Readers' Rating:

Rate this article

The following article is a special collaboration from AFMA (Animal Feed Manufacturers Association) www.afma.co.za
We thank their kind support.

All the current nutritional recommendation systems for farm animals (and not only those relating to minerals) refer to an "average animal" under "normal" feeding conditions and in a state of good health. Actually, the progress in genetics and in standards and practices of nutrition and sanitation effectively mean that the "normal" conditions used for these recommendations are sometimes outdated. It does not seem impossible that some day one of the limits of these systems may be reached.

The factorial method is actually the most efficient one for the assessment of phosphorus requirements of animals. It consists of establishing net physiological requirements (maintenance, growth, pregnancy and lactation) and true absorption coefficients (TAC).

This classical approach has been and remains very useful in ruminant nutrition but it does not really take into account the variation of its parameters (for example there is generally only one value for TAC, or faecal endogenous losses). Moreover, cattle feeding must integrate other objectives such as the quality of animal products or the control of excreted elements in terms of pollution. In other words, we have to be able to predict the laws of responses of animals to the dietary variations; as a first step, a better assessment of the main parameters of the factorial approach (maintenance requirement and TAC) taking into account its variation, is needed.






Maintenance requirement

The net physiological requirement for maintenance is often assimilated with the endogenous faecal and urinary losses (the latter are often negligible for Ca and P in ruminants). It is convenient to consider this aspect from a relative standpoint. The endogenous faecal loss of P actually corresponds to an irreducible fraction that may represent the actual net maintenance requirement and also an excretory component that permits the organism to eliminate the P absorbed in excess of its requirement. This explains the quasi-linear relationship between the endogenous faecal loss and the level of P intake (Grace, 1981; Braithwaite, 1985; Ternouth, 1989; Scott et al, 1987; Scott et al, 1995).

The relationship between the endogenous faecal losses and P intake level presents one of the major difficulties in estimating a net maintenance level. It has resulted in widely varying values between systems in the past.

Other possible factors of variation can also affect this. These are strongly related to P recycling in the saliva, and, in particular, the non-reabsorbed fraction. In ruminants, large amounts of P return to the digestive tract via this route and may represent up to 2 or 3 times the quantity of P actually ingested (Durand & Kawashima, 1980; Scott & McLean, 1981). This recycling P is the major contributor of P for the microorganisms of the rumen.

The type of feed also plays a determining role in the saliva flow, both in terms of the proportion of forage/concentrate (Scott & Buchan, 1985) and in the granulometry (Scott & Buchan, 1987, Meschy & Peyraud, 1998).

It is now well established that the maintenance requirement of phosphorus is more closely related to the quantity of dry matter intake (DMI) than to the body weight (BW) of the animals (Braithwaite, 1983; Field et al, 1985; Chrisp et al, 1989; Ternouth, 1989). Due to insufficient experimental evidence, it is more difficult to take into account the effect of the type of feed.


True Absorption

The TAC values vary considerably for P (between 50 and 70%). These differences can be partly explained by the fact that the values given by some of the recommendation systems (INRA 1988 and NRC 1989) include a safety margin in this coefficient to take into account individual variations (the coefficient of variation for inter-and intra-trials is around 10% for P). AFRC 1991, however, used experimental results (Gu‚guen, 1962; Grace, 1981; Field et al, 1984; Scott et al, 1984) distinguishing between forage and concentrates. This was a new approach and was an initial step towards taking into consideration the variations in true P absorption. More work is needed to clarify certain points, e.g.: can the abrupt difference between the values for forage or concentrate be modified? In other words, would it be possible to take a weighted average of the TAC values of forage and concentrate? Should complementary inorganic sources be considered in the same way? What is the actual utilisation of phytate phosphorus by ruminants when the proportion of grain in their diet is large (30-50%)? There is only a small amount of contradictory data present in the literature relating to this last question.

The values of TAC for P and Ca in FAG 1994 and INRA 1988 vary somewhat over the production cycle in order to take into account a slight increase in absorption capacity of lactating animals. This has been shown in ewe (Braithwaite, 1983), but more experiments are needed to quantify this phenomenon in other more productive dairy species (cows and goats). In addition, while this adaptation does not seem to occur immediately in ewes, it was shown to have been more rapid in cows (Van´t Klooster, 1976).

Experimental results permit the identification and sometimes begin the quantification of a certain number of factors of variation for the endogenous losses and the true absorption. It should be possible to establish laws of animal responses to dietary variations given the volume of published data that exists (and also the vast amount of non-published material that remains stockpiled in laboratories) and to integrate these factors. Current computational methodologies permit such relationships to be taken into consideration in future requirement recommendations.
This will probably entail, if only for validation purposes, renewed investigations in this area, which has been curiously quiescent these past few years in many countries.

Another point that should be considered concerns the eventual goals of modern farming. In addition to the production pressure that has been felt for many years (and continues to be a real pressure today), farming is faced with an increasing number of new dilemmas to consider. Paramount among these are concerns about the environment, particularly in terms of controlling P-waste in farming run-offs.

The mechanistic modelling approach for mineral metabolism should respond, at least in the long term, to these questions. Short-term improvements in these recommendation systems probably depend on a better understanding of the parameters involved in the factorial method.


Dr. FRANCOIS MESCHY
-- INRA Laboratoire de Nutrition et d´Alimentation INA PG, 16, rue Claude Bernard F-75231 Paris


REFERENCES
Agricultural and Food Research Council (AFRC). 1991. Technical commitee on responses to nutrients, Report 6. A reappraisal of the calcium and phosphorus requirements of sheep and cattle. Nutrition Abstracts and revues (Series B). 61: 573-612.
Braithwaite, G. D. 1983. Calcium and phosphorus requirements of the ewe in pregnancy and lactation. 2. Phosphorus. British Journal of Nutrition. 50: 723-736.
Braithwaite, G. D. 1985 Endogenous faecal loss of phosphorus in growing lambs and the calculation of phosphorus requirements. Journal of Agricultural Science. 105: 67-72.
Chrisp, J. S.; Sykes, A. R. & Grace, N. D. 1989. Faecal endogenous loss of calcium in young sheep. British Journal of Nutrition. 61: 59-65.
Durand, M.;& Kawashima, R. 1980. Influence of minerals in rumen microbial digestion. In: Digestive physiology and metabolism in ruminants. pp 375-408 (Ed.) Ruckebush &. Thivend P. MTP Press Ltd., Lancaster
Field, A. C.; Williams, J. A.& Dingwall, R. A. 1985. The effect of dietary intake of calcium and dry matter on the absorption and excretion of calcium and phosphorus by growing lambs. Journal of Agricultural Science. 105(2): 237-243.
Grace, N. D. 1981. Phosphorus kinetics in sheep. British Journal of Nutrition. 45: 367-374.
Gu‚guen, L. 1962. Utilisation digestive r‚elle du phosphore du foin de luzerne par le mouton en croissance, mesur‚e … l´aide de 32P. Annales de Biologie Animale Biochimie Biophysiologie. 2: 143-149.
Gu‚guen, L. 1982. French recommended dietary allowances for calcium and phosphorus in cattle and sheep. 33rd Meeting of European Association for Animal Production; Leningrad, USSR .
Gu‚guen, L.; Meschy, F.& Durand, M. 1989. Recent progress in the assessment of phosphorus and calcium requirements of ruminants. International Meeting on Mineral Nutrition and Mineral Requirements in Ruminants; Kyoto, Japan. 123-126.
Institut National de la Recherche Agronomique (INRA). 1988. Alimentation des bovins, ovins et caprins. Ed. R. Jarrige, INRA, Paris.
National Research Council (NRC). 1989. Nutrient requirements of dairy cattle. Washington: National Academy Press.
Scott, D.& Buchan, W. 1985. The effect of feeding either roughage or concentrate diets on salivary phosphorus secretion, net intestinal absorption and urinary excretion in the sheep. Quarterly Journal of Experimental Physiology. 70: 365- 375.
Scott, D.& Buchan, W. 1987. The effect of feeding either hay or grass diets on salivary phosphorus secretion, net intestinal absorption and on the partition of phosphorus between urine and faeces in the sheep. Quarterly Journal of Experimental Physiology. 72: 331-338.
Scott, D.& McLean, A. F. 1981. Control of mineral absorption in ruminants. Proceedings of the Nutrition Society. 40: 257- 266.
Scott, D.; McLean, A. F.& Buchan, W. 1984. The effect of variation in phosphorus intake on net intestinal phosphorus absorption, salivary phosphorus secretion and pathway of phosphorus excretion in sheep fed roughage diets. Quarterly Journal of Experimental Physiology. 69: 439-452.
Scott, D.; Rajaratne, A. A. J.& Buchan, W. 1995. Factors affecting faecal endogenous phosphorus loss in the sheep. Journal of Agricultural Science. 124: 145-151.
Station F‚d‚rale de Recherches sur la Production Animale (FAG). 1994. Apports alimentaires recommand‚s et tables de la valeur nutritive des aliments pour les ruminants. Zollikofen: LMZ.
Ternouth, J. H. 1989. Endogenous losses of phosphorus by sheep. Journal of Agricultural Science. 113: 291-297.
Van´t Klooster, A. Th. 1976. Adaptation of calcium absorption from the small intestine of dairy cows to changes in the dietary calcium intake and at the onset of lactation. Zeitschrift Tierphysiologie Tierern„hrung Futtermittelkunde. 37: 169- 182.

Readers' Rating:

Rate this article