W.M. Stoop

Effect of lactation stage and energy status on milk fat composition of Holstein-Friesian cows

The effects of lactation stage, negative energy balance (NEB), and milk fat depression (MFD) were estimated on detailed milk fat composition in primiparous Holstein-Friesian cows. One morning milk sample was collected from each of 1,933 cows from 398 commercial Dutch herds in winter 2005. Milk fat composition was measured using gas chromatography, and fat and protein percentage were measured using infrared spectrometry. Each fatty acid changed 0.5 to 1 phenotypic standard deviation over lactation, except odd-chain C5:0 to C15:0, branched-chain fatty acids, and trans-10, cis-12 conjugated linoleic acid (CLA). The greatest change was an increase from 31.2 to 33.3% (wt/wt) for C16:0 from d 80 to 150 of lactation. Energy status was estimated for each cow as the deviation from each average lactation fat-to-protein ratio (FPdev). A high FPdev (>0.12) indicated NEB. Negative energy balance was associated with an increase in C16:0 (0.696 +/- 0.178) and C18:0 (0.467 +/- 0.093), which suggested mobilization of body fat reserves. Furthermore, NEB was associated with a decrease in odd-chain C5:0 to C15:0 (-0.084 +/- 0.020), which might reflect a reduced allocation of C3 components to milk fat synthesis. A low FPdev indicated MFD (<-0.12) and was associated with a decrease in C16:0 (-0.681 +/- 0.255) and C18:0 (-0.128 +/- 0.135) and an increase in total unsaturated fatty acids (0.523 +/- 0.227). The study showed that both lactation stage and energy balance significantly contribute to variation in milk fat composition and alter the activity of different fatty acid pathways.

Genome-wide scan for bovine milk-fat composition. I. Quantitative trait loci for short- and medium-chain fatty acids

A genome-wide scan was performed to identify quantitative trait loci (QTL) for short- and medium-chain fatty acids (expressed in wt/wt %). Milk samples were available from 1,905 cows from 398 commercial herds in the Netherlands, and milk-fat composition was measured by gas chromatography. DNA was available from 7 of the paternal half-sib families: 849 cows and their 7 sires. A genetic map was constructed comprising 1,341 SNP and 2,829 cM, with an average information content of 0.83. Multimarker interval mapping was used in an across-family regression on corrected phenotypes for the 7 half-sib families. Four QTL were found: on Bos taurus autosome (BTA) 6, a QTL was identified for C6:0 and C8:0; on BTA14, a QTL was identified for fat percentage, all odd-chain fatty acids, and C14:0, C16:0, C16:1, and their unsaturation indices; on BTA19, a QTL affected C14:0; and on BTA26, a QTL was identified for the monounsaturated fatty acids and their unsaturation indices. The QTL explained 3 to 19% of phenotypic variance. Furthermore, 49 traits with suggestive evidence for linkage were found on 21 chromosomes. Additional analyses revealed that the QTL on BTA14 was most likely caused by a mutation in DGAT1, whereas the QTL on BTA26 was most likely caused by a mutation in the SCD1 gene. Quantitative trait loci that affect specific fatty acids might increase the understanding of physiological processes regarding fat synthesis and the position of the causal genes.

Short communication: Genome-wide scan for bovine milk-fat composition. II. Quantitative trait loci for long-chain fatty acids

We present the results of a genome-wide scan to identify quantitative trait loci (QTL) that contribute to genetic variation in long-chain milk fatty acids. Milk-fat composition phenotypes were available on 1,905 Dutch Holstein-Friesian cows. A total of 849 cows and their 7 sires were genotyped for 1,341 single nucleotide polymorphisms across all Bos taurus autosomes (BTA). We detected significant QTL on BTA14, BTA15, and BTA16: for C18:1 cis-9, C18:1 cis-12, C18:2 cis-9,12, CLA cis-9,trans-11, C18:3 cis-9,12,15, the C18 index, the total index, total saturated fatty acids, total unsaturated fatty acids (UFA), and the ratio of saturated fatty acids:unsaturated fatty acids on BTA14; for C18:1 trans fatty acids on BTA15; and for the C18 and CLA indices on BTA16. The QTL explained 3 to 19% of the phenotypic variance. Suggestive QTL were found on 16 other chromosomes. The diacylglycerol acyltransferase 1 (DGAT1) K232A polymorphism on BTA14, which is known to influence fatty acid composition, most likely explains the QTL that was detected on BTA14.

Genetic correlation between composition of bovine milk fat in winter and summer, and DGAT1 and SCD1 by season interactions

Milk fat composition shows substantial seasonal variation, most of which is probably caused by differences in the feeding of dairy cows. The present study aimed to know whether milk fat composition in winter is genetically the same trait as milk fat composition in summer. For this purpose, we estimated heritabilities, genetic correlations, effects of acyl-coenzyme A:diacylglycerol acyltransferase 1 (DGAT1) K232A, and stearoyl-coenzyme A desaturase 1 (SCD1) A293V polymorphisms for milk fat composition in winter and summer, and tested for genotype by season interactions of DGAT1 K232A and SCD1 A293V polymorphisms. Milk samples were obtained from 2,001 first-lactation Dutch Holstein-Friesian cows, most with records in both winter and summer. Summer milk contained higher amounts of unsaturated fatty acids (FA) and lower amounts of saturated FA compared with winter milk. Heritability estimates were comparable between seasons: moderate to high for short- and medium-chain FA (0.33 to 0.74) and moderate for long-chain FA (0.19 to 0.43) in both seasons. Genetic correlations between winter and summer milk were high, indicating that milk fat composition in winter and in summer can largely be considered as genetically the same trait. Effects of DGAT1 K232A and SCD1 A293V polymorphisms were similar across seasons for most FA. Allele DGAT1 232A in winter as well as in summer milk samples was negatively associated with most FA with less than 18 carbons, saturated FA, saturated FA to unsaturated FA ratio, and C10 to C16 unsaturation indices, and was positively associated with C14:0, unsaturated C18, unsaturated FA, and C18 and conjugated linoleic acid unsaturation indices. Allele SCD1 293V in winter as well as in summer milk samples was negatively associated with C18:0, C10:1 to cis-9 C14:1, trans-11 C18:1, and C10 to C14 unsaturation indices, and positively associated with C8:0 to C14:0, cis-9 C16:1, and C16 to conjugated linoleic acid unsaturation indices. In addition, significant DGAT1 K232A by season interaction was found for some FA and SCD1 A293V by season interaction was only found for trans-11 C18:1. These interactions were due to scaling of genotype effects.