Conjugated linoleic acid (CLA) represents a mixture of positional and geometric isomers of C18 fatty acid with two double bonds. The CLA isomers cis-9, trans-11 and trans-10, cis-12 have been shown to have a range of positive health effects that include anti-carcinogenic, anti-atherogenic, anti-obesity, anti-diabetic and immune system enhancement (McGuire & McGuire, Reference McGuire and McGuire2000; Larsson et al. Reference Larsson, Berkgvist and Wolk2005). Ruminant dairy products are the major dietary sources of CLA, and its isomer cis-9, trans-11 is approximately 78–89% of the total CLA in ovine milk fat (Antongiovanni et al. Reference Antongiovanni, Mele, Buccioni, Petacchi, Serra, Mellis, Cordeddu, Banni and Secchiari2004).
The cis-9, trans-11 CLA in milk fat is produced primarily in the mammary gland by Δ9-desaturase from vaccenic acid (trans-11 18:1, VA), an intermediate formed in the rumen by biohydrogenation of linolenic (cis-9, cis-12, cis-15 18:3, LNA) and linoleic (cis-9, cis-12 18:2, LA) acids (Bauman et al. Reference Bauman, Corl, Peterson, Sebedio, Christie and Adlof2003). This cis-9, trans-11 CLA is also an intermediate in rumen biohydrogenation of LA, and a portion of it is absorbed to provide the remainder of the cis-9, trans-11 CLA in milk fat (Bauman et al. Reference Bauman, Baumgard, Corl and Griinari2000).
Studies on milk fat CLA in sheep and goats have mainly focused on dietary sources of variation (Addis et al. Reference Addis, Pinna, Molle, Fiori, Spada, Decandia, Scintu, Piredda and Pirisi2006; Nudda et al. Reference Nudda, Battacone, Usai, Fancellu and Pulina2006), seasonal variation (Nudda et al. Reference Nudda, McGuire, Battacone and Pulina2005; Tsiplakou et al. Reference Tsiplakou, Mountzouris and Zervas2006a) and physiological factors (Tsiplakou et al. Reference Tsiplakou, Mountzouris and Zervas2006b). Diet is the main factor that influences milk fat contents of CLA and VA, the highest content of both fatty acids being found in milk of sheep and goats when their diet contains large amounts of LNA and LA (Chilliard et al. Reference Chilliard, Ferlay, Rouel and Lamberet2003; Nudda et al. Reference Nudda, Battacone, Usai, Fancellu and Pulina2006).
Olive tree leaves (OTL) and grape marc (GM) have traditionally been used in the Mediterranean area as alternative feeds for sheep and goats during periods of scarce feed supplies. OTL and GM are high in LNA and LA respectively, which can be used as substrate for cis-9, trans-11 CLA and VA biohydrogenation in the rumen. The studies reported with these by-products focused on their nutritive value (Fegeros, Reference Fegeros1985; Zervas et al. Reference Zervas, Fegeros, Michalochristas and Vastardis1993; Martín García et al. Reference Martín, Yáñez Ruiz and Molina Alcalde2003) but no information is available on the effects of these by-products on milk fatty acid profile in sheep and goats.
Thus the objective of this study was to determine the effect of dietary inclusion of OTL and GM on milk FA profile in sheep and goats, with emphasis on cis-9, trans-11 CLA and VA.
Materials and Methods
Animals and experimental design
Sixteen 2–4-year-old Friesian dairy ewes weighing 65±2·3 kg and sixteen 2–5-year-old Alpine dairy goats weighing 55±2·7 kg were kept indoors in two groups under the same management throughout the experimental period. The body condition score (BCS) of ewes, assessed according to Russel et al. (Reference Russel, Doney and Gunn1969), was 3·2±0·15 and that of goats 2·7±0·16 in a range from of 1 (very poor) to 5 (very fat). Lambing and kidding started in December and lasted 3 weeks. From parturition to the third month of lactation [days in milk (DIM) 1–90], sheep and goats were fed a diet based on alfalfa hay, wheat straw and concentrates (control diets, C). The composition and the average daily amount of diets consumed by sheep and goats are in Table 1. Three months post partum (DIM=91) the ewes and the goats were divided into two homogenous sub-groups (n=8) that were balanced by age, body weight and milk yield.
Table 1. Chemical composition of dietary ingredients and the whole diet, and average daily intake of dietary ingredients in sheep and goatsFootnote †
† Means with different superscript (a, b, c) in each row for each parameter differ significantly (P⩽0·05)
‡ DM, dry matter
The control groups of both species continued to be fed the C diets, whereas the treated groups were gradually switched over a 2-week period (DIM=91–105) from the C diets to that of treatment 1, containing air-dried OTL from olive cleaning. Thus, the OTL diets consisted of OTL and concentrates (Table 1) and were fed ad libitum for one month (DIM=106–135).
After that period, the same treated groups after 2 weeks of adaptation (DIM=136–150) were gradually switched from OTL to treatment 2 diet, which contained air-dried GM. GM diets consisted of alfalfa hay, wheat straw, GM and concentrates (Table 1) and were fed ad libitum for one month (DIM=151–180).
The average daily milk yield and composition in sheep and goats throughout the experimental period are shown in Table 2. All animals were fed twice a day at 8.00 and 17.00 ad libitum with daily feed intake recorded. Water was freely available at all times. C, OTL and GM diets were formulated according to animal species requirements (Zervas et al. Reference Zervas, Kalaisakis and Fegeros2004) by taking into account their maintenance (average body weight) and lactation (milk yield and milk composition, Table 2) requirements. The control groups of sheep and goats were fed a constant amount of 2·0 and 2·6 kg of the C diets, respectively, throughout the experimental period; there were no refusals.
Table 2. Average (±sem) daily milk yield and composition in sheep and goats throughout the experimental periods
† Days in milk
‡ Olive tree leaves
OTL and GM were offered for ad-libitum consumption during the adaptation periods (DIM=91–105 for OTL and DIM=136–150 for GM) where the potential of their consumption was determined. Thus, OTL were consumed at 1·5 kg DM/d and 2·0 kg DM/d and GM at 0·47 kg DM/d and 0·49 kg DM/d by sheep and goats respectively. The offered amount of concentrates (Table 1) was calculated based on these OTL and GM intakes and on the respective requirements of the sheep and goats (Table 2). There were no refusals at each feeding of the OTL and GM diets.
The concentrate (g/kg as fed) consisted of: maize grain, 340; barley grain, 380; soybean meal, 150; wheat middlings, 110; calcium phosphate, 15; salt, 3; mineral and vitamin premix, 2. The mineral and vitamin premix contained (per kg as mixed): 150 g Ca, 100 g P, 100 g Na, 100 mg Co, 300 mg I, 5000 mg Fe, 10 000 mg Mn, 20 000 mg Zn, 100 000 mg Se, 5 000 000 i.u. retinol, 500 000 i.u. cholecalciferol and 15 000 mg α-tocopherol.
All animals were milked twice a day with a milking machine. Individual milk samples were collected from sheep and goats on DIM 125, 134, 170 and 179, after mixing the yield from the evening and the morning milking, on a volume basis of 50 ml/l.
Measurements and analytical methods
Alfalfa hay, wheat straw, concentrates, OTL and GM samples were also collected and analysed for organic matter (OM; 7·009), dry matter (DM; 7·007), N (7·016) and ether extract (7·060) according to AOAC (1984) and for neutral detergent fibre (NDF) assayed without a heat stable amylase and expressed inclusive of residual ash and acid detergent fibre (ADF) expressed exclusive of residual ash according to Van Soest et al. (Reference Van Soest, Robertson and Lewis1991). Milk was analysed for fat and protein with IR spectrometry (Milkoscan 133/; Foss Electric, Hillerød, Demark) after appropriate calibration of the instrument according to Gerber (BSI, 1955) and Kjeldahl (IDF, 1993). Milk samples, OTL and GM were analysed for fatty acid methyl esters by GC as described by Tsiplakou et al. (Reference Tsiplakou, Mountzouris and Zervas2006a).
Calculations
The different groups of fatty acids were defined as follows according to Stockdale et al. (Reference Stockdale, Walker, Wales, Dalley, Birkett, Shen and Doyle2003) :
Short-chain saturated fatty acids (SC-SFA)=6:0+8:0+10:0+11:0,
Medium-chain saturated fatty acids (MC-SFA)=12:0+13:0+14:0+15:0,
Long-chain saturated fatty acids (LC-SFA)=16:0 +18:0+20:0+22:0+23:0+24:0,
Polyunsaturated fatty acids (PUFA)=cis-9, trans-11 CLA+18:2n6c+18:2n6t+18:3n3c+18:3n6c+20:2+20:3n3c+20:3n6c +20:4+20:5 +22:2,
Mono-unsaturated fatty acids (MUFA)=14:1+15:1+16:1+17:1+18:1+VA+20:1,
Saturated/unsaturated ratio (S/U)=(SC-SFA+MC-SFA+LC-SFA)/(PUFA+MUFA) and
VA=trans-11 18:1. This value is not included in the 18:1 content.
Atherogenicity index (AI) was defined as (12:0+4×14:0+16:0)/(PUFA+MUFA) as described by Ulbricht & Southgate (Reference Ulbricht and Southgate1991). The Δ−9 desaturase activity indexes were calculated by the following four ratios: 14:1/14:0, 16:1/16:0, 18:1/18:0 and CLA/VA.
Statistical analysis
Results are presented as least square means±sem. Treatments effects (C v. OTL or C v. GM) on milk fatty acid profile in each animal species (sheep/goat) was tested by one-way ANOVA, using SPSS statistical package (release 9.0.0). Post-hoc tests were performed using Duncan's multiple range test and significance was set at P<0·05. The effect of stage of lactation (DIM=125, 134, 170 and 179) on milk fatty acid profile in each animal species was determined when consuming the C diets. Additionally, Pearson correlations were performed between milk fatty acids in each animal species and the most important, according to R-square and P-values are presented in this study.
Results
Diets
The intake of OTL, as a proportion of total DM intake, was 751 and 702 g/kg DM in sheep and goats respectively, while the intake of GM was 196 and 189 g/kg DM in sheep and goats respectively (Table 1). The chemical composition and the fatty acid profile of alfalfa hay, OTL, GM and concentrates are in Table 3. The ether extract content of the OTL and GM diets was significantly higher than the respective control diets for each animal species (Table 1).
Table 3. Chemical composition (g/ kg dry matter) and the main fatty acids (% of total fatty acids) of alfalfa hay, olive tree leaves, grape marc and concentrates
Apart from the relatively high ether extract content of both by-products, OTL fat was high in LNA (37% of total fatty acids) and that of GM was high in LA (61·1% of total fatty acids) and 18:1 (21·7% of total fatty acids) (Table 3).
Milk fatty acids
The individual fatty acid profile (% of total fatty acids) of milk from sheep and goats consuming C and OTL diets is presented in Table 4. Relative to C, OTL decreased significantly the concentrations of 6:0, 8:0, 10:0, 12:0, 14:0, 14:1, 15:0 and 16:0 in sheep milk fat whereas the concentrations of 18:1, VA, LA, LNA and cis-9, trans-11 CLA increased significantly. For goats, again relative to C, OTL decreased significantly the concentrations of 11:0, 12:0, 14:0, 14:1, 15:0 and 16:0. The opposite trend was observed with the concentrations of 18:0, 18:1 and LNA in milk of goats fed the OTL diet.
Table 4. The mean (±sem) fatty acid profile (% of total fatty acids)Footnote † of milk fat from sheep and goats consuming the control and the olive tree leaves diets. Values are the mean of two different samplings at days in milk (DIM) 125 and 134
† Means with different superscripts (a, b) in each row between Control and Olive tree leaves treatments, for each fatty acid within an animal species differ significantly (P<0·05)
‡ VA, vaccenic acid
§ SC-saturated, short-chain saturated fatty acids; MC-saturated, medium-chain saturated fatty acids; LC-saturated, long-chain saturated fatty acids; PUFA, polyunsaturated fatty acids; MUFA, mono-unsaturated fatty acids
¶ AI, atherogenicity index (see text)
The fatty acid groups (SC-SFA, MC-SFA, LC-SFA, PUFA and MUFA) and the values of S/U and AI of in milk fat of sheep and goats fed with OTL v. C diets, are also presented in Table 4. Concentrations of SC-SFA, MC-SFA and LC-SFA and the S/U value decreased significantly when the sheep were fed with the OTL v. C diet. A similar trend was observed in milk of goats fed the OTL diet, but the results were significant only for MC-SFA and S/U ratio. On the contrary, the concentrations of PUFA and MUFA increased significantly in milk fat of sheep fed with OTL v. C diets but only PUFA was increased in milk fat of goats. The AI value was decreased significantly only for goats fed the OTL diet.
The individual milk fatty acid profile (% of total fatty acids) in sheep and goats fed with C and GM diets are presented in Table 5. Relative the control diet, the GM diet decreased significantly the concentrations of 11:0, 13:0, 14:0 and 16:0 in sheep milk fat. In addition, the GM diet increased significantly the concentrations of 18:0, VA, LA and cis-9, trans-11 CLA in sheep milk fat.
Table 5. The mean (±sem) fatty acid profile (% of total fatty acids)Footnote † of milk fat from sheep and goats consuming the control and the grape marc diets. Values are the means of two different samplings at days in milk (DIM) 170 and 179
† Means with different superscripts (a, b) in each row between Control and Grape marc treatments, for each fatty acid within an animal species differ significantly (P<0·05)
‡ VA, vaccenic acid
§ SC-saturated, short-chain saturated fatty acids; MC-saturated, medium-chain saturated fatty acids; LC-saturated, long-chain saturated fatty acids; PUFA, polyunsaturated fatty acids; MUFA, mono-unsaturated fatty acids
¶ AI, atherogenicity index (see text)
The fatty acid groups and the values of S/U and AI of milk fat in sheep and goats fed with GM v. C diets are also presented in Table 5. Concentrations of MC-SFA decreased significantly while the concentrations of PUFA increased significantly only for sheep fed with GM diets. The GM v. C diet in goats had no significant effect on fatty acid profile (apart from 13:0), on fatty acid groups or on S/U, AI and Δ−9 desaturase activity indexes values.
The trans-10, cis-12 isomer of CLA was not detected in milk fat of either animal species for any of the dietary treatments. The statistical analysis of the data showed that the sampling time (stage of lactation) had no effect on the milk fatty acid profile of either animal species consuming the C diets.
Δ−9 Desaturase
The Δ−9 desaturase activity indexes in both animal species fed the C, OTL and GM diets are in Tables 4 and 5. The 18:1/18:0 and cis-9, trans-11 CLA/VA Δ−9 desaturase activity indexes were higher than 14:1/14:0 and 16:1/16:0 for both animal species, irrespective of diet. The only significant Δ−9 desaturase activity index was that of 18:1/18:0 ratio in sheep, with opposite effects of OTL and GM diets on that ratio, compared with the respective controls. The 14:1/14:0 ratio was affected significantly by the OTL diet only in sheep.
Discussion
It is very difficult in practice, or even under experimental conditions, to have isoenergetic and isonitrogenous diets, particularly between sheep and goats, owing to their different milk yield, milk composition, lactation persistency and dietary preferences. Thus, a great effort was made to keep the differences between treatments (C v. OTL or C v. GM) in sheep and goats as small as possible, aiming at the same time to meet the animals' nutritional requirements. Therefore, a strict statistical comparison between sheep and goats was not possible. The same concerns apply to the comparison, between OTL and GM diets, their effects on milk fatty acid profile of sheep or goats, owing to their different dietary characteristics (Tables 1 and 3).
Despite the fact that the diets (C v. OTL and C v. GM) fed to each animal species were not isoenergetic and isonitrogenous (per kg DM), the average daily energy and crude protein intakes were comparable between treated and control groups for each animal species (Table 1). Thus, it would seem unlikely that these small differences in crude protein of the diets (C v. OTL and C v. GM) in each animal species would have affected the milk fatty acid profile and especially the cis-9, trans-11 CLA and VA concentrations (Leonardi et al. Reference Leonardi, Stevenson and Armentano2003). The small differences in energy and crude protein intakes between C v. OTL and C v. GM in each animal species were unavoidable owing to the inclusion of OTL and GM in the diets.
Further to that, the OTL and GM diets had significantly higher ether extract contents than their respective control diets (Table 1). In addition, the LNA and LA content of OTL and GM respectively was also higher than the respective control diets (Table 3). These two differences in ether extract and in LNA and LA contents of the diets should have affected the cis-9, trans-11 C18:2, CLA in milk fat (Nudda et al. Reference Nudda, Battacone, Usai, Fancellu and Pulina2006; Zhang et al. Reference Zhang, Mustafa and Zhao2006).
Diet has a major effect on milk fat CLA content. Increasing the supply of LNA and LA acids in the diet is an efficient way of improving cis-9, trans-11 CLA concentration in milk fat. Many authors have reported that pasture and linseed play a key role in improving the cis-9, trans-11 CLA content of milk fat (Atti et al. Reference Atti, Rouissi and Othmane2006; Tsiplakou et al. Reference Tsiplakou, Mountzouris and Zervas2006a; Chilliard et al. Reference Chilliard, Glasser, Ferlay, Bernard, Rouel and Doreau2007). Pasture and linseed increase the cis-9, trans-11 CLA content of milk fat because of their high LNA (60% of total fatty acids) content (Cabbidu et al. Reference Cabiddu, Decandia, Addis, Piredda, Pirisi and Molle2005; Chilliard et al. Reference Chilliard, Glasser, Ferlay, Bernard, Rouel and Doreau2007). As well as pasture and linseed, by-products such as OTL also have high LNA content (Table 3). The OTL diets increased the cis-9, trans-11 CLA content in milk fat, compared with controls, but the results were significant only for sheep (Table 4). The increase in cis-9, trans-11 CLA was 71% in sheep milk and only 21% in goats' milk compared with their respective controls, reflecting the difference in VA content (3·89 v. 8·46 in the C v. OTL-fed sheep, and 4·25 v. 4·35 in the C v. OTL-fed goats, Table 4). These results for goats' milk do not agree with those of Nudda et al. (Reference Nudda, Battacone, Usai, Fancellu and Pulina2006) who observed increases in cis-9, trans-11 CLA of 52–67% when the goats consumed extruded linseed cake supplements. Chilliard et al. (Reference Chilliard, Ferlay, Rouel and Lamberet2003) did not observe increases in the VA of the milk fat of goats given whole linseed supplements, but did find an increase of VA of 123% when free linseed oil was used.
The increase in cis-9, trans-11 CLA and VA milk fat content in sheep fed the GM diet was significantly higher by 56% and 42% respectively than controls (Table 5). The respective changes observed in cis-9, trans-11 CLA (50%) and VA (−9·8%) milk fat content in goats were not significant (Table 5). When diets are supplemented with LA-rich seeds or oil the cis-9, trans-11 CLA and VA milk fat concentrations are affected. Comparing the effects of oils and seeds on cis-9, trans-11 CLA and VA production, paradoxically fatty acids from seeds are hydrogenated more strongly to 18:0 than are those from oils, the latter being recovered more in the form of cis-9, trans-11 CLA and VA in milk (Chilliard et al. Reference Chilliard, Ferlay, Rouel and Lamberet2003). In our study the GM diets which have high LA content, gave significantly higher cis-9, trans-11 CLA and VA contents in milk fat only for sheep. In addition, the high content of 18:1 in GM (Table 3) seems to have had no effect on 18:1 content in milk fat in sheep or goats in this experiment.
The changes in milk fatty acid composition that are obtained by lipid supplementation of ruminant diets are linked to the lipid source (animal fat, plant or marine oil) and to the form in which it is presented, technological treatment and amount included in the diet (Chilliard et al. Reference Chilliard, Glasser, Ferlay, Bernard, Rouel and Doreau2007). However, the responses are also largely dependent on both the forage source and the diet forage/concentrate ratio (Chilliard & Ferlay, Reference Chilliard and Ferlay2004).
The 18:0 concentration in milk fat of sheep fed the GM diet showed a sharp increase and was significantly higher than the respective C diet, while there was no difference for C v. OTL diets (Tables 4 and 5). On the contrary, the 18:0 concentration in milk fat of goats fed with OTL diet was significantly higher than for the respective C diet, with no significant difference between C and GM diets (Tables 4 and 5). It seems likely that OTL or GM diets may have different effects on the rumen environment between sheep and goats, which influence the build-up of biohydrogenation intermediates or limit the conversion of VA to 18:0. Possible factors include changes in rumen pH, type of lipid ingested and type and amount of bacteria inhabiting the rumen (Lock & Garnsworthy, Reference Lock and Garnsworthy2003).
The apparent different effects of OTL and GM diets on the milk fatty acid profile in sheep and goats are difficult to explain solely on the basis of the unavoidable differences in the amounts and chemical composition (ether extracts, NDF, ADF etc.) among the diets offered. These dietary differences were unavoidable because, to maximize the relevance of the results to practice, it was a priority to have maximum consumption of OTL and GM by sheep and goats, to reflect their availability and low cost as raw feeding materials.
Further to diet effects (OTL/GM) the stage of lactation had no effect on milk fatty acid profile, since the C v. OTL or C v. GM were fed in both animal species at the same DIM. In addition to that, Tsiplakou et al. (Reference Tsiplakou, Mountzouris and Zervas2006b) found no effect of the stage of lactation on cis-9, trans-11 in milk fat when four sheep breeds were kept indoors and fed the same diet.
The present results for cis-9, trans-11 CLA, VA, 18:0 and 18:1/18:0 in milk fat suggest that sheep and goats responded differently when OTL or GM were included in their diets. This suggestion supports the notion of species differences already hypothesized by Jahreis et al. (Reference Jahreis, Fritsche, Mockel, Schone, Moller and Steinhart1999), and highlights the need for further investigation. To test that hypothesis rigorously would require an experimental design incorporating strictly identical diets and comparable sheep and goat requirements.
Correlations
A positive relationship was found between cis-9, trans-11 CLA and VA in milk of sheep fed the OTL diet: cis-9, trans-11 CLA=1·74+0·22∗VA, R 2=0·73 (P<0·05). In addition, a positive relationship was also found between cis-9, trans-11 CLA and VA in milk of goats fed OTL: cis-9, trans-11 CLA=0·23+0·23∗VA, R 2=0·94 (P<0·001). A positive relationship between cis-9, trans-11 CLA and VA in milk fat of sheep grazing different pastures has also been reported by Cabiddu et al. (Reference Cabiddu, Decandia, Addis, Piredda, Pirisi and Molle2005) (R 2=0·74, P<0·05) and Addis et al. (Reference Addis, Cabiddu, Pinna, Decandia, Piredda, Pirisi and Molle2005) (R 2=0·67, P<0·001) in sheep, and by Chilliard & Ferlay (Reference Chilliard and Ferlay2004) in goats (r=0·99) fed with hay-based diets with or without lipid supplementation (sunflower oil or soybeans or untreated lupin seeds or sunflower seeds). Additionally, a negative correlation between 18:0 and VA was found only in goats fed with the OTL diet: 18:0=16·42−1·01∗VA, R 2=0·82 (P<0·01), which may show that the last step of biohydrogenation in the goat rumen was more extensive than that occurring in sheep, despite the fact that the passage rate of digesta is usually higher in goats (Van Soest, Reference Van Soest1997) and despite the fact that the goats consumed higher amounts of OTL than did the sheep. However, as mentioned above, under the constraints of the present experimental design no strict comparison of sheep and goats is possible.
Δ−9 Desaturase
Most cis-9, trans-11 CLA in milk fat is of endogenous origin, synthesized via the enzyme Δ−9 desaturase from VA. The best indicator of Δ−9 desaturase activity is the 14:1/14:0 because all 14:0 in milk fat is produced by synthesis de novo in the mammary gland, whereas the other acid substrates can be absorbed from the gut (Cabiddu et al. Reference Cabiddu, Decandia, Addis, Piredda, Pirisi and Molle2005). Lock & Garnsworthy (Reference Lock and Garnsworthy2003) found that an increase of CLA in cow milk was related to an increase of 14:1/14:0 ratio. In this study no relationship between cis-9, trans-11 CLA and 14:1/14:0 ratio in milk fat of sheep or goats was found irrespective of diet consumed. Our results agree with those of Cabiddu et al. (Reference Cabiddu, Decandia, Addis, Piredda, Pirisi and Molle2005) who found no relationship between cis-9, trans-11 CLA and 14:1/14:0 ratio in sheep milk fat. It was also observed that the 14:1/14:0 and 16:1/16:0 ratios in milk fat of sheep and goats fed either the OTL or the GM diets, were lower than the other two Δ−9 desaturase ratios (Tables 4 and 5). These results also agree with those of Addis et al. (Reference Addis, Cabiddu, Pinna, Decandia, Piredda, Pirisi and Molle2005). The Δ−9 desaturase activity indexes, expressed by the 14:1/14:0 and 16:1/16:0 ratios, are very low because only a small proportion of 14:0 and 16:0 is desaturated to 14:1 and 16:1 respectively (Chilliard et al. Reference Chilliard, Ferlay, Mansbridge and Doreau2000). The discrepancy between cows and small ruminants (sheep and goats) could be due to an effect of animal species on Δ−9 desaturase activity in the mammary gland. In the present study a higher efficiency of Δ−9 desaturase activity was estimated in both animal species and with both diets (OTL and GM), on the basis of the ratio 18:1/18:0 (Tables 4 and 5). This might be explained by the fact that 18:0 is the most preferred substrate of Δ−9 desaturase in the mammary gland (Chilliard et al. Reference Chilliard, Ferlay, Mansbridge and Doreau2000; Mosley & McGuire, Reference Mosley and McGuire2003). However, the GM and OTL diets had opposite effects on 18:1/18:0 ratio. This interesting observation cannot be easily explained.
In the present study the cis-9, trans-11 CLA/VA ratio showed a declining tendency in both animal species fed with OTL diets. A decrease in Δ−9 desaturase activity in the mammary gland of goats, as indicated by the cis-9, trans-11 CLA/VA ratio, has been reported by Nudda et al. (Reference Nudda, Battacone, Usai, Fancellu and Pulina2006) when the goats fed with extruded linseed cake which has high LNA content.
Atherogenicity index
The AI characterizes the atherogenicity of dietary fat; fat with a higher AI value is assumed to be more detrimental to human health. In the human diet, lipids (particularly saturated fatty acids) are known to contribute to coronary disease (Williams, Reference Williams2000). On the contrary, some unsaturated fatty acids in milk have a protective effect against the risk of cardiovascular disease, including trans-10, cis-12 CLA, MUFA (in particular oleic acid) and PUFA. The present study found a significant effect of the OTL diet on AI value only in goats (Table 4).
Conclusions
When included in sheep and goat diets, OTL and GM increased significantly the concentrations of cis-9, trans-11 CLA and VA in milk fat only in sheep, showing an apparent difference in response between sheep and goats. These results support the hypothesis that there are species differences, which should be tested in further studies.
The authors are grateful to Dr G Papadomichelakis, from the Animal Nutrition Department of the Agricultural University of Athens, for his assistance in statistical analysis of the data.
