaining weight on a low density — high quality pasture on which energy intake will be the major limiting factor. A target of 1 lb. gain per day might be decided on the theoretical energy requirement for this being about 2 lb. of starch equivalent per day above maintenance. This would be supplied by say 5 lb. of hay or 3 lb. of grain. In practice, feeding these amounts usually results in little or no increase in gain, due to a decrease in grazing time and pasture intake when the supplement is given (McClymont, 1956). However, where stock are rapidly losing weight due to energy deficiency, the response to a given amount of supplementary energy may approach the theoretical expectation, and if poor performance is due primarily to protein deficiency in the pasturage a small amount of protein supplement may give a marked response.
Observations such as these can only be given a rational explanation in the light of evidence which has led to the following conclusions:—
— that the activity of a central nervous system appetite centre is involved in the complex act of seeking and ingesting feed.
— that the activity of this centre is stimulated by the total energy demand for growth, activity, resisting cold stress,etc.).
— that activity of the appetite centre is inhibited by another centre, the satiety centre, which can be stimulated by a large number of different factors, the most important of which, for the appear to be energy intake, grazing fatigue, abdominal pressure (due largely to ruminal fill), and protein deficiency (McClymont, 1958).
On low density pastures grazing fatigue appears to be the major stimulus to the satiety centre — for even on very low density pastures, on which stock will eventually die, they will not extend their grazing time beyond about twelve or thirteen hours per day. On high density — low digestibility mature pasturage, ruminal fill and/or protein deficiency appear to be the major stimuli to the satiety centre, and on first class pasturage energy intake appears to be the major stimulus. On moderately low density and/or low digestibility pasturage a summation of stimuli from energy intake, grazing fatigue, ruminal fill and protein deficiency appear to limit intake.
Supplementary feeding provides, a stimulus to the satiety centre by the provision of energy and ruminal fill. Where energy intake and ruminal fill from pasture are already appreciable, as is the case with animals just maintaining weight on low density high quality pastures, the added stimuli to the satiety centre from the supplementary feeding is apparently sufficient to shorten the grazing time and so restrict pasture intake, so that a poor response to the supplementary feeding is recorded. The "supplementary" feeding is in fact then, largely "alternative" or "replacement" feeding. Of course if sufficient supplementary feed is given, major growth responses can be effected, but in these cases the situation approaches provision of supplementary grazing, rather than supplementary feeding.
Thus where energy deficiency is the major limiting factor, response to an input of supplementary feed is not predictable on the basis of feeding standards and in general is inversely proportional to the performance of the stock without supplementation.
Supplementary Feeding and Stocking Rates.
A corollary of the above is that supplementary feeding may allow stocking rates to be increased during periods of low pasture density or quality without increasing the grazing pressure i.e. the rate of pasture consumption. So that such feeding can be of greater interest from the point of view of increasing stocking rates to allow optimal economic use of the flush growth, without causing overgrazing during other periods, rather than through improving individual animal performance. (See Lloyd, 1959 for an analysis of economic problems involved).
Compensatory Growth, and Reproductive Efficiency.
When growth has been inhibited or weight lost due to under-nutrition, return to a high plane of nutrition is associated with hyper-normal appetite and hyper-normal, or "compensatory" growth. So that when good pasturage is again available after a period of poor pasturage, unsupplemented animals may, in a few months largely catch up in weight to animals supplemented during the period of poor pasturage. However, any loss of reproductive efficiency through under-nutrition, such as by low conception rates, resorption, prenatal lamb mortality, and ewe mortality is of course irreversible. Carcase quality is apparently little effected, as shown by experiments with identical thin beef calves, by up to nine months of growth cessation provided that compensatory growth is allowed. So that improvement in final carcass quality cannot be a major economic justification for supplementary feeding.
Thus unless the relatively expensive extra growth due to supplementary feeding on poor pastures has immediate, or eventually certain, economic and/or biological advantages, compensatory growth when pastures improve may quickly eliminate most or even all of the growth response obtained by the supplementary feeding and make the feeding operation uneconomic. Supplementary feeding for ensuring reproductive efficiency is more likely to give a permanent benefit.
Supplementary Feeding for Wool Production.
Supplementary feeding may be undertaken with the primary object of maintaining the rate of wool growth and so increasing the annual wool clip.
However, in all cases where data of input of feed and extra output of wool are available, the conversion ratio is so low, even with high protein supplements supplementing low protein feed, that it is highly unlikely, unless with a particularly favourable feed price/wool price ratio, to be economic. For example in the experiments of Franklin, Briggs and McClymont (1955) cited above an input of 24 lb. of linseed meal (assessed value about ten shillings) was required for an extra one pound output of wool.
Extra wool production can be a significant factor in assessing the economics of supplementary feeding but can rarely be a sufficient justification by itself.
Effects of cutting for conservation on pasture growth and subsequent animal performance.
In many discussions on the value of cutting surplus pasture growth for feeding back when pastures decline in density and/or quality, it is tacitly assumed that the cutting will have little or no effect on the subsequent growth of the pasture, and that the pasturage would have no value if left uncut. Under particular circumstances both of these assumptions may be unwarranted, particularly the latter. Thus Willoughby (1958) has shown that under some circumstances animal performance on pastures cut for conservation could be considerably depressed in comparison with animals on uncut pastures, with the result that there could be very little net gain in animal performance by cutting, conserving and feeding back.
Opportunity Costs.
Supplementary feeding, whether it be by conservation and feeding back or purchase of feed, is essentially an investment of capital with expectancy of profit. But profit alone may not justify it if there are alternative avenues of investment, particularly pasture improvement, watering sites, or irrigation, and which might yield a higher return and/or have a lower risk factor. Thus in much Tableland country it is fairly certain that limited capital is far better invested in pasture improvement than in fodder conservation machinery and conserved fodder.
GENERAL DISCUSSION
It can be seen that supplementary feeding, at first consideration a simple logical proposition, is in fact a highly complex biological and economic problem. So that rational decisions on such questions as whether to feed, when to feed, what to feed, and how to feed must depend on critical assessment of many factors such as the major nutritional deficiency of the stock, the present and possible future performance of the stock if not supplemented, the likely response per input of supplementary feed, the expectation of compensatory growth, the effect of cutting for conservation on the immediate and future food value of the pasture, and the comparative economic returns from supplementary feeding compared to other possible avenues of investment.
REFERENCES:
- Franklin, M. C., Briggs, P. K. and McClymont, G. L. (1955) "The Utilization of Low Quality Pasture" — J. Aust. Inst. Ag. Sci., 21: 216
- Lloyd, A. F. (1959) "Fodder Conservation in the Southern Tablelands Wool Industry". — Rev. of Mark. and Ag. Ec., 27: 5
- McClymont, G. L. (1956). "Response of Stock to Supplementary Feeding on Pastures". — Proceedings of the Australian Society of Animal Production, 1: 63
- McClymont, G. L. (1958) “Physiology of Regulation of Pasture Intake". — Proc. Aust. Agrostol. Conf., p.66, 1
- Norman, M. J. T. (1960) “Grazing and Feeding Trials with Beef Cattle at Katherine, N.T." — C.S.I.R.O. Div. of Land Res. & Reg. Survey, Techn. Paper No. 12
- Willoughby, W. M. (1958) "Conservation as an Aid to Increased Pasture Utilization". — Proc. Aust. Agrostol. Conf., p.54, 1