Jean Noblet, Bernard Sève and Catherine Jondreville

Foreword

The concepts and data used for evaluating feeds for pigs in FeedTables.com are similar to those used in the 2002-2004 INRA-AFZ Tables by J. Noblet, B. Sève and C. Jondreville (Sauvant et al., 2004) and presented in the latter document. The present text includes additional references published after 2004, either produced at INRAE (Noblet and van Milgen, 2004; Noblet and van Milgen, 2013) or from other laboratories, with major contributions of MAFIC (Chinese Agricultural University, Beijing, PRC; D.F. Li and collaborators) and the Department of Animal Sciences at University of Illinois (Urbana; USA; H.H. Stein and collaborators). The reference to the EvaPig software published in 2008 (www.evapig.com) is also used; this free software implements the concepts and some additional equations for recalculating the nutritional values of a new ingredient according to its actual chemical composition, listed or not in the feeding tables.

Energy value

The estimation of the energy value of feed ingredients for pigs requires several steps. The first one is the estimation of digestible energy (DE), calculated as the gross energy multiplied by the apparent faecal digestibility coefficient for energy (Ed). This coefficient varies according to the characteristics of the feeds but also with the live weight of the pig. Two main physiological statuses were considered: the 50-70 kg growing pig (the data can be applied to fast growing animals between 10 to 150 kg live weight) and the adult sow (the results can be used for both gestation and lactation) (Le Goff and Noblet, 2001). The energy losses in urine are calculated using the amount of nitrogen excreted in the urine and the losses in the form of gas from degraded cell walls; the latter energy loss differs between the two physiological statuses used to estimate DE. The metabolisable energy content (ME) is the difference between the DE value and the energy losses in urine and gas. The net energy (NE) value is estimated using the equations proposed by Noblet et al. (1994) which can be applied to both the growing pig and the sow (Noblet et al. 1994).

Estimation of the digestibility of energy and nutrients

Growing pig

Energy digestibility (Ed) was estimated using prediction equations specific for each feed material. These equations used one or two chemical characteristics that were variable enough and able to discriminate between different feedstuffs. Establishing those equations was achieved using literature values and unpublished INRA data. However, for the majority of feed materials, there were not enough original digestibility values available for a single ingredient, and we had to group the data from feed materials having similar characteristics, such as botanical origin and anatomical structure. For example, the data from wheat and its by-products (bran, shorts, middlings, gluten feed, wheat distillery by-product, etc…) were combined (n = 52) and the Ed was calculated using cell wall constituents (crude fibre, NDF or ADF) as predictors. This method is illustrated in Noblet and Le Goff (2000) for wheat and maize products. Similar equations were established for the protein digestibility coefficient (Nd). These equations are reported by Noblet et al. (2003) and available in digital format on the EvaPig website at https://www.evapig.com/documents.

However, for several (families of) ingredients given in the tables, there were insufficient or no data in the literature or the results had been obtained using products that had very similar compositions (i.e., no variability). It was then impossible to establish specific prediction equations for Ed and Nd based on chemical composition. Therefore, we used either the average values calculated from literature data - if the results were consistent – or, in the case of DE, values predicted by the following global equation (Le Goff and Noblet, 2001 and Noblet, unpublished data; n= 77 diets):

DE = 0.2247 CP + 0.3171 EE + 0.1720 Starch + 0.0318 NDF + 0.1632 Residue (RSD = 0.35)

DE is expressed in MJ/kg dry matter; CP (crude protein), EE (ether extract), NDF, starch and Residue are expressed in % dry matter. Residue corresponds to the difference between the quantity of organic matter and the sum of the other constituents used in the equation.

For some feed materials, none of the previously described methods could be used, so we chose a likely value. In all cases, Ed or DE content for each feed material were calculated using the chemical characteristics published in the tables. In addition, when several equations to predict Ed or Nd were obtained, the estimate provided by the most precise equation(s) was used.

The faecal digestibility coefficients for starch and sugars are considered to be equal to 100%, both in the growing pig and the reproductive sow. There is a paucity of data for faecal digestibility of fats (EEd) in the literature and the values found are sometimes incoherent and quite imprecise for products with less than 5% fat, as are the majority of the feed materials in the tables. Except for fat sources (oils and fats, see below), we then decided to predict the digestible fat content (DEE) from an equation established by Le Goff and Noblet (2001) using 77 diets. EEd corresponds to the ratio between DEE and the fat content (x 100). The following equation was used:

DEE = 0.82 EE – 0.02 NDF – 0.7 (RSD = 0.33)

where DEE, EE and NDF are expressed in % dry matter. This equation gives very low EEd values (which can even be negative) for products with low levels of fat.

For numerous reasons, there are few reliable data concerning cell wall digestibility in the pig. Therefore, it was not possible to estimate directly the digestibility of this fraction. The indirect method used consisted in estimating the faecal digestibility coefficient of organic matter (OMd) or the content of digestible organic matter (DOM). Firstly, a residue (Res) which corresponds to the difference between organic matter and the sum of crude protein, ether extract, starch and sugars was calculated. Secondly, a digestible residue (DRes), equal to the difference between the DOM content and the sum of DCP, DEE, starch and sugars (calculated according to the methods described above) was also calculated. The components Res and DRes are theoretically equivalent to the cell walls and the digestible cell walls fractions, respectively. OMd was estimated using the following equation (Noblet, unpublished data, n = 270 diets):

OMd = 7.0 + 0.955 Ed – 0.05 DCP – 0.03 DEE (RSD = 0.4)

The following equation has the same precision as the previous one:

OMd = 7.9 + 0.915 Ed + 0.031 (Starch + Sugars) (RSD = 0.4)

OMd and Ed are expressed in %; DCP, DEE, starch and sugars are expressed in % of dry matter.

For all feed materials with high fat content (oils and fats), the EEd, Ed and OMd were assumed to be 85%, both in the growing pig and the adult sow. This value is the same as the average of the literature values and does not take into account the potential (but unlikely) differences in digestibility associated with the degree of fatty acid unsaturation. However, it cannot be used for products rich in free fatty acids (e.g. acid oils) for which the EEd (and the Ed) are much lower. Finally, the Ed of synthetic amino acids was fixed at 100% and the DE value was therefore considered to be the same as the gross energy concentration of the pure amino acid.

Adult sow

It is stated in the literature that energy digestibility is higher for adult sows than for growing pigs. This effect depends on the quantity and botanical origin of cell walls and it clearly justifies the choice of two distinct energy values for feedstuffs (Le Goff and Noblet, 2001). However, due to a shortage of literature data, energy digestibility for the sow cannot be estimated by regression, unlike for the growing pig. In addition, the few available data do not necessarily correspond with the feed materials defined in the tables. The approach described by Le Goff and Noblet (2001) in which the DE content for the sow is estimated using the DE content measured or estimated in the growing pig was possible for some families of feed materials (wheat, maize and soybean; Noblet and Le Goff, 2000 and Le Goff and Noblet, 2001). However, such equations were not available for all the feed materials in the tables. In addition, there was a risk that a bias would be introduced if the same equation were used for all feed materials.

A further analysis of the data used in the publication of Le Goff and Noblet (2001) shows that the difference in DE content between the sow and the growing pig is directly proportional to the level of indigestible organic matter in the growing pig. In their study, concerning 77 diets, an increase in the DE concentration per g of indigestible organic matter in the growing pig (DEdiff) was on average 4.2 kJ per g. This extra 4.2 kJ of DE is associated with an additional supply of 0.195 g of DOM, made up of 0.058 g DCP and 0.137 g DRes. However, a comparison of digestibility measurements in the sow and growing pig shows that the DEdiff varies according to the (families of) feed ingredients (Noblet et al., 2003 and unpublished data). For example, DEdiff is 2.9 kJ for wheat products compared to 7.5 and 8.0 kJ for soybean and maize products, respectively. The data obtained by INRA for about 50 feed materials (Noblet et al., 2003 and unpublished data) made possible to estimate DEdiff for all the products in the tables (values vary between 0 to 8.4). It has also allowed the calculation of the differences in DE, DOM, DCP and DRes contents between the adult sow and the growing pig using the level of indigestible organic matter in the growing pig (as has been previously defined). It was assumed that the amount of DOM per kJ (0.047 g/kJ) and the repartition of the surplus DOM between DCP and DRes were constant whatever the value of DEdiff. The levels of DE, DOM, DCP and DRes in the adult sow were then obtained by adding the calculated differences to the levels of DE, DOM, DCP and DRes estimated in the growing pig. It was assumed that the digestibility coefficients for fat, starch and sugars are identical for the growing pig and the adult sow.

Estimation of ME content

As indicated in the introduction, the energy losses in urine (Euri) and in fermentation gases (methane; Egas) were taken into account in the calculation of the ME content of feed materials. An analysis of the data obtained in 50-70 kg growing pigs and in the adult sow (n = 610; Noblet, unpublished data; Noblet and van Milgen, 2004) showed that Euri (MJ/kg ingested dry matter) depends on the quantity of nitrogen measured in the urine (Nuri; g/kg of ingested dry matter). The prediction equations are:

Growing pig: Euri = 0.19 + 0.031 Nuri (RSD = 0.05)

Adult sow: Euri = 0.22 + 0.031 Nuri (RSD = 0.05)

The quantity of nitrogen excreted in urine is directly proportional to the difference between the daily supply and the capacity of the pig to fix nitrogen in the form of protein. We can assume that for most stages of pig production, when the protein supply has a correct amino acid balance and meets the animal's requirements, close to 50% of digestible nitrogen is fixed or the quantity of nitrogen found in the urine represents 50% of digestible nitrogen. This assumption was applied to each feed material and for the level of DCP (N x 6.25) estimated according to the methods described above. For simplification, it could also be assumed that Nuri is close to 40% of N in the feed material with Nd averaging 80%.

The quantity of energy lost in the form of gas (Egas) was calculated using the quantity of fermented cell walls. This was considered to be equal to the DRes value obtained in the nutrient digestibility method. The compilation of data obtained in respiration chambers (Le Goff, 2001) allows estimations of Egas equal to 0.67 and 1.34 kJ per g of DRes in the growing pig and the adult sow, respectively.

For feed materials containing neither cell walls nor crude protein (oils and fats), this method produces a ME value which is very close to that of DE, as observed in animal experiments. The synthetic amino acids generally represent a limiting factor for nitrogen retention and it can be supposed that the retention coefficient for the nitrogen supplied by these amino acids is higher than that for total nitrogen. We have estimated it to be 65% when calculating their ME values.

Estimation of NE content

The NE content of feedstuffs has been estimated using equations established by Noblet et al. (1994) with 61 diets. Three equations were preferentially used:

NE2 = 0.121 DCP + 0.350 DEE + 0.143 Starch + 0.119 Sugars + 0.086 DRes (RSD = 0.25)

NE4 = 0.703 DE + 0.066 EE + 0.020 Starch – 0.041 CP – 0.041 CF (RSD = 0.18)

NE7 = 0.730 ME + 0.055 EE + 0.015 Starch – 0.028 CP – 0.041 CF (RSD = 0.17)

NE, ME and DE are expressed in MJ/kg dry matter. The chemical constituents are expressed in % dry matter.

Equation NE2 is actually a variant of the equation NE2 proposed by Noblet et al. (1994), as the “Weende” analysis was not used here to define the values of digestible elements. In practice, the NE value given in the tables is the average of the three NE values obtained using the above equations and applied to the feed materials for which the chemical characteristics are given in the tables. The values of digestible nutrients or DE or ME were obtained using the methods described above. For sources of fat (oils and fats) and feed materials that contain practically only starch (maize starch), equation NE2 was used to calculate the NE value. In the case of synthetic amino acids, it was assumed that the efficiency of ME use was 85% for the fraction fixed in body protein (65% of DE) and 60% for the fraction which was deaminated (35% of DE).

Conclusion

The approach proposed for the calculation of the energy values of feedstuffs for pigs generates six energy values appropriate to the physiological status of the animal – growing pig and adult sow – according to three different systems (DE, ME and NE).

The NE system should be preferred, because it results in the estimation of an energy value which is the closest to the “true” value and thus allows the formulator to differentiate more precisely between feed materials when calculating diets. Finally, it should be noted that the NE value of a feedstuff is highly dependent on its DE and ME values, which are themselves dependent on the chemical characteristics of the feed, the animal that consumes the feedstuff and the technology used (milling, granulation etc…) to produce the diet. The values given in the tables are principally for ground feeds, rapeseeds being the sole exception with table values given for pelleted rapeseed as the non-pelleted form has a very low digestibility (Skiba et al., 2002). In general, pelleting improves energy and nutrient digestibilities (Noblet and van Milgen, 2004; Le Gall et al., 2009). However, literature data are insufficient to propose, for all the materials used in pig feeding, energy values that take into account the different types of processing, in particular pelleting.

Nutritional value of proteins and ileal digestibility of amino acids

The nutritional availability of amino acids (AA) can be estimated by measuring their digestibility at the end of the small intestine, or ileum. Indeed, in the large intestine, AA are not absorbed and microorganisms can metabolise some undigested amino acids, which prevents them from appearing in the faeces. Therefore, “ileal” digestibility is used and fecal digestibility of AA has no meaning (Stein et al., 2007). The data for ileal digestibility of AA given in the tables are derived from experiments started in the early 1980s and all conducted in France by Adisseo, by Arvalis and Ajinomoto Animal Nutrition and by INRA (Rennes). These data obtained from 1987 to 1997 in the three groups were collated between 1996 and 1999, combined and analysed, and published in the AmiPig software in 2000 (AFZ et al., 2000). Measurements were conducted on most cereals grains (wheat, maize, barley, sorghum, rye, triticale, oats) and their by-products (wheat bran, maize and wheat by-products from starch and ethanol industries, etc.), legumes (pea, faba bean, lupin), oilseeds (soybean, rapeseed), oilseed meals (soybean, sunflower, cotton seed, etc.), animal and dairy products (fish meal, meat and bone meals, whey, etc.), protein concentrates (soybean, potatoes), etc. In total, 350 samples were measured representing 62 different ingredients.

Ileal digestibility can be determined in pigs fitted with an ileal cannula, after measuring the concentrations of an indigestible marker, or in pigs with an ileo-rectal anastomosis (IRA), after collecting the totality of the ileal output. The data presented in the tables were obtained using the termino-terminal ileo-rectal anastomosis technique, validated by Laplace et al. (1994), where the large intestine is completely isolated. The way in which ileal digestibility is expressed depends on how the endogenous losses have been taken into account in the calculations (Sève, 1994) and “apparent” and “standardised” digestibility values can be calculated. But "apparent" values are rather difficult to interpret and are not additive for ingredients in a diet. A consensus proposed by Stein et al. (2007) consisted then to prefer "standardized" values. This concept has been universally adopted in the literature and in feeding tables published since the 2000s.

The concept of biological value proposed by H. H. Mitchell in the 1920s distinguished the nitrogen losses due to dietary proteins from the endogenous losses due to maintenance requirements or basal endogenous losses. These basal losses are independent from the composition of the feedstuff, not proportional to the quantity of protein ingested but proportional to the total quantity of dry matter ingested (Figure 1).

Figure 1. Effect of the quantity of ingested amino acid on the ileal flow of amino acids, at a constant level of dry matter intake

By subtracting these losses from the measured indigestible fraction, the "true" digestibility as defined by H. H. Mitchell, and commonly called "standardized" (ileal) digestibility (SID) is calculated. Unlike "apparent" digestibility values, SID values are additive (Furuya and Kaji, 1991). In addition, unlike energy values, the AA digestibility values are not by affected by the metabolic status of the animal; the values obtained in growing pigs can then be applied to reproductive sows.

The SID values depend on the estimation of basal endogenous losses of N or of each AA. If the animals do not receive the diet for a long period, the use of a protein-free diet is the most appropriate method to measure these losses. But it has been shown that the basal endogenous losses, when measured using protein-free diets of similar composition (Sève et al. 2000), depend on the laboratory where they were measured.

Table 1. Basal endogenous losses (g/kg ingested dry matter) in the three laboratories that produced the digestibility data used in the tables.

ND = not determined

The tables provide the averages of measurements taken at three different laboratories. Each feed material was usually the only protein source in the diet. For each individual SID value, the apparent ileal digestibilities of the diet (AID in %) and the basal endogenous losses of each site (EndobDMIsite), expressed in g/kg of ingested dry matter (tab. 1) and the AA content of the diet (AADietDM), expressed in % of dry matter were required. The following equation was used: SID = AID + (EndobDMIsite x 10/AADietDM). It is assumed that the SID of AA, for a given ingredient, is constant whatever the CP and AA level in the ingredient and is not changed by changes of the chemical composition (Dietary fibre, starch, ash, etc.) of the ingredient when compared to the ingredient(s) that were measured in the experiments. Finally, for ingredients without known SID values of their AA, default values are proposed; they correspond to average values obtained on a large number of diets at INRA in the 1990s (http://www.evapig.com).

Phosphorus digestibility

The principle used for the calculation of the “phosphorus value” of a feed material is its digestible phosphorus concentration. It is calculated by multiplying the total phosphorus concentration by the apparent faecal digestibility coefficient of phosphorus. The digestibility coefficients were obtained in most cases from published or unpublished results produced during the 1990s by Arvalis – Institut du Végétal (formerly ITCF) using pigs weighing approximately 45 kg (Barrier-Guillot et al. 1996; Chauvel et al. 1997; Skiba et al. 2000). We have also used additional data from the literature based on the same concept (Jongbloed et al. 1993; Jongbloed et al. 1999) and from more recent publications. In some cases, due to the lack of recent reliable references, the digestibility coefficient can be absent from the tables.

In some feed materials, the presence of endogenous phytase causes a problem concerning the additive nature of digestible phosphorus values calculated in this way. The endogenous phytase found in a feed material can increase not only the digestibility of its phytate phosphorus but also the digestibility of the phytate phosphorus found in the other diet ingredients. This is why two values for apparent faecal digestibility are given for feed materials with a significant endogenous phytase activity (wheat and its by-products, rye, barley and triticale). The first value (Pd) corresponds to the feed material when phytase has been denatured, e.g. by heating. The second value (PPhyd), which is higher, corresponds to the same feed material in cases where it is processed in a way that does not affect phytase activity, milling for instance. Only the first value allows the calculation of additive digestible phosphorus concentrations; the second value only gives an indication of phosphorus digestibility.

Two steps are therefore necessary in order to estimate the concentration of apparent digestible phosphorus in a diet. In the first step, apparent digestible phosphorus is estimated in a diet made up of feed materials where the phytase has been denatured. This is done by multiplying the phosphorus content of each feed material by its apparent faecal digestibility. The second step is to take into account the phytase activity of the diet by adding to the previously calculated value an estimation of the quantity of apparent digestible phosphorus released by the phytase present in the diet. The second step is problematic for several reasons. Firstly, the phytase activity in a given feed material is variable. Secondly, the phytase present in a diet is sensitive to any technological treatments it has undergone. Finally, in the case of plant phytase, the estimation of a relationship between phytase activity and the level of apparent digestible phosphorus remains difficult in the light of present knowledge.

References

• AFZ, Ajinomoto Eurolysine, Aventis Animal Nutrition, INRA, ITCF, 2000. Ileal standardised digestibility of amino acids in feedstuffs for pigs (AmiPig). Association Française de Zootechnie Ed., Paris (France). www.feedbase.com
• Barrier-Guillot B., Casado P., Maupetit P., Jondreville C., Gatel F., 1996. Wheat phosphorus availability: 2. In vivo study in broilers and pigs; relationship with endogenous phytasic activity and phytic phosphorus content in wheat. J. Sci. Food Agric., 70, 69-74. DOI: 10.1002/(SICI)1097-0010(199601)70:1<69::AID-JSFA466>3.0.CO;2-C
• Chauvel J., Granier R., Jondreville C., Williatte I., 1997. Use of isophosphorus diets differing in phytase activity: effects on phosphorus digestibility, growth performance, bone resistance and phosphorus excretion in growing-finishing pigs. 48th Annual Meeting of the European Association of Animal Production (EAAP), Vienna, Austria, 25-28 August 1997.
• Furuya S., Kaji Y., 1991. Additivity of the apparent and true digestible amino acid supply in barley, maize, wheat or soya bean based diets for growing pigs. Anim. Feed Sci. Technol., 32, 321-331. DOI: 10.1016/0377-8401(91)90036-R
• Hess V., Sève B., 1999. Effect of body weight and feed intake level on basal endogenous losses in growing pigs. J. Anim. Sci., 77, 3281-3288. DOI: 10.2527/1999.77123281x
• Jongbloed A. W., Everts H., Kemme P. A., 1993. Phosphorus availability and requirements in pigs. In : Recent developments in pig nutrition, 2nd edition, Cole D.J.A., Haresign W., Garnsworthy P.C. (Eds), Nottingham University press, Loughborough, UK, 163-178.
• Jongbloed A. W., Everts H., Kemme P. A., Mroz Z., 1999. Quantification of absorbability and requirements of macroelements. In: Quantitative biology of the pig, Kyriazakis I. (Ed), CAB International, Edinburgh, UK, 275-298.
• Laplace J. P., Souffrant W. B., Hennig U., Chabeauti E., Février C., 1994 Measurement of precaecal dietary protein and plant cell wall digestion in pigs; comparison of four surgical procedures. Livest. Prod. Sci., 40, 313-328. DOI: 10.1016/0301-6226(94)90098-1
• Le Gall M., Warpechowski M, Jaguelin-Peyraud Y., Noblet J., 2009. Influence of dietary fibre level and pelleting on digestibility of energy and nutrients in growing pigs and adult sows. Animal 3: 352-359. DOI: 10.1017/S1751731108003728
• Le Goff G., 2001. Etude des mécanismes impliqués dans l'évolution de la digestion et de l'utilisation métabolique des fibres alimentaires au cours de la vie du porc ; conséquences sur l'estimation de la valeur énergétique des aliments. Thèse de Doctorat de l'Ecole Nationale Supérieure Agronomique de Rennes.
• Le Goff G., Noblet J., 2001. Comparative total tract digestibility of dietary energy and nutrients in growing pigs and adult sows. J. Anim. Sci., 79, 2418-2427. DOI: 10.2527/2001.7992418x
• Noblet J., Shi X. S., Fortune H., Dubois S., Lechevestrier Y., Corniaux C., Sauvant D., Henry Y., 1994. Teneur en énergie nette des aliments chez le porc: mesure, prédiction et validation aux différents stades de sa vie. Journées Rech. Porcine en France, 26, 235-250. Journées de la recherche porcine.
• Noblet J., Fortune H., Shi X. S., Dubois S., 1994. Prediction of net energy value of feeds for growing pigs. J. Anim. Sci., 72, 344-354. DOI: 10.2527/1994.722344x
• Noblet J., Le Goff G., 2000. Utilisation digestive et valeurs énergétiques du blé, du maïs et leurs co-produits chez le porc en croissance et la truie adulte. Journées Rech. Porcine en France, 32, 177-184. Journées de la recherche porcine
• Noblet J., Bontems V., Tran G., 2003. Estimation de la valeur énergétique des aliments pour le porc. INRA Prod. Anim., 16: 197-210. INRAE
• Noblet J., van Milgen J., 2004. Energy value of pig feeds: Effect of pig body weight and energy evaluation system. J. Anim. Sci. 82 Number 13, E. Suppl., E229-E238. DOI: 10.2527/2004.8213_supple229x
• Noblet J., van Milgen J., 2013. Chapter 2: Energy and Energy Metabolism in Swine. In Sustainable Swine Nutrition, Ed. L.I. Chiba, John Wiley & Sons, Ames, Iowa, USA, pp. 23-57. DOI: 10.1002/9781118491454.ch2
• Sauvant, D., Perez, J.-M., & Tran, G. (2004). Tables of composition and nutritional value of feed materials. Pigs, poultry, cattle, sheep, goats, rabbits, horses, fish. Paris: INRA Editions. DOI: 10.3920/978-90-8686-668-7
• Sève B., 1994. Alimentation du porc en croissance: intégration des concepts de protéine idéale, de disponibilité digestive des acides aminés et d’énergie nette. Prod. Anim., 7, 275-291. HAL Archives ouvertes
• Sève B., Tran G., Jondreville C., Skiba F., Van Cauwenberghe S., Bodin J.C., Langer S. 2000. Measuring ileal basal endogenous losses and digestive utilisation of amino acids through ileo-rectal anastomosis in pigs: ring test between three laboratories. In: Digestive physiology of pigs, J.E. Lindberg and B. Ogle Eds, Proceedings of the 8th. International symposium on digestive physiology in pigs, 2000/06/20-22, Uppsala (SWE), p. 195-197, CABI Publishing, Oxon, UK. DOI: 10.1079/9780851995175.0195
• Skiba F., Hazouard I., Bertin, J. M., Chauvel J., 2000. Digestibilité du phosphore de 14 matières premières et influence de la phytase végétale dans l’alimentation du porc charcutier. Journées Rech. Porcine en France,. 32, 169-175. Journées de la recherche porcine
• Skiba F., Noblet J., Callu P., Evrard J., Melcion J.P., 2002. Influence du type de broyage et de la granulation sur la valeur énergétique de la graine de colza chez le porc en croissance. Journées Rech. Porcine en France 34: 67-74. Journées de la recherche porcine
• Stein H.H., Sève B., Fuller M.F., Moughan P.J., de Lange C.F.M., 2007. Invited review: amino acid bioavailability and digestibility in pig feed ingredients: terminology and application. J. Anim. Sci. 85: 172-180. DOI: 10.2527/jas.2005-742