Case study: Effects of milled rapeseed on milk production, milk fat composition and ruminal CH4 emissions of dairy cows in practical farm conditions

Introduction. Besides mitigating ruminal CH4 production, unsaturated lipids such as 18:1n-9-rich oil from rapeseeds have great potential to modify the lipid composition of ruminant meat and milk by decreasing the proportion of saturated fatty acids and increasing that of unsaturated fatty acids. This is significant as milk and dairy products contribute significantly to human 12:0, 14:0 and palmitic acid (16:0) consumption, with excessive intake of these saturated fatty acids increasing the risk of cardiovascular disease as well as lowered insulin sensitivity (see review by Shingfield et al., 2013). The form of lipid inclusion in the dairy cow diet affects lipid bioavailability and final product composition. Milling of rapeseeds has been found to be necessary to release lipids within seeds for efficient absorption (Kairenius et al., 2009). Milled rapeseeds in the diet resulted in a similar fatty acid profile in bovine milk as free rapeseed oil, with the exception of lower increase in trans-fatty acids. Milling whole oilseeds as needed also minimises the risk for oxidative deterioration of unsaturated lipid during feed storage relative to pure oil. The aim of the study described here was therefore to examine the effects of milled rapeseed on milk fat composition and ruminal CH4 emissions of dairy cows in practical farm conditions.

Materials and methods. The study was conducted at the University of Helsinki Viikki research farm in Finland (for details, see Halmemies-Beauchet- Filleau et al., 2019). The whole Finnish Ayrshire milk herd was fed a control diet for 3 weeks (period 1) followed by rapeseed lipid-rich diet for 4 weeks (period 2). After this, all cows were switched back to the control diet (3 weeks, period 3). Forage-rich dairy cow TMR based on high-quality grass silage (digestible OM 696 g/kg DM, 60% in TMR DM) were fed ad libitum. The pre-wilted grass silage (predominantly timothy and meadow fescue) was from the first cut and ensiled with formic acid-based additives in big bales. Concentrates in TMR (40% in TMR DM) comprised home-grown cereals, rapeseed feeds as protein supplement, molassed sugar beet pulp and vitamins and minerals. Rapeseed protein was isonitrogenously supplied either as a lipid extracted meal (control diet) or full- fat seeds milled daily during TMR preparation using an ordinary hammer mill (sieve pore size 6-8 mm) (test diet).

The amount of additional rapeseed lipids in the test diet was ca. 50 g/kg diet DM. Cereal in the control diet was barley and in the test diet oats. When visiting the milking robot (Lely Astronaut A3, Lely, Maassluis, the Netherlands), cows producing less than 30, between 30 and 40 and over 40 kg of milk per day at the beginning of the trial received 3, 4 or 5 kg of standard concentrate per day throughout the study. The milking robot was equipped with Green Feed system (С-Lock Inc., Rapid City, SD, USA) that measures ruminal CH4, carbon dioxide and H2 emissions.

Results and discussion. Cows had no health concerns when fed the test diet, but DM intake was decreased by, on average, 4% relative to control diet (for details, see Halmemies-Beauchet-Filleau et al., 2019). This is not unexpected because lipid supplementation often suppresses DM intake at high inclusion rates (review of Huhtanen et al., 2008; Halmemies-Beauchet-Filleau et al., 2017). As ECM was unaffected by the test diet, the FE was marginally improved from 1.34 to 1.40 (ECM kg per DM intake kg) compared with control diet. Protein yield and milk urea content were also unaffected by the form of rapeseed protein in the diet. Though the test diet had no effect on mil к fat yield, it altered milk fat composition (Table 5). The total saturated fatty acid content of milk fat from the test diet was 17% lower than from the control diet (Table 5). Furthermore, the 10- to 16-carbon-saturated fatty acids, regarded as the key blood cholesterol-increasing fatty acids in humans, were substantially lower in milk from the test than in the milk from the control diet. Indeed, increased supply of long-chain fatty acids is known to inhibit de novo synthesis of saturated fatty acids in the mammary gland (see review of Shingfield et al., 2010). The total monounsaturated fatty acids were 58% higher in milk fat from the test

Table 5 Fatty acid composition of tank milk

Fatty acid, g/100 g total fatty acids

Control diet

Test diet

Change in %

10:0

3.9

2.0

-49

12:0

4.6

2.2

-52

14:0

13

8.5

-35

16:0

31

21

-31

18:0

9.7

18

+82

18:1n-9

16

28

+70

18:2n-6

1.3

1.1

18:3n-3

0.4

0.4

Total saturated fatty acids

74

61

-17

Total monounsaturated fatty acids

23

36

+58

Total polyunsaturated fatty acids

2.6

2.3

Total trans-fatty acids

3.6

5.0

Source: adapted from Halmemies-Beauchet-Filleau et al. (2019).

diet than the control diet that principally originated from 18:1n-9. The effect of milled rapeseeds on polyunsaturated fatty acids in milk was marginal. Furthermore, milk fat and fat-rich dairy products with a high monounsaturated fatty acid content are less susceptible to oxidative deterioration (Lin et al., 1996) compared with milk fat enriched with polyunsaturated fatty acids (Havemose et al., 2006).

Ruminal CH4, carbon dioxide and H, emissions were decreased by 18%, 5% and 36%, respectively, with the test diet relative to the control. Milled rapeseeds substantially decreased H, load and CH4 formation in the rumen of dairy cows fed diets rich in highly digestible grass silage (Fig. 1). The small decrease in DM intake cannot account for all the diminution in the ruminal H2 and CH4 emissions observed in the test diet. It is likely that the rumen fermentation pattern shifted towards propionate that increases H2 utilisation. Rapeseed or other unsaturated lipids in the dairy cow diet have been shown to decrease the ruminal acetate to propionate ratio in some (Hristov et al., 2011a; Table 2), but not in all studies (Table 2). Though ruminal biohydrogenation of unsaturated fatty acids is an alternative H2 sink as well, its significance to overall ruminal methanogenesis is generally considered very low (Martin et al., 2010). The CH4 emission intensity was 12.1 versus 15.1 g CH4 per kg ECM for test and control diet, respectively. In an average dairy herd in Finland that produces 10 300 ECM per year per cow and has 45 dairy cows, a change from the control diet typical to the production system in the area to test diet would thus represent an annual decrease of 1 390 t in ruminal CH4 emissions. This corresponds to the withdrawal of CH4 production of the whole herd of about 2 months in a year.

Ruminal gas emissions of dairy cows fed control diet in period 1, test diet in period 2 and again control diet in period 3. Source

Figure 1 Ruminal gas emissions of dairy cows fed control diet in period 1, test diet in period 2 and again control diet in period 3. Source: adapted from Halmemies-Beauchet- Filleau et al. (2019).

Conclusions. Replacing rapeseed meal with milled rapeseeds (supplying 5% of lipid in diet DM) in a dairy cow diet based on highly digestible grass silage had no adverse effects on milk production, FE or animal health. Milled rapeseeds improved milkfat profile by decreasing the proportion of cholesterolraising medium-chain saturated fatty acids and increasing that of 18:1n-9 in a whole herd level. In addition, milled rapeseeds fed at a commercially practical level substantially suppressed ruminal CH4 production.

Acknowledgements. This study was financed in part by the European Institute of Innovations and Technology (EIT) (EIT Food Project 18095: Dairy products with reduced saturated fatty acids) and made in co-operation with Valio Ltd and the University of Reading.

Summary and future trends

Nutritional strategies available for mitigating GHG emissions from dairy cow production include various rumen modifiers (currently under development) and forage and/or concentrate-based dietary strategies currently available for practical use. Given the concern for reducing human-edible feed ingredients such as grains in animal feed, forage-based dietary strategies should be emphasised, especially with ruminant animals specialised in fibre digestion. In temperate areas, major plant species available for making silage include grasses, forage legumes and maize but their availability in various areas depends on local climatic conditions.

Grass silages. Altering forage maturity at harvest has the greatest potential to reduce the environmental footprint of cool-season grass silages in dairy production. Harvesting grass herbage at an early rather than late maturity stage has led to increased DM intake, ECM, FE and reduced CH4- emission intensity with dairy cows, though at the expense of reduced NUE. The trade-offs between reduced CH4 emissions and reduced NUE are complex and clearly warrant further research. Other management factors such as N fertilization rate, use of additives in ensiling or high-sugar grass cultivars were of minor importance.

Forage legume silages. Limited data on the effects of forage legume silages on CH4-emission intensity in comparison to grasses together with reduced NUE suggest minor potential for forage legumes to reduce the environmental footprint of dairy production. In contrast, the literature suggests lower CH4-emission intensity for forage legumes than grasses, provided that higher DM intake potential and ruminal passage rates characteristic to forage legumes take place. Contrasting results may be attributable to the practice of growing and feeding forage legumes in mixtures with grasses or other plants as well as the large variation in silage nutritive and fermentation quality between years. Further research on the potential of forage legumes to reduce the environmental footprint of dairy production is needed to fully exploit their beneficial effects on forage production, feed DM intake and animal performance.

Maize silages. Starch containing maize silage is characterised by high metabolizable energy and low CP contents, which makes it a highly valuable forage crop and compatible to be mixed with grasses and legumes high in CP. The means available for reducing the environmental footprint of maize silage include advancing the maturity of maize crop at harvest to a late stage (40% DM) and using maize cultivars developed for higher cell wall digestibility and intake properties such as brown midrib maize. These methods may have the potential to reduce CH4-emission intensity up to 10% on high-forage diets.

Replacing grass or legume forage silages with maize silage consistently leads to environmental benefits in terms of reducing CH4-emission intensity on high-forage grass silage-based diets, and improving NUE especially on forage legume silages high in CP without compromises in milk production. Nevertheless, further research is needed to optimise the use of these forages in dairy production. There is a particular need for more research on the effects of forage legume N fractions on NUE and forage carbohydrate type on CH4 emissions. There also need to be life cycle analyses comparing the environmental effects of using maize and perennial silage crops.

Lipids in concentrate. Unsaturated plant lipids at levels up to 5% in diet DM have the potential to mitigate ruminal CH4 emissions in a dose-dependent manner by 20-40% in diets based on conserved grass or forage legumes without negative effects on animal performance in terms of ECM yield and FE. The effect of lipids seems to persist throughout the entire lactation, but more long-term studies are needed to confirm this. At high lipid inclusion rates, feeding lipid as a part of TMR is preferred to separate concentrate feeding. In contrast, on starchy diets (based on maize silage or rich in concentrate starch), lipid supplementation is of limited interest due to the negative effect on ECM yield. This is probably linked to more detrimental effect of unsaturated lipid on rumen fibre digestion when basal diet contains significant amounts of starch.

Carbohydrates in concentrate. Increasing the proportion of cereal starch in the dairy cow diet in general improves feed intake, ECM yield and NUE. A critical dietary concentration of starch of 20-22% in the diet DM is required to mitigate ruminal methanogenesis. Decreases of 20-25% have been reported when the starch content has reached 20-32% in the diet DM. However, high inclusion of readily fermentable carbohydrates from cereals predisposes cows to SARA and competes with human nutrition. Fibrous, human-inedible by-products of food and bioenergy industries provide a cost-effective and ethically sound feeding strategy that promotes a circular economy. Soya bean hulls, sugar beet pulp and cereal bran have partly or totally replaced starch-rich cereal grains in the diet of dairy cows without a decrease in animal performance or increase in ruminal CH4. However, the production level of midlactation cows has not exceeded 30 kg/d in these studies, so more research is needed at higher levels of milk production and at early lactation to confirm these promising findings.

Protein in concentrate. Good-quality protein sources such as rapeseed and soya bean meals typically increase DM intake of dairy cows though the effects have been negligible in some cases. Low to medium inclusion of protein feed (dietary CP content of 15-18% depending on the CP of the basal forage) results in the smallest ruminal CH4-emission intensity (at best around -15%) together with the biggest improvement in lactation performance. The excess of dietary CP in dairy cow ratios (CP above 18-20% in DM) is unwanted since protein feeds are expensive, and improvements in milk and CH4 production diminish or even reverse at the highest CP levels, leading only to a more significant N load to environment via manure and urine. Interestingly, the protein in conventional dairy cow protein feeds using rapeseed and soya bean is superior to that in forage in enhancing milk production. Rapeseed protein is slightly superior to soya bean and faba bean and pea in terms of lactation performance but in terms of ruminal CH4 emissions, the differences between these protein sources are negligible. Faba beans and peas are promising home-grown protein and energy sources for dairy cow feed in temperate areas due to their relatively high CP and starch content. More research is needed to find ways to improve the CP utilization of forage and alternative grain legumes to improve their NUE in milk production.

 
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