SACCHAROMYCES CEREVISIAE PREPARATIONS IN THE FEEDING OF COWS AND THEIR EFFECT ON MILK YIELD AND COMPOSITION AS WELL AS RUMEN MICROORGANISMS

Aleksander Dobicki¹, Jerzy Preś², Andrzej Zachwieja¹, Roman Kwaśnicki^3
1 Institute of Animal Breeding, Faculty of Biology and Animal Science, Wrocław University of Environmental and Life Science, Poland
² Department of Animal Nutrition and Feed Management, Faculty of Biology and Animal Science, Wrocław University of Environmental and Life Science, Poland
³ Institute of Animal Breeding, Wrocław University of Environmental and Life Sciences, Poland

ABSTRACT

The aim of present investigations was to determine the effect of dried yeasts and a preparation isolated from cell membranes of beer yeasts (MOS) on the yield and composition of milk from Polish Holstein-Friesian cows during the first 100 days of lactation (variety Red-and-White), fed according to the PMR system. The development of the rumen microflora was also taken into consideration. The experiment was conducted on 75 Polish Holstein-Friesian cows maintained in tied stalls. The cows calved during the summer season. The experimental design included two experimental and one control group (Table 1). The number of protozoa and bacteria were determined in the rumen fluid of 8 cows selected at random from each group at the end of the experiment (24 animals in total). The addition of dried beer yeast preparations (dried yeasts – 80 g per head per day and extract of yeast cell membranes – 20 g MOS) to the feed of high-producing dairy cows resulted in a statistically significant increase in milk yield in comparison to animals of the control group, (by 3 and 2.2 kg milk per day). A significant (by 34-39%) increase was observed in the number of bacteria in the rumen fluid, what indicates that the feed additives used lead to a more efficient utilisation of fibre from the daily ration, improved the energy balance and the use of nutrients for milk production with a simultaneous decrease of the Somatic Cell Count (SCC). These results confirm that the yeast preparations offered positively stimulated the immunological system.

Key words: cows feeding, milk composition, rumen microorganisms, yeast culture.

INTRODUCTION

Bacterial and fungus probiotics are used in the nutrition of cows and calves [21, 23]. During the early period of feeding milk or milkroplacers it is live bacteria cells that are most often used as a precaution against scours. In turn, during the adaptation of the rumen to the process of digestion, most often priobiotics are used, as for instance live beer yeast cells (Saccharomyces cerevisiae), because they stimulate the development of microflora. Probiotics lead to complex, morphological changes in the digestive tract of cattle, changes more pronounced that in monogastric animals or birds [23]. Fungus probiotics increase the consumption of feed and growth rate of calves and lambs [10, 23, 33].

In the feeding of dairy cows are used preparations of beer yeasts, containing beside live yeast cells also other components [11, 27, 36, 38]. An increase was observed of feed consumption, milk yield during the first 100 days of lactation, fat content in milk, breakdown of organic matter and protein in the rumen and a higher share of bacterial N in the duodenum. The results were better with a higher level of NDF in the ration. The supplement had no effect on the milk composition and somatic cell count [11]. Similar results when testing an identical preparation were obtained in Poland [24]. A turning point in the studies on beer yeast cultures are the works of Öztűrk et al. [29], which demonstrated that live and dead (autoclaved) yeast cells have an almost identical effect on the metabolism of proteins and carbohydrates in the rumen – increase of LKT, propionic acid, N-NH₃ and synthesis of microbial protein [32]. Those studies open up new possibilities of using dried beer yeasts (dead cells) in place of yeast cultures.

An addition of beer yeast cultures leads to a change of nitrogen metabolism in the rumen by active peptides [17, 18, 19]. Erasmus et al. [15] stated that beer yeasts clearly increase the flow of bacterial nitrogen into the small intestine. This phenomenon may be to an extent explained by the occurrence of partial defaunation [13, 16]. Only a few works performed on dairy cows referred to the use of components of yeast cell membranes (beta-glucanes or mannanes). Field experiments, conducted by Christensen [9] and Devegowd and Chennegowd [12] demonstrated a unit increase of the cows’ daily milk field from 0.5 kg to 1.6 kg, when offering preparation Mykosorb (Alltech). When offering yeast cultures to dairy cows as a rule the blood biochemical and physiological components were not determined and neither was the immunomodulating effect.

Mannano-oligo-sacharides (MOS) are complex carbohydrates, isolated from the cell membranes of beer yeasts. Only the cell membranes of yeasts of the Saccharomyces cerev-isiae species contain the valuable and highly effective mannanes and β-glucanes. Mannanes are linked in the cell membrane with glycoproteins, lipids, chitin and numerous other biologically active substances. Beta-D-glucanes which absorb mycotoxins of field and store-room fungus, are components of yeast cell membranes of special importance for dairy cows [18]. A work by Jouan et al. [21] and Yoon and Stern [39] explains the mechanism of the absorption of two mycotoxins: aflatoxin B and Zearalenon, by a beta-glucane helisa (1-3) and (1-6). It consists of a structural, electrostatic and hydrophobic complementarity with hydrogen and van der Vals bonds.

The present investigations aimed at determining the effect of dried yeasts and a preparation isolated from cell membranes of beer yeasts (MOS) on the yield and composition of milk from Polish Holstein-Friesian cows during the first 100 days of lactation (variety Red-and-White), fed according to the PMR system. The development of the rumen microflora was also taken into consideration.

MATERIAL AND METHODS

Material: the experiment was conducted on 75 Polish Holstein-Friesian cows (variety Red- -and-White), maintained in tied stalls. The cows calved during the summer season. The experimental design included two experimental and one control group (Table 1). The number of protozoa and bacteria were determined in the rumen fluid of 8 cows selected at random from each group at the end of the experiment (24 animals in total).

Nutrition: the basic ratio (PMR), distributed twice a day by a feeding wagon, was calculated to meet the requirements for the production of 15 liters of milk per day; it contained forages and concentrates of the following joint nutrition value: 68 MJ NEL, 1085 g crude protein, 80 g Ca, 30 g P and 2.5 kg fiber (NDF). A higher milk yield was rewarded by an additional dose of concentrate (1 kg concentrate per 2 kg milk) containing in 1 kg 6.53 MJ NEL, 174 g crude protein, 2.15 g Ca and 7.06 g P. Together with the concentrate the cows were offered yeast preparations (Table 1).

Table 1. Experimental arrangement and feed rations

Groups	n	Body weight of cows, kg,		Additives supplemented g per head per day1)	Basic PMR feed (ad libitum) up to 15 kg milk per day4)	PREMIUM: 1 kg concentrate per every further 2 kg milk
		x	δ
Dried beer yeasts Leiber® BT	25	577.8	65.5	200 g 2), including 80 g yeast	sugar beet leaves (silage): 3 kg; silage (grass+alfalfa): 10.5; corn silage (whole plants): 23.5; concentrate: 2 kg	corn meal (25%), solvent extracted rapeseed oilmeal (25), wheat bran (25), barley meal (25%)+ mineral mixture IN-R 18 PLUS (150 g head/day) 5)
MOS Biolex® MB 40	25	580.1	66.5	50 g 3), including 20g MOS
Control	25	582.8	79.1	No additives
Total cows	75	580.2	70.4	-

1) Supplements were offered to experimental cows 7 days before the expected calving date. 2) Leiber® BT contains 40% yeasts; chemical composition (%): crude protein 31,0; crude fat 7.0; crude fiber 9.5 and crude ash 6.5; amino acids (%): lisin 1.8, methionin 0.6, cystin 0.6, tryptophan 0.40, threonine 1.40; mineral components (mg/100 g): Ca 230.8, P 1038.4, Mg 153.5, Fe 7.95, Mn 0.45, Zn 44.5, Cu 0.9, Na 25.0. 3) Biolex® MB 40 contains 40-50% Mannans and ß-Glucans; chemical composition (%): mannanes 20-25, (1.3)-(1.6)-β-D-Glucanes 25-30, crude protein 25, crude fat 7.0, crude fiber 0.5 and crude ash 4.0; amino acids (%): lisin 2.00, methionin 0.55, cystin 1.00, tryptophan 0.20, threonine 1.60; mineral components (mg / 100 g): Ca 0.20, P 0.45, Na 0.12. 4) PMR nutrition value (1 kg dry matter): 5,23 MJ NEL, 84 g crude protein, 6,15 g Ca and 2,30 g P. 5) PREMIUM nutrition value (1 kg): 6.53 MJ NEL, 174 g crude protein, 2.15 g Ca and 7.06 g. P;

Feed analyses were performer at the laboratories of the Wrocław Agriculture University.

The feeding of cows and the nutritive value of the ration offered was controlled once a month, the routine analyses being performed at the Laboratory of the Animal Nutrition Department (components and nutritive value).

Vitamins in the yeast preparations were determined at the Department of Fruit, Vegetable and Grain technology: vitamins B₁ and B₂ – after rectification on a mini-column SEP-PAK C-18 and HPLC; B₆, B₁₂, biotin, pantothenic acid, folic acid and niacin according to the method described by Switalski et al. [35], on a liquid chromatograph Lachrom (Merck-Hitachi), with a diode detector; the vitamins were identified on the basis of retention times and spectra, compared with standards and the registration was conducted at a maximum absorption for each vitamin.

Mineral elements were determined at the University Analytical Laboratory: the samples were mineralized (Mars 5, CEM) and the elements determined on a ICP-AES Liberty 220 spectrometer (Varian).

Milk analysis. The SCC was analyzed every fortnight on a Somacant –120 apparatus (Bentley) at the Laboratory of Milk Analysis and Evaluation. The daily yield, milk composition and content of urea were determined once a month at the Milk Analysis Laboratory in Opole.

Rumen bacteria and protozoa. The rumen contents were obtained through an esophagus probe and analyzed for bacteria colonies and protozoa: samples of 150 – 200 ml were taken from the central and lower part of the rumen.

The bacteria count in the rumen fluid (cfu) was determined indirectly, i.e. on a Bactoscan-70 apparatus, which uses the method of flow cytometry for counting colony forming bacteria. The RNA of the bacterial nucleus was stained with acridine orange, which in a laser light renders it possible to observe a fluorescence of stained bacterial nuclei. Five ml of an non-filtrated rumen fluid was drawn and conserved in a 4% formalin at a 1:1 ratio. Next the fluid was filtrated at the Laboratory, 1 ml of the filtrate was added to a 25 ml standard milk sample (200 000 LKS and 100 tys. OLB-jtk) and finally, units forming colonies were determined in relation to the standard. The number of colony forming bacteria in the rumen fluid was calculated for a 50-fold dilution.

Protozoa were identified within four types: Entodinium, Diplodinium, Epidinium and Holotricha, using Dogiel’s key and taking into consideration traits typical for protozoa, such as length, width, ratio between the two, location of the micro- and macro-nucleus and width of the cytoskeletal plate. For identification a light microscope was used with a Fuchs–Rosenthal camera. Photographs of yeasts and rumen microflora were made using an electron microscope (scanning microscope S.E.M. Zeiss; Laboratory of Electron Microscopy, Wrocław University of Environmental and Life Sciences).

Statistical calculations. The results obtained were elaborated using the GLM procedure [40], applying the least squares method according to the following model:

Y_ijk = ľ + a_i + b_j + e_ijk

where:
µ – overall mean,
a_i – effect of subsequent lactation (1, 2, 3 and further),
b_j – effect of yield of first control milking (kg milk),
e_ijk – error.

The significance of differences between groups of cows was determined using Scheffe’s test [40].

RESULTS AND DISCUSSION

Milk yield and composition (Table 2). A supplement containing dried beer yeasts or preparation of yeast cell membranes (MOS) resulted in a statistically highly significant increase in milk yield (by about 3 and 2.2 kg per head per day, respectively) as compared to the milk production of cows from the control group, receiving no yeast additives. The content of basic milk components: fat (4.156 ± 0.735), protein (3.149 ± 0.293), lactose (4.837 ± 0.216) and dry matter (12.599 ± 0.659), was similar in all the groups examined. Also, the content of urea in milk remained on a mean physiological level for high producing cows (183.99 ± 86.49 mg · l), thus confirming the daily ration offered contained an optimum energy to protein ratio.

Table 2. Daily milk yield and composition

Group		Milk yield, kg	Milk components yield and content
			Fat %	Protein %	SCC thousand	Lactose %	Dry matter,%	Urea mg x l
Dried beer yeasts Leiber® BT	x	26.780A	4.196	3.142	129.80A	4.872	12.936	187.51
	δ	4.459	0.74	0.239	142.38	0.192	0.74	91.73
MOS Biolex® MB 40	x	25.993A	4.048	3.136	244.02B	4.794	12.955	175.33
	δ	5.707	0.755	0.271	219.37	0.234	0.503	77.54
Control	x	23.810C	4.224	3.171	216.44B	4.846	12.906	189.13
	δ	4.765	0.709	0.369	251.05	0.223	0.736	90.21
Total Mean	x	25.528	4.156	3.149	196.75	4.837	12.599	183.99
	δ	4.977	0.735	0.293	204.26	0.216	0.659	86.49

A,B,C – groups differ significantly at P < 0.01.

A clear increase in the milk production of cows was recorded by Korniewicz et al. [24] when the animals were fed a preparation containing beer yeasts (Diamond V). In turn, Wang et al. [36] observed a significant increase of milk yield during month 2 and 3 of lactation when adding the same preparation to a daily ration with a higher level of NDF (21% in DM of forages). Skórko-Sajko et al. [31], when adding yeast cultures, observed a higher milk yield as well as a higher content of fat and protein in milk, what was also confirmed by Wang et al. [35], Adams et al. [1] oraz Piva et al. [30].

The increased milk production arises from stimulating cows to a greater feed consumption, principally within the TMR system; Dann et al. [11], offering the preparation Diamong to Jersey cows (140 g per head per day) after calving, observed an increased feed intake during the first month and a higher lactation peak, while the milk composition did not change. An increased feed consumption after offering a yeast probiotic Yea Sacc was recorded by Adams et al. [1] and Piva et al. [30].

Somatic cell count. The available literature indicates that milk with a high somatic cell count (SCC) contains less casein, the proteins are of a lower cheese-making value and the milk cannot be stored long. The basic reason for an increased somatic cell count in milk is an infection of the mammary gland [26]. Statistically significant relations were observed between the content of urea and SCC in milk [4]. Extensive investigations were conducted by Barkema et al. [2], who reported that a negative correlation exists between SCC and milk yield (SCC<150 000; the highest quality). In turn, a high SCC (250 – 400 thousand) was correlated with the technology of cow management (dry period, milking technique, antibiotic treatment). Cows with a small SCC in milk received more mineral-and-vitamin supplements.

The SCC level in the milk of cows tested in the present experiment was within the lower interval of Polish Standards for extra class milk (<400 thousand). It was shown that a supplement of dried yeasts highly significantly decreased the level of SCC in milk (by about 87 000), while the SCC in the milk of cows receiving a preparation of yeast cell membranes (MOS) was similar to that recorded for the control group of cows (244 and 216 thousand, respectively).

The higher level of the local resistance of the mammary gland may thus result in a significant decrease of the constant threat of sub-clinical and clinical mastitis and thus also in a lowering of the SCC. This takes place in accordance with the well known idea of causing an inflammation as a constant factor existing on the micro- and macro-organism line, in which the efficiency of the general immune system and that within the mammary gland, is of significant importance [20].

The results presented here are compliant with the studies reported by Karniewicz et al. [24], who fed Holstein-Friesian cows a yeast preparation (Diamond V XP) obtained during fermentation of grain with the use of Saccharomyces cerevisiae. The daily milk yield during the first three months of lactation was higher by a mean of 3.7 kg and during the first two months of lactation the content of mineral elements in milk and blood serum increased. This may be related to the feeding tolerance, that results in a lack of a decisive overall immune response to feed antigens, such as feed yeasts and their metabolites which enter the blood circulation through mucous membranes [5]. This mechanism is part of the mucosal immune system against a hyper-sensitivity to different, usually non-pathogenic antigens, affecting the mucous membrane of the digestive tract. According to the author cited the second part of this system consists of secretion antibodies stimulated by pathologic antigens, pathologic microorganisms or other antigens, accompanied by a general immune response of the organism. A full value, well balanced ration may, in an immune aspect, be treated as immunomodulation.

Lasek [25], explains that released fats, oligosaccharides or polysaccharides, as well as some proteins which are not digested and unchanged reach the intestines, stimulate the intestinal flora according to the rules regulating the activity of probiotics and thus start the immune processes. Of special importance is here the idea of a joint immunological system of mucous membranes, consisting of a stimulation of their Mucosal Associated Lymphatic Tissue (MALT) and including, beside the mucosa of the digestive, respiratory, urinary and sexual tracts, also the mammary gland. Thus the stimulation of the lymphatic system of the digestive tract results party in an overall resistance but also in the appearance of effector plasmatic cells in various mucous membranes, including the mammary gland.

When offering beer yeasts to animals it is necessary to conduct biochemical and physiological blond tests, as it is known that beta-glucanes from cell membranes stimulate the immunological system. According to Drochner [14], to determine the immunologic status of animals it is necessary to determine leucocytes, erythrocytes as well as hemoglobin and cholesterol in the blood. In studies conducted by Beauchenim et al. [3] there was fund no significant effect of a supplement of live cells of beer yeasts (10 g per head) on the level of glucose, Ca and Mg in the blood of fattened steers. Piva et al. [30] also did not observe a significant effect of the yeast probiotic offered (10 g head) to cows on the basic biochemical and physiological blood indicators. An increase was observed only for the level of protein and its fractions. Hematological indicators were not determined, so it is impossible to describe the immunological status of the organism. One should emphasize that the studies were conducted on Holstein-Friesian cows being in the middle of lactation and producing about 25-26 kg of milk per day.

The number of bacteria in the rumen fluid (Table 3, Photo 1) proved to be statistically significantly higher in groups of cows receiving yeast preparations than in animals from the control group, the dried yeasts being more conductive to bacteria multiplication than the preparation containing yeast cell membranes (1.004, 0.753 and 0.615 jtk x 10⁸, respectively). The authors own investigations proved that the addition of yeast feed supplements results In an increased number of bacteria In the rummen fluid and a better utilisation of energy (higher NDF digestion).

Table 3. Number of protozoa (p x 105) and bacteria (jtk x 108) in 1 ml of rumen fluid

Grup		Protozoa types					Total (p)	Number of bacteria
		Entodinium	Diplodinium	Epidinium	Poliplas	Holotricha
Leiber® BT	x	1.152	0.478A	0.021	0.050	0.106A	1.807A	1.004A
	δ	0.5724	0.2510	0.0099	0.0277	0.3005	0.8919	0.231
MOS BiolexŽ MB 40	x	1.077	0.236B	0.041	0.028	0.0012B	1.383B	0.753 B
	δ	0.3849	0.1822	0.0412	0.0216	0.0035	0.4761	0.502
Control	x	1.140	0.291B	0.033	0.053	0.003B	1.520C	0.615C
	δ	0.6329	0.1630	0.0206	0.0311	0.0074	0.7626	0.267
Mean	x	1.123	0.335	0.032	0.044	0.037	1.572	0.791
	δ	0.3360	0.1136	0.0127	0.0131	0.1199	0.5091	0.377

A,B,C – groups differ significantly at P < 0.01.

The considerable increase of cellulolytic bacteria caused by beer yeast cultures (Saccharomyces cerevisiae) in in vitro experiments was demonstrated by Newbold et al. [28]. However, in studies conducted in vivo on fistulated steers Beauchenim et al. [3] recorded no effect of the same yeast cultures on the number of bacteria and protozoa. Thus one may observe a clear difference in the activity of small (live cells) and large (dried yeasts) quantities of yeasts. The first are usually offered in doses of 10g, while the second of 100 g per head. Similarly as in the experiment on calves [13], the addition of dried yeasts lead to a statistically confirmed (P = 0.001) high increase of the number of bacteria in the rumen contents. This increase was significant when offering the MOS preparation, but not as high as when feeding dried yeasts. According to Kirchgessner [22] rumen bacteria in the first stage of proliferation require supply of group B vitamins. Beer yeasts – rich in these vitamins, added to the fodder, stimulate the development of rumen bacteria. That may explain the differences in cows milk yield and in the number of rumen bacteria when dried beer yeasts or mannano-oligo-sacharides (low in B vitamins) are added.

The concentration of protozoa in the rumen fluid (Table 3, Photo 2) was lowest in the group of cows receiving the preparation from yeast cell membranes, what indicates the existence of a statistically significant effect of MOS on the defaunation of rumen protozoa in comparison with the group of cows receiving dried beer yeasts (1.383 and 1.807 x 10⁵, respectively). In the latter group was observed a higher number of protozoa in the rumen fluid than in cows of the control group (1.520 x 10⁵). This was caused by changes in the number of only two types of protozoa: Diplodinium and Holotricha. The number of protozoa in the rumen fluid and their quantitative composition may differ in different animal species and even in animals of the same species. Determinations of four groups of protozoa show a considerable inter-group variability, what in effect erased the statistical significance of differences between groups. This variability may arise from nutrition, but may also be the effect of other physiological factors and individual digestion characteristics, typical of the given animal [39]. The number of micro-organisms and time of their presence in the rumen is closely related to the level of nutrients (in particular of protein and energy) and the source of their origin. The addition of dried beer yeasts lead to defaunation and an increase in the number of rumen bacterial colonies [27, 33].

Photo 1. Yeast cell, fodder molecules and multiplying bacterial colonies of rumen

Photo 2. SEM: Protozoa of Diplodinium and Eidinium type attached to fodder molecules in rumen liquid

A partial defaunation of the rumen microfauna is clearly favourable for nitrogen metabolism – the supply of bacterial protein to the small intestine increases by 10-20%. Moreover, the energy balance is more favourable for the rumen ecosystem – the protozoa turnover cycle is long [16]. This situation, however, leads to a decrease of the quantity of starch passing from the rumen to the small intestine – protozoa store about 20% of starch [34]. This means, that when offering larger amounts of dried beer yeasts (on their own or with dried brewer’s grains) this process should be recompensed by an increase in the daily ration of feeds from which starch is broken down in the rumen at a slower rate (e.g. corn as silage or grain). On the other hand, Yang and Varga [37] stated that changing the quantities of concentrates in the feeding of Holstein Friesian cows did not affect the concentration of protozoa in the rumen.

There are not many publications to be found on the efficiency of adding yeasts to the daily ration of ruminants, especially cattle, and its effect on the basic parameters of rumen metabolism [6, 8]. The authors of the works cited stated moreover, that the use of the Saccharomyces boulardi yeasts in the RUSITEC model rumen digestion did not lead to any digestive disturbances. Neither of the biochemical rumen parameters changed significantly after adding 500 or 1500 mg of yeasts. Only the level of ammonia increased from about 5 mmol x d to 8.5 mmol x d in response to an addition of yeasts. When using another artificial rumen model the addition of the Saccharomyces cerevisiae yeasts resulted in a decrease of the oxygen concentration in the rumen contents [7].

SUMMARY AND CONCLUSIONS

The addition of dried beer yeast preparations (dried yeasts – 80 g per head per day and extract of yeast cell membranes – 20 g MOS and β-glucanes) to the feed of high-producing dairy cows resulted in a statistically significant increase in milk yield in comparison to animals of the control group, receiving no supplements (by 3 and 2.2 kg milk per day). A significant (by 34-39%) increase was observed in the number of bacteria in the rumen fluid, what indicates that the feed additives used lead to a more efficient utilisation of fibre from the daily ration, improved the energy balance and the use of nutrients for milk production with a simultaneous decrease of the Somatic Cell Count (SCC). These results confirm that the yeast preparations offered positively stimulated the immunological system.

Used were yeast preparationLeiber® BT and Biolex® MB 40, produced by: Leiber GmbH (Hafenstrasse 24, 49565 Bramsche) and InterYeast® (99-340 Krosniewice, Łęczycka 38), respectively.

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Accepted for print: 18.12.2006

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Aleksander Dobicki
Institute of Animal Breeding, Faculty of Biology and Animal Science, Wrocław University of Environmental and Life Science, Poland
Chełmońskiego 38 C, 51-630 Wrocław, Poland

Jerzy Preś
Department of Animal Nutrition and Feed Management, Faculty of Biology and Animal Science, Wrocław University of Environmental and Life Science, Poland
Chełmońskiego 38 C
51-630 Wrocław, Poland
Phone +48 71 32-05-839
fax: +48 71 32-05-845

Andrzej Zachwieja
Institute of Animal Breeding, Faculty of Biology and Animal Science, Wrocław University of Environmental and Life Science, Poland
Chełmońskiego 38 C
51-630 Wrocław, Poland
Phone +48 71 32-05-765
fax: +48 71 32-05-765
email: andrzej.zachwieja@up.wroc.pl

Roman Kwaśnicki
Institute of Animal Breeding,
Wrocław University of Environmental and Life Sciences, Poland
Chełmonskiego 38 C, 51-630 Wrocław, Poland
phone (071) 32-05-770, fax: (071) 32-05-764

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Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.

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