Stearoyl-CoA desaturase 1 genotype and stage of lactation influences milk fatty acid composition of Canadian Holstein cows
Summary
Single nucleotide polymorphisms in the coding region of the bovine stearoyl-CoA desaturase 1 gene have been predicted to result in p.293A (alanine at amino acid 293) and p.293V (valine at amino acid 293) alleles at the stearoyl-CoA desaturase1 locus. The objectives of this study were to evaluate the extent to which genotypes at the stearoyl-CoA desaturase 1 locus and stage of lactation influence milk fatty acid composition in Canadian Holstein cows. Cows with the p.293AA genotype had higher C10 index, C12 index and C14 index and higher concentrations of C10:1 (10 carbon fatty acid with one double bond), C12:1 (12 carbon fatty acid with one double bond) and myristoleic acid (C14:1) compared with the p.293AV or p.293VV cows. Cows had higher C18 index and total index, and lower C10 index, C12 index, C14 index and CLA index during early lactation compared with the subsequent lactation stages. Early lactation was also characterized by higher concentrations of oleic acid (C18:1 cis-9), vaccenic acid (C18:1 trans-11), linoleic acid (C18:2), monounsaturated fatty acids and total polyunsaturated fatty acids, and lower concentrations of capric acid (C10:0), C10:1, lauric acid (C12:0), C12:1, myristic acid (C14:0), myristoleic acid (C14:1), palmitic acid (C16:0) and total saturated fatty acids compared with the subsequent lactation stages. Neither the stearoyl-CoA desaturase 1 genotype nor the stage of lactation had an influence on conjugated linoleic acid concentrations in milk.
Introduction
The fatty acid composition of bovine milk influences its physical and organoleptic properties (Chilliard et al. 2000) and also presents some potential health benefits and/or health risks to consumers (Williams 2000). Bovine milk fat typically contains 70% saturated fatty acids (SFA), 25% monounsaturated fatty acids (MUFA) and 5% polyunsaturated fatty acids (PUFA) (Grummer 1991). Saturated fatty acids are characterized by the absence of double bonds in their molecular structure while unsaturated fatty acids have one (MUFA) or more (PUFA) double bonds in their molecular structure. Saturated fatty acids, in particular lauric (C12:0), myristic (C14:0) and palmitic (C16:0) acids in bovine milk have been reported to increase plasma total and low-density lipoprotein (LDL) cholesterol concentrations, which are considered important biomarkers for cardiovascular diseases (Mattson & Grundy 1985). PUFA and MUFA have the opposite effect of reducing plasma total and LDL cholesterol concentrations, and increasing the MUFA concentrations of bovine milk at the expense of SFA remains an important selection objective (Williams 2000).
Current strategies for changing the composition of bovine milk towards increased MUFA and other functional components such as conjugated linoleic acid (CLA) are mainly based on nutritional manipulation of the animals’ diet. However, genetic selection for increased MUFA and CLA content of milk is possible in view of the recent moderate heritability estimates of the concentrations of individual MUFA and desaturase indices in dairy cattle (Soyeurt et al. 2007; Schennink et al. 2008). In comparison with nutritional manipulation, genetic selection has the advantage of permanent and accumulative improvements in milk composition.
In cattle, stearoyl-CoA desaturase 1 (SCD1) is responsible for the synthesis of MUFA and CLA from precursor molecules in the mammary gland. Three SNPs in complete linkage disequilibrium, of which one non-synonymous SNP resulted in a p.Ala293Val polymorphism, have been identified in cattle (Taniguchi et al. 2004; Kgwatalala et al. 2007). An association analysis between the p.Ala293Val polymorphism and the fatty acid composition of intramuscular fat in Japanese Black cattle revealed a significant effect of the p.Ala293Val polymorphism on fatty acid composition (Taniguchi et al. 2004). Recent association studies on the effect of the p.Ala293Val polymorphism on the milk fatty acid composition and desaturase indices in Holstein cattle are, however, inconsistent or differed between countries (Mele et al. 2007; Schennink et al. 2008). Furthermore, the effect of stage of lactation on desaturase indices remains unknown, while the literature on the effect of stage of lactation on milk fatty acid composition is limited (Karijord et al. 1982; Auldist et al. 1998; Kay et al. 2005; Mele et al. 2007).
As a result of the inconsistency of the effect of the SCD1 p.Ala293Val polymorphism on milk fatty acid composition of Holsteins and the limited information on the effect of stage of lactation on milk fatty acid composition and desaturase indices, the objectives of our study were to investigate the effects of the SCD1 p.Ala293Val polymorphism and stage of lactation on milk fatty acid composition and desaturase indices in Canadian Holstein cows.
Materials and methods
Experimental animals
Eight-hundred and sixty-two Canadian Holstein cows representing 17 different herds in Southern Quebec were used in this study. All animals were enrolled in the Quebec Dairy Production Centre of Expertise (VALACTA) program (http://www.valacta.com). A single composite milk sample comprising the morning and evening milk was collected for each cow for both fatty acid determination and DNA extraction. The cows used in the study ranged from first to fifth parity, of which 290, 223, 159 and 190 cows were of first, second, third and forth parity or greater respectively. The cows ranged from 21 to 350 days in milk (DIM) and 199, 261, 402 cows were in early (<100 DIM), mid (100–200 DIM) and late (>200 DIM) lactation stages respectively.
Fatty acid determination
Lipid extraction was performed according to Hara & Radin (1978) and milk fatty acids were derivatized to methyl esters according to Christie (1982) with modifications as described by Chouinard et al. (1999). Fatty acid methyl esters were analysed (split inlet 100:1) by gas chromatography (Varian, CP 3900 GC) equipped with Supelco-100 m column (100 m × 0.25 mm × 0.2 μm film thickness) and flame ionization detector. Oven temperature was programmed from 60 to 165 °C at 3 °C per min and held for 10 min, followed by an increase to 220 °C at 5 °C per min and held for 28 min. Total running time was 89 min and the injector and detector temperatures were maintained at 250 and 255 °C respectively. The gas chromatograph calculated peak areas for individual fatty acids automatically and the standard fatty acid mixture comprising 36 known individual fatty acids (Nu Check Prep, Inc.) was used to provide reference retention times for the identification of fatty acids in the milk samples. Heptadecanoic acid (C17:0; Nu Check Prep Inc.) was used as the internal standard.
Desaturase indices were used as a proxy for SCD1 activity. Individual desaturase indices were calculated according to Schennink et al. (2008) and the total desaturase index (TI) was calculated according to Mele et al. (2007).
DNA isolation and determination of SCD1 genotypes by RFLP
Somatic cells were obtained from the 1 ml of sediment following centrifugation of 13 ml of milk at 21 000 g at 4 °C for 30 min. Somatic cells were washed three times with 1X PBS and genomic DNA samples were isolated by proteinase K digestion followed by three phenol–chloroform extractions. The primer pair, forward – 5′-CCCATTCGCTCTTGTTCT GT-3′ and reverse – 5′-CGTGGTCTTGCTGTGGACT-3′, designed based on GenBank Accession No. AY241932 and previously used by Kgwatalala et al. (2007), was used to generate a 400-bp amplicon that contained three SNPs in linkage disequilibrium in exon 5 of the SCD gene. The PCR mixture contained 50 ng of genomic DNA, 0.6 μm of each primer, 0.2 mm dNTP mixture, 2.25 mm MgCl2 and 1.0 unit of Taq DNA polymerase in a final reaction volume of 25 μl. The PCRs were performed in a programmable thermal cycler, PTC-100TM (MJ-Research, Inc.) with the following protocol: 94 °C for 3 min; followed by 34 cycles of 94 °C for 45 s, 54 °C for 30 s and 72 °C for 1 min; with a final extension step of 72 °C for 10 min.
The genotypes at the SCD1 locus were determined by restriction digestion of the PCR products with NcoI (New England BioLabs Inc.) based on the c.702A>G polymorphism, which is in complete linkage disequilibrium with the c.878C>T (p.Ala293Val) polymorphism (Kgwatalala et al. 2007). The p.293AA genotype was digested into two fragments of 200 bp each. In contrast, the PCR product for the p.293VV genotype remained undigested (400 bp) and the heterozygote had both the 200- and 400-bp fragments. The fragments were separated by electrophoresis through a 2% agarose gel stained with ethidium bromide and visualized under UV rays with FX Phosphoimager (Bio-Rad Laboratories INC).
Statistical analysis
Least squares means were determined using a mixed model procedure of sas version 9.2.1 (Littell et al. 2006). The model included fixed effects of herd (17 herds), parity (first, second, third, fourth or greater), SCD1 genotype (p.293AA, p.293AV and p.293VV), stage of lactation [<100 DIM (early), 100–200 DIM (mid), >200 DIM (late)], the interaction of SCD1 genotype and stage of lactation, and the random effect of the sire (336 sires with 1–36 progeny per sire). All other interactions were not significant and were dropped from the model. Means were separated using pairwise t-tests with Scheffe’s adjustment and determined to be different at P ≤ 0.05.
Results
Genotypes at the SCD1 locus
The SCD1 p.293A and p.293V alleles were present at a frequencies of 69.3% and 30.7%, respectively. Among the 862 cows genotyped, 46.8% had the p.293AA genotype, 45.0% were heterozygous p.293AV and 8.2% were homozygous for the p.293VV genotype. The observed genotypic frequencies were consistent with those expected from Hardy–Weinberg’s law, suggesting random mating and a lack of new mutations at the SCD1 locus.
Association of SCD1 genotype with milk fatty acid composition
Effects of SCD1 genotype on desaturase indices and the concentrations of individual fatty acids are shown in Table 1. Desaturase indices were generally lower for C10 index (C10I), C12 index (C12I), C14 index (C14I) and C16 index (C16I) and comparatively higher for C18 index (C18I) and CLA index (CLAI). The SCD1 p.Ala293Val polymorphism had a significant effect on C10I, C12I and C14I. The C10I, 12I and C14I were the highest in the p.293AA genotype, intermediate in the p.293AV genotype and the lowest in the p.293VV genotype. Significantly higher concentrations of C10:1, C12:1 and C14:1 were observed for the p.293AA genotype compared with the p.293AV and p.293VV genotypes. The SCD1 genotype had no effect on C16 index (C16I), C18 index (C18I), CLA index (CLAI) and Total index (TI), as well as on the concentrations of C4:0–C8:0 and C14–C18 fatty acids except C14:1. The SCD1 genotype had no significant effect on the concentrations of total SFA, MUFA and PUFA, and on fat yield and fat %, protein yield and protein %, total milk yield, somatic cell count, lactose %, milk urea nitrogen, 305-day fat yield, protein yield and milk yield (results not shown).
| Fatty acid | AA(n = 403) | AV(n = 388) | VV(n = 71) | Overall P-value |
|---|---|---|---|---|
| C4: 0 | 3.68 ± 0.04 | 3.68 ± 0.04 | 3.72 ± 0.09 | 0.848 |
| C6:0 | 2.56 ± 0.02 | 2.26 ± 0.03 | 2.63 ± 0.06 | 0.485 |
| C8:0 | 0.12 ± 0.003 | 0.13 ± 0.003 | 0.12 ± 0.01 | 0.688 |
| C10:0 | 5.57 ± 0.05 | 5.62 ± 0.06 | 5.85 ± 0.13 | 0.064 |
| C10:1 | 0.63a ± 0.01 | 0.61b ± 0.01 | 0.56b ± 0.02 | 0.001 |
| C12:0 | 5.34 ± 0.05 | 5.31 ± 0.06 | 5.54 ± 0.13 | 0.200 |
| C12:1 | 0.19a ± 0.003 | 0.18b ± 0.003 | 0.15c ± 0.01 | <0.00 |
| C14:0 | 14.53 ± 0.09 | 14.61 ± 0.09 | 14.82 ± 0.22 | 0.360 |
| C14:1 | 1.19a ± 0.01 | 1.15b ± 0.01 | 1.14b ± 0.03 | 0.040 |
| C16:0 | 27.52 ± 0.15 | 27.43 ± 0.15 | 26.88 ± 0.35 | 0.274 |
| C16:1 | 1.29 ± 0.01 | 1.31 ± 0.01 | 1.34 ± 0.03 | 0.494 |
| C18:0 | 8.50 ± 0.14 | 8.75 ± 0.14 | 8.63 ± 0.33 | 0.636 |
| C18:1 cis-9 | 19.06 ± 0.23 | 18.81 ± 0.23 | 18.86 ± 0.54 | 0.278 |
| C18:1 trans-11 | 1.14 ± 0.02 | 1.14 ± 0.02 | 1.16 ± 0.05 | 0.783 |
| CLA | 0.28 ± 0.01 | 0.27 ± 0.005 | 0.28 ± 0.012 | 0.683 |
| C18:2 cis-9, cis-12 | 1.84 ± 0.03 | 1.88 ± 0.03 | 1.91 ± 0.07 | 0.636 |
| C18:3 cis-9, cis-12, cis-15 | 0.40 ± 0.01 | 0.41 ± 0.01 | 0.41 ± 0.02 | 0.424 |
| SFA2 | 67.83 ± 0.23 | 68.07 ± 0.23 | 68.19 ± 0.53 | 0.393 |
| MUFA3 | 22.36 ± 0.22 | 22.05 ± 0.22 | 22.04 ± 0.52 | 0.359 |
| PUFA4 | 3.66 ± 0.05 | 3.71 ± 0.05 | 3.76 ± 0.11 | 0.924 |
| C10 Index5 | 10.38a ± 0.08 | 9.85b ± 0.09 | 8.97c ± 0.20 | <0.000 |
| C12 Index | 3.48a ± 0.05 | 3.30b ± 0.05 | 2.62c ± 0.12 | <0.000 |
| C14 index | 7.67a ± 0.08 | 7.37b ± 0.08 | 7.20b ± 0.18 | 0.003 |
| C16 Index | 4.51 ± 0.05 | 4.60 ± 0.05 | 4.74 ± 0.11 | 0.171 |
| C18 Index | 69.15 ± 0.48 | 68.37 ± 0.49 | 68.52 ± 1.13 | 0.641 |
| CLA Index | 22.63 ± 0.44 | 21.98 ± 0.45 | 21.89 ± 1.04 | 0.733 |
| Total Index | 26.45 ± 0.24 | 26.13 ± 0.25 | 26.14 ± 0.57 | 0.388 |
- CLA, conjugated linoleic acid.
- Mean values with different superscript letters within a row differ significantly (P < 0.05).
- 1Values are expressed as least squares means ± standard error. Fatty acid content expressed as g/100 g of total fatty acids.
- 2Total saturated fatty acids.
- 3Total monounsaturated fatty acids.
- 4Total polyunsaturated fatty acids.
- 5Indices were determined by calculating the ratios of cis-9 unsaturated to cis-9 unsaturated + saturated for each fatty acid pairs and multiplied by 100 (Kelsey et al. 2003).
Stage of lactation had a significant effect on several desaturase indices and concentrations of several fatty acids in the milk fat of Canadian Holstein cows (Table 2). Lower C10I, C12I, C14I and CLAI and lower concentrations of C10:0, C10:1, C12:0, C12:1, C14:0, C14:1, C16:0 and total SFA were found during early lactation compared with either mid lactation or late lactation. There was no significant difference in C12I, C14I and the concentrations of C10:1, C12:0, C12:1, C14:0 and C14:1 in milk fat between mid lactation and late lactation. Early lactation was, however, associated with significantly higher C18I and higher concentrations of C18:1 cis-9, C18:1 trans-11, C18:2, MUFA and PUFA in milk fat compared with either mid lactation or late lactation. Stage of lactation had no significant effect on C16I and milk fat concentrations of C4:0, C8:0, C16:1, C18:0 and CLA. The animals’ parity did not affect the fatty acid composition of milk. Significant genotype by stage of lactation interaction was observed for C12:0 and C12I only.
| Fatty acid | Early2(n = 199) | Middle3(n = 261) | Late4(n = 402) | Overall P-value |
|---|---|---|---|---|
| C4:0 | 3.72 ± 0.06 | 3.73 ± 0.05 | 3.62 ± 0.04 | 0.275 |
| C6:0 | 2.58a ± 0.048 | 2.64a ± 0.03 | 2.53ab ± 0.03 | 0.003 |
| C8:0 | 0.12 ± 0.004 | 0.13 ± 0.004 | 0.12 ± 0.003 | 0.677 |
| C10:0 | 5.44a ± 0.09 | 5.77b ± 0.08 | 5.53ac ± 0.07 | 0.033 |
| C10:1 | 0.53a ± 0.01 | 0.64b ± 0.01 | 0.64b ± 0.01 | <0.000 |
| C12:0 | 5.10a ± 0.08 | 5.61b ± 0.07 | 5.48ab ± 0.06 | 0.003 |
| C12:1 | 0.15a ± 0.004 | 0.18b ± 0.004 | 0.19b ± 0.003 | <0.000 |
| C14:0 | 13.85a ± 0.14 | 15.15b ± 0.13 | 14.96b ± 0.11 | <0.000 |
| C14:1 | 1.04a ± 0.02 | 1.20b ± 0.02 | 1.23b ± 0.02 | <0.000 |
| C16:0 | 26.96a ± 0.22 | 27.89b ± 0.20 | 26.98ab ± 0.17 | 0.005 |
| C16:1 | 1.33 ± 0.02 | 1.33 ± 0.02 | 1.28 ± 0.02 | 0.123 |
| C18:0 | 8.94 ± 0.21 | 8.56 ± 0.19 | 8.83 ± 0.16 | 0.540 |
| C18:1 cis-9 | 20.87a ± 0.34 | 17.05b ± 0.31 | 18.80c ± 0.27 | 0.010 |
| C18:1 trans-11 | 1.22a ± 0.03 | 1.11b ± 0.03 | 1.12b ± 0.02 | 0.010 |
| CLA5 | 0.28 ± 0.01 | 0.27 ± 0.01 | 0.28 ± 0.01 | 0.458 |
| C18:2 cis-9, cis-12 | 1.98a ± 0.04 | 1.84b ± 0.04 | 1.81b ± 0.03 | 0.001 |
| C18:3 cis-9, cis-12, cis-15 | 0.39 ± 0.01 | 0.42 ± 0.01 | 0.43 ± 0.01 | 0.340 |
| SFA6 | 66.75a ± 0.34 | 69.61b ± 0.31 | 67.73c ± 0.26 | <0.000 |
| MUFA7 | 23.91a ± 0.33 | 20.40b ± 0.30 | 22.14c ± 0.26 | <0.000 |
| PUFA8 | 3.86a ± 0.07 | 3.64b ± 0.06 | 3.64bc ± 0.05 | 0.007 |
| C10 Index9 | 8.92a ± 0.13 | 9.92b ± 0.12 | 10.33c ± 0.10 | <0.000 |
| C12 Index | 2.85a ± 0.08 | 3.22ab ± 0.07 | 3.34b ± 0.06 | <0.000 |
| C14 index | 7.08a ± 0.11 | 7.44ab ± 0.10 | 7.71b ± 0.09 | 0.004 |
| C16 Index | 4.72 ± 0.07 | 4.57 ± 0.06 | 4.57 ± 0.05 | 0.063 |
| C18 Index | 69.96a ± 0.73 | 66.66b ± 0.66 | 69.43a ± 0.56 | 0.012 |
| CLA Index | 20.87a ± 0.66 | 22.40ab ± 0.60 | 23.23b ± 0.51 | <0.000 |
| Total Index | 28.14a ± 0.36 | 24.26b ± 0.33 | 26.31c ± 0.28 | <0.000 |
- Mean values with different superscript letters within a row differ significantly (P < 0.05).
- 1Values are expressed as least squares means ± standard error. Fatty acid content expressed as g/100 g of total fatty acids.
- 2<100 DIM.
- 3100–200 DIM.
- 4>200 DIM.
- 5Conjugated linoleic acid = cis-9, trans-11 CLA.
- 6Total saturated fatty acids = C4:0 + C6:0 + C8:0 + C10:0 + C12:0 + C14:0 + C16:0 + C18:0.
- 7Total monounsaturated fatty acids = C10:1 + C12:1 + C14:1 + C16:1 + C18:1 cis-9 + C18:1 trans-9.
- 8Total polyunsaturated fatty acids = CLA+C18:2 cis-9, cis-12 + C18:3 cis-9, cis-12, cis-15.
- 9Indices were determined by calculating the ratios of cis-9 unsaturated to cis-9 unsaturated + saturated for each fatty acid pairs and multiplied by 100 (Kelsey et al. 2003).
Discussion
The higher frequency of the p.293A allele compared with the p.293V allele (69.3% vs. 30.7%) at the SCD1 locus confirms previously reported frequencies of 83% and 17% respectively, in a limited sample of 44 Canadian Holstein cows (Kgwatalala et al. 2007). Mele et al. (2007) also reported a higher frequency of the p.293A allele relative to the p.293V allele (57% vs. 43%) in Italian Holstein cows, and a similar trend (73% vs. 27%) was reported in the Dutch Holstein Friesian heifers (Schennink et al. 2008).
The desaturase indices obtained in this study are very comparable with those of Schennink et al. (2008). The significant positive effect of the p.293A allele compared with the p.293V allele on C10I, C12I and C14I and milk fat concentrations of C10:1, C12:1 and C14:1 found in this study is consistent with Schennink et al. (2008) who found similar results in the Dutch Holstein Friesen heifers. Schennink et al. (2008) also reported a significant negative effect of the p.293A allele on C16I, C18I and CLAI, and milk fat concentrations of C16:1 and CLA, while we found no significant differences between the two alleles for all those parameters. In agreement with our results, Moioli et al. (2007) also reported a significant positive effect of the p.293A allele on C10I and C14I and on milk fat concentrations of C10:1 and C14:1 in Piedmontese and Valdostana cows, and no significant effect on CLA, MUFA and PUFA in either breed. The significant effect of SCD1 genotype on C10I, C12I and C14I, and on C10:1, C12:1 and C14:1 might be related to the origin of fatty acids in bovine milk. All fatty acids with 12 carbons or less, most of C14:0 and about 50% of C16:0 are synthesized endogenously in the mammary gland from acetate and butyrate produced in the rumen, and a minor fraction of C14:0, 50% of C16:0 and all C18 fatty acids are extracted from arterial blood (Enjalbert et al. 1998). The SCD1 p.293A allele thus seems to have a significant positive effect only on the desaturation of endogenously synthesized fatty acids (C10:0, C12:0 and C14:0) and consequently on the concentrations of their respective MUFA in milk, but no significant effect on the desaturation of fatty acids derived mostly or exclusively from blood lipids. Lower C10I, C12I, C14I and C16I compared with C18I and CLAI are consistent with low SCD1 substrate preference and consequently low SCD1 activity with fatty acids shorter than 18-carbon chain length (Chilliard et al. 2000). The lack of effect of SCD1 genotype on C18I might be related to the low heritability of C18I, which was previously estimated at 3% (Soyeurt et al. 2008), implying that the production of C18:0 and C18:1 cis-9 in bovine milk are more influenced by the environment (feeding) than by genetic factors. Higher C18I and a higher concentration of C18:1 in milk could also be because of selective uptake of stearic acid (C18:0) by the mammary gland and its preferential transport to the endoplasmic reticulum membrane for desaturation by fatty acid binding protein-3, which has high affinity for stearic acid (Whetstone et al. 1986; Hanhoff et al. 2002). Furthermore, higher indices for C18I and CLAI could be because of some contribution of intestinal SCD1 activity on rumen-derived precursors before their extraction from the blood and further desaturation in the mammary gland.
Results on the effect of stage of lactation on the fatty acid composition of milk obtained in this study are fairly consistent with those of Mele et al. (2007). Mele et al. (2007) reported significantly lower C14:0 and C14:1 and higher C18:1 and MUFA during early lactation relative to either mid lactation or late lactation, which was also the case in the current investigation. In addition, we report significantly lower C10:0, C10:1, C12:0, C12:1, C16:0 and SFA, and significantly higher C18:1 cis-9, C18: l trans-11, C18:2 and PUFA are present during early lactation compared with either mid lactation or late lactation in Canadian Holstein cows. Auldist et al. (1998) also reported significantly higher milk MUFA and significantly lower SFA during early lactation compared with either mid or late lactation stages in New Zealand Holsteins. Similarly, Kay et al. (2005) reported significantly lower concentrations of C10:0, C12:0, C16:0 and a higher concentration of C18:1 cis-9 during early lactation (week 1 of lactation) compared with mid lactation (week 16 of lactation) in Holsteins. The concentrations of most fatty acids (C10:1, C12:0, C14:0, C14:1, C18:0, C18:1 cis-9, C18:1 trans-11, CLA, C18:2 and C18:3) were not significantly different between mid lactation and late lactation, consistent with the results of Auldist et al. (1998), who reported that the effect of stage of lactation on fatty acid profiles was mostly because of differences in early lactation milk. Stage of lactation had no effect on concentrations of C16:1, C18:0 C18:3 and CLA as reported by Mele et al. (2007) in Italian Holstein cows. We also report significantly lower C10I, C12I, C14I and CLAI, and higher C18I and TI during early lactation relative to either mid lactation or late lactation. Changes in the fatty acid profile during the entire lactation could be related to the energy balance or status of the cows. During early lactation, dairy cows might be in a negative energy balance, leading to the mobilization of adipose tissue fatty acids (mainly palmitoleic acid, oleic acid and other long chain fatty acids) and their eventual secretion into milk, hence the higher concentrations of C18:1 cis-9 (oleic acid), C18:1 trans-11 and C18:2 during early lactation. Oleic acid is the preferred substrate for mammary SCD1, and increased availability of oleic acid during early lactation resulted in higher C18I during early lactation compared with mid lactation. C18:1 cis-9 and C18:1 trans-11 have been shown to depress the activity of enzymes responsible for mammary synthesis of saturated fatty acids in the MAC-T cell line (Jayan & Herbein 2000), and the increased concentrations of these fatty acids during early lactation might therefore explain the relatively lower concentrations of de novo-synthesized C10:0, C12:0 and C14:0 during early lactation compared with the subsequent lactation stages. Drackley et al. (2007) also reported a linear increase in the concentration of C18:1 cis-9 and a concomitant linear decrease in the concentrations of C12:0, C14:0 and C16:0 with increasing abomasal infusions of high oleic (C18:1 cis-9) sunflower fatty acids in Holstein cows. A limited supply of some of the substrates for SCD1 (C10:0, C12:0 and C14:0) might explain lower C10I, C12I and C14I and lower concentrations of C10:1, C12:1 and C14:1 in milk fat during early lactation compared with the subsequent lactation stages. Attainment of a positive energy balance during mid lactation is expected to reverse the inhibitory effects of C18:1 fatty acid on de novo fatty acid synthesis, and hence lead to an increase in the concentrations of C10:0, C10:1, C12:0, C12:1, C14:0, C14:1 and to higher C10I, C12I and C14I during mid lactation compared with early lactation.
Acknowledgements
The authors would like to thank Dr Robert Moore and Mr. Brian Corrigan at VALACTA for organizing sample collections. We would also like to thank Mr. Benjamin Olaniyan for assisting with the preparation of fatty acid methyl esters and Dr Roger Cue for his expert advice on statistical analysis. The study was supported by the Dairy Cattle Genetics Research and Development Council of Canadian Dairy Network and the Natural Sciences and Engineering Research Council of Canada.
