Changes in fatty acid composition of milk from lactating dairy cows during transition to and from pasture,☆☆

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Abstract

Five Holstein cows producing an average of 25.4 ± 6.4 kg/d and 347 ± 111 days in milk were used to study the changes in fatty acid composition in milk from lactating dairy cows during transition to and from pasture. The 45-d experiment was divided into 3 periods. During the first 2 d of the experiment (Period I), cows were fed a TMR diet containing 50% conserved forage and 50% grain. On day 3, cows were turned out to pasture and remained on all-pasture diet for 29 d (Period II). On day 32, cows were withdrawn from pasture and offered a TMR diet until day 45 (Period III). Milk yield was recorded daily, and milk fat content and fatty acid (FA) composition were determined daily for composite samples collected from a.m. and p.m. milkings. Data were analyzed using spline regression (H0 = zero slope) to determine whether there was any change in the concentration of a particular fatty acid after a specific time or whether it had stabilized. Cows produced an average of 25.2 ± 5.5, 13.7 ± 5.6, and 12.1 ± 4.8 kg/d of milk with 3.6, 4.2, and 3.6% fat during pre-pasture, pasture, and post-pasture diets, respectively. The conjugated linoleic acid (CLA) content was 0.45% of total fat during pre-pasture, reached to a maximum of 2.53% on day 23 into pasture and plateaued thereafter for the period. Milk fat C18:1 t-11 content was 2.89% of total fat during pre-pasture, reached a maximum of 7.95% after day 22 in pasture, and plateaued afterwards. The C18:2 content declined gradually from pre-pasture to pasture diet with no further decrease observed after day 22, while C18:3 content increased until day 7 on pasture. The post-pasture CLA content in milk fat reached a value similar to pre-pasture within 4 days after the cows were withdrawn from pasture. No change in other fatty acids was observed after day 7 once the cows were switched to post-pasture diet. In the present study, it took 23 days to establish the highest level of CLA in milk fat after turning cows out to pasture, whereas only 4 days were needed to bring it back to the original level once the cows were withdrawn from pasture. Other milk fatty acids were stabilized around day 23 after turning cows out to pasture and by day 7 after being withdrawn from pasture and put back on a TMR diet inside the barn.

Introduction

Conjugated linoleic acid (CLA) is a mixture of positional and geometric isomers of linoleic acid (C18:2) with conjugated unsaturated double bonds. Although a product of incomplete biohydrogenation of C18:2 fatty acid (FA) in the rumen (Kepler and Tove, 1967), its post-ruminal synthesis from C18:2 t-11 derived from both C18:2 and C18:3 FA has also been elucidated (Griinari and Bauman, 1999). Of the several isomers identified, c-9, t-11 and t-10, c-12 are the predominant ones with known biological significance with the first comprising 80% or more in foods from ruminants (Parodi, 2003). The increasing interest in CLA is attributed to its various reported health benefits, such as anticarcinogenic, antidiabetic, antiadipogenic, and antiatherogenic effects (Banni et al., 2003, Pariza, 1999). Positive effects on bone formation and immune response have also been reported (Terpstra et al., 2002, Watkins and Seifert, 2000).

In the United States, a majority of the dairy cows are kept in confinement and fed a total mixed ration (TMR) containing conserved forage and grain in about equal proportions. With the increasing awareness about the importance of CLA on human health, a possibility exists for more dairies switching their cows from confinement feeding to grazing on pastures during summer. The change from a conserved forage and grain based diet to grazing on pasture (with or without grain supplementation) leads to adjustment in the rumen biohydrogenation process resulting a change in the FA composition of milk, including the CLA. Such changes in FA composition and the presence of conjugated dienes in milk fat have been noticed several decades ago (Booth et al., 1935). Total conjugated dienes in milk fat increased more than 3-fold when cows on a winter diet were turned out to pasture in summer (Kuzdal-Savoie and Kuzdal, 1961, Riel, 1963). A change in FA composition of milk also occurs when cows are withdrawn from pasture at the end of the grazing season owing once again to the changes in the rumen biohydrogenation of FA.

Various studies conducted thus far for increasing the milk fat CLA content have been related to manipulation of cows' diet (Shingfield et al., 2006, Khanal et al., 2005, Kay et al., 2004), and involve either grazing cows on pasture or supplementation of oils and oil seeds to mostly TMR based diets. Kelly et al. (1998) observed the CLA content in the milk from cows grazing on pasture during the last two weeks of the experiment when cows were fully on pasture was higher compared with the previous two weeks of transition to pasture from a TMR based diet. Loor et al. (2002) found an increase in the milk fat CLA content from wk 4 to 8 when grazing cows were supplemented with soybean meal. With an all-pasture diet, Dhiman et al. (1999) showed a 500% increase in CLA content in the milk when sampled after three months of grazing. More recently, Shingfield et al. (2006) showed peak CLA content after d 5 when corn silage based TMR diets were supplemented with a mixture of fish and sunflower oil (1:2) at 45 g/kg DM. However, AbuGhazaleh et al. (2004) observed the highest milk fat CLA content in 3 wk, which declined slightly and remained relatively constant after 5 wk in dairy cows fed TMR based diets supplemented with fish meal and extruded soybeans. Another study on the effect of turning cows out to pasture on FA composition of milk involved supplementation of TMR while grazing and sampling being done only on d 1, 8, and 29 of the experiment (Agenäs et al., 2002). These and various other studies on increasing the milk fat CLA content, however, have shown neither the duration required for establishing the highest CLA content in the milk nor have they shown the daily changes in milk FA composition once the cows are turned out to pasture from the conserved forage and grain based diet. Literature is also limited on the daily changes in FA composition of milk once the cows are withdrawn from pasture, which might have lost its relevance to the search for ways to enhance milk fat CLA content. Therefore, the objective of the current study was to investigate the daily changes in milk FA composition, including the CLA, once cows were turned out to pasture for grazing and after they were withdrawn from it.

Section snippets

Selection of cows, feeding, and management

Five lactating Holstein cows with an initial daily milk yield of 25.4 ± 6.4 kg and 347 ± 111 days in milk were used in a 45-d experiment. All cows were pregnant and had previous exposure to pasture during previous lactation(s). The entire experiment was divided into 3 periods. During first 2 d of the experiment (Period I), cows were fed a TMR containing 50% conserved forage and 50% grain; TMR being the same to that on pre-trail. On day 3, cows were turned out to a predominantly perennial ryegrass (

Pasture quality

Botanical and chemical composition of pasture samples is given in Table 1 and their FA composition in Table 2. For ease of interpreting pasture quality, weekly values are presented. Overall, pasture clippings contained 22.1, 21.4, 48.2, 32.6, and 3.49% DM, CP, NDF, ADF, and total FA, respectively. Botanical composition of pasture showed 3.34, 2.15, 0.18, 0.54, and 0.48 t/ha of total DM, live grass [(∼ 95% perennial rye grass (L. perenne L.) with some fescue (Festuca sp.) and brome grass (Bromus

Conclusion

In the present study, milk fat content and most of the fatty acids changed daily before stabilizing around day 23 after switching cows to an all-pasture diet. A decrease in most of the short and medium chain FA occurred after turning cows out to pasture and the trend reversed soon after cows were withdrawn from pasture and fed total mixed ration. Although fatty acid C8:0 decreased after switching cows to pasture, it showed no specific pattern in terms of stabilization, whereas C10:0 and C16:1

Acknowledgement

Authors would like to acknowledge Dr. P. Amatya for his help with the spline regression.

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Approved as journal paper number 7803 of the Utah Agricultural Experiment Station, Utah State University, Logan.

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