Flores-Calvete G.1, Dagnac T.1, Resch-Zafra C.1, Pereira-Crespo S.2, Botana A.1, Veiga M.1, Agruña M.J.2, González-González L.1, Barreal M. 2, Lorenzana-Fernández R.2 and Fernández-Lorenzo B.1
1Centro de Investigacións Agrarias de Mabegondo, P.O. Box 10, 15080 A Coruña, Spain; 2Laboratorio Interprofesional Galego de Análise do Leite (LIGAL), Mabegondo, 15318 Abegondo, A Coruña, Spain;
gonzalo.flores.calvete@xunta.es
Abstract
From a collection of 217 tank cow’s milk samples taken in Galician dairy farms with varying feeding regimes, it was investigated the relationships between milk trans fatty acids (FA) content, milk composition and diet fed to the lactating cows. Samples were grouped into four clusters based on the content of trans C18:1 (t6-t9, t10, t11 and t12 isomers) and rumenic acid (c9t11CLA) in the milk fat.
Average typical diets for each cluster were identified in terms of the relative importance of grass (fresh and ensiled) and starch (from maize silage and concentrate) in the ration fed to the cows. The herbage-based diets had higher mean values of the t11/t10 C18:1 ratio compared with the maize-herbage-based diets.
In contrast, high-starch diets had higher t10 C18:1 and lower milk fat contents compared with pasture and silage diets. It was concluded that pasture-based diets produced the healthiest milk FA profile even compared with the linseed oil supplemented diet.
Keywords: pasture, silage, linseed supplement, milk, fatty acids
Introduction
The predominant trans fatty acids (trans-FA) in human diet are the trans-C18:1 isomers, coming either from industrial (partially hydrogenated vegetable oils, which excessive intake is deemed negative for human health) or natural sources from ruminant products (Krettek et al., 2008). Whilst the predominant industrial trans-FAs are isomers of t9 and t10 C18:1, TFAs from ruminants consist mainly of vaccenic (t11 C18:1) and rumenic acids (c9,t11 CLA) that are associated with health benefits (Field et al., 2009).
Since fat content and FA composition of dairy milk are mainly dependent on the cows’ diet (Elgersma 2015), a study was carried out with the aim of gaining insight in the relationships between diets fed to the cows in commercial dairy farms and trans-FA profile of milk.
Materials and methods
This study was carried out using survey data and feed and milk sampling in dairy farms from Galicia (Atlantic-humid NW Spain). Farms (n=45) were selected to represent the broad range of intensification existing in the dairy production system of the region, including both extensive (grazing-based) and intensive (maize-silage based) feeding regimes. Feed components of the ration consumed by lactating dairy cows and bulk tank milk samples were collected five times between October 2013 and September 2014. During the visits, the farmers were asked to describe the lactating cows’ feeding as well as milk production, herd number and management. Diet composition was expressed in terms of percentage of each component of the ration in the total dry matter (DM) intake by cow. When the ration included grazed herbage, pasture intake was estimated as the difference between the potential DM intake (DMI) and the total DM of the ration consumed in the barn. DMI (kg d-1) was estimated following the expression DMI=0.372 × Yc +12, where Yc is the average milk yield per cow (L d-1) corrected to the 4% fat. This equation was adapted from NRC (2001) considering cows with 620 kg in the 21th week of lactation. Chemical composition of feed and tank milk samples were estimated in the LIGAL laboratory, respectively, using NIRS (Foss NIRSystems 5000) and FTMIR (Foss Milkoscan FT 6000) calibrations.
FA composition was determined by gas chromatography (GC-FID) following the standards ISO°14156/
IDF 172 and ISO°15885/IDF 184. On a final data set of 217 valid observations, a cluster analysis on trans C18:1 values and c9t11 CLA (expressed in % of total FA) was performed (Proc Cluster and Proc Tree of SAS), followed by an ANOVA analysis (Proc GLM of SAS) for testing the differences between clusters in diet composition and milk physical components and FA profile, after the transformation of the percentage data by means of the arcsine function.
Results and discussion
As can be seen in Table 1, diet composition was different among clusters (P<0.0001) except for the dry forage component. In terms of the importance of grass (fresh and ensiled) and starch (from maize silage and concentrate) in the ration, expressed in %DMI, the four typical diets of clusters 1 to 4 were respectively characterized as high grass-low starch (HGLS: 60.0 and 10.7%), medium grass-medium Table 1. Diet composition and milk fatty acids composition by cluster.1
n Cluster P
Number of dairy cows 33.4b 51.1a 33.3b 44.5a <0.0001
Milk yield (corrected 4% fat) kg cow-1 d-1 27.7b 32.1a 23.9c 31.9a <0.0001
Diet composition (% of each ingredient in total DM intake)
Fresh herbage 12.6b 2.8c 27.4a 9.2bc <0.0001
Maize silage 18.2b 31.1a 4.0c 34.3a <0.0001
Grass silage 29.8a 19.3b 32.6a 15.2b <0.0001
Dry forage 8.5 6.6 7.2 4.0 0.294
Linseed oil 0.0b 0.1b 0.0b 1.0a <0.0001
Concentrate 30.9b 40.0a 28.7b 36.3a <0.0001
Milk fatty acids composition (% total FA unless stated otherwise)
Saturated (SFA) 69.92a 68.62b 66.51c 64.30d <0.0001
Monounsaturated (MUFA) 25.81d 26.91c 28.35b 30.46a <0.0001
trans MUFA 1.87d 2.30c 3.17b 3.92a <0.0001
t6-t9 C18:1 0.43d 0.64b 0.55c 0.84a <0.0001
t10 C18:1 0.27c 0.57b 0.32c 0.92a <0.0001
t11 C18:1 1.04c 0.84d 2.09a 1.73b <0.0001
t12 C18:1 0.14d 0.25b 0.21c 0.43a <0.0001
t11/t10 C18:1 ratio 4.35b 1.57c 7.45a 2.24c <0.0001
Polyunsaturated (PUFA) 3.58d 3.85c 4.40b 4.65a <0.0001
Linoleic 1.82b 2.25a 1.81b 2.30a <0.0001
c9-t11 CLA 0.62b 0.54b 1.15a 1.00a <0.0001
Alpha-Linolenic 0.43b 0.38b 0.69a 0.66a <0.0001
Omega-6/Omega-3 ratio 3.52b 4.23a 2.48c 3.21b <0.0001
Milk composition
Fat (%) 3.89a 3.69b 3.74b 3.47c <0.0001
Protein (%) 3.23 3.23 3.20 3.23 0.664
Lactose (%) 4.70b 4.76a 4.67b 4.79a <0.0001
1 n = number of observations; HGLS = high grass-low starch; MGMS: medium grass-medium starch; LGHS: low grass-high starch; LGHSL: low grass-high starch supplemented with extruded linseed; Figures affected by different letters in the same row are significantly different.
starch (MGMS: 42.4 and 15.7%), low grass-high starch (LGHS: 22.1 and 22.5%) and low grass-high starch, linseed supplemented (LGHSL: 24.4 and 22.2%, with 1.0% of linseed oil). As expected, FA profile was markedly different between clusters. Based on the presence of c9-t11 CLA and alpha-linolenic acid in milk, both HGLS and LGHSL diets showed the healthiest FA profile and, whilst high grass diet showed the lowest value of the Omega-6/Omega-3 ratio, the linseed supplemented diet had the lowest saturated FA proportion, all differences being significant (P<0.0001). Milk could be classified, from the point of view of human health, as follows: HGLS > LGHSL > MGMS > LGHS, in line with the observation of various researchers (e.g. Elgersma, 2015) indicating that milk from grazing-based systems and from linseed supplemented diets has more n-3 polyunsaturated FA and c9,t11 CLA, which is considered beneficial for health. Relative percentages of t11 and t10 C18:1 (expressed on the total trans C18:1) were, respectively 66.0 and 10.0% in HGLS, 55.4 and 14.4% in MGMS, 44.3 and 23.4%
in LGHSL and 36.7 and 24.6% in LGHS diets, indicating that t11/t10 C18:1 ratio increases almost linearly with the proportion of fresh and ensiled grass in the ration and decreases in the same fashion with the starch content of the diet. HGLS and LGHSL diets showed the highest values of t11 C18:1, followed by MGMS and LGHS, with mean values of 2.09, 1.73, 1.04 and 0.84% total FA, respectively (all values different at P<0.0001). In contrast, the highest values of the isomer t10 C18:1 were observed in the two high-starch diets, with mean values of 0.92, 0.57, 0.32 and 0.27% total FA for, respectively, LGHSL, LGHS, HGLS and MGMS diets. An inverse lineal relationship was observed between milk fat content and t10 C18:1 content (r=-0.60, P<0.0001), corresponding the lowest milk fat value (3.47%) to the linseed-supplemented, high-starch diet. Results are in accordance with the findings indicating that feeding a grass-based diet enhances the levels of t11-C18:1 and c9,t11-CLA, while feeding cereal-based diets supplemented with oil increases t10-C18:1 (e.g. Chilliard and Ferlay, 2004) and this fact is frequently associated with a depression of milk-fat content (e.g. Griinari et al., 1998).
Conclusions
High maize silage-high concentrate diets, supplemented with extruded linseed showed the highest t10 C18:1 milk concentrations and the lowest milk fat values. High grass-low concentrate diets showed the highest t11 C18:1 milk concentrations. Milk produced in extensive systems, based on grass, showed a healthier FA profile, compared with high starch diets, even when these are supplemented with extruded linseed.
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