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Sustainable intensification of dairy farming - the specific role of grassland- based forage production

Nowadays, dairy farming has become a globalized business, and thus related environmental impacts need to be analysed on a global scale. To meet the food demand of future generations, the production and use of food of animal origin in general, and of dairy prod- ucts in particular, require higher efficiency. The three- component model of sustainability requires holistic assessment approaches aiming at increasing the pro- ductivity (economy) and compliance of social stan- dards (social welfare), while also reducing environmental impacts (ecology). In the following, social welfare will not be considered as it is outside the classical discipline of natural science and agron- omy and thus not part of the authors' expertise. Instead, we focus on interdependencies between increasing productivity and consequential impacts on the environment. However, whether or not increased economic benefit through higher productivity justifies increased environmental impacts has to be assessed by appropriate methodology, e.g. through numerical indices (Del Prado et al., 2011). The net effect of increases in productivity and resulting environmental impacts on the sustainability of a system can be described by the term 'eco-efficiency'. Eco-efficiency thus determines the ratio between economic benefit and additional environmental impacts. The eco-effi- ciency of a product or process can be assessed by eco- logical footprinting, i.e. a method to assess the life cycle (LCA) of a product or process (Del Prado et al., 2011; Nemecek et al., 2011). The ecological footprint considers any environmental impact (such as emis- sions of GHGs) of a certain production process accord- ing to previously defined impact categories (e.g. global warming, eutrophication, biodiversity). To combine productivity and environmental impact of a certain production process, emissions are referred to as the production unit, the so-called functional unit (e.g. kg of milk). Thus, abiotic resource-use efficiency of cer- tain products or processes can be quantified at differ- ent temporal and regional scales using homogenized LCA methodology. This allows for comparative LCA studies, provided that the same methodological approach and measurement standards were applied. In contrast, biotic resource-use efficiency, i.e. the impact of agronomic land use on biodiversity, is difficult to quantify because its determination implies the qualita- tive rating or classification of species and functional groups within an ecosystem (Del Prado et al., 2011). A review of European studies on plant diversity in grass- land-based dairy systems has shown that increasing requirements for forage quality demand highly intensified and specialized grassland management, which is associated with negative effects on biodiver- sity (Treyse et al., 2007).Converting to organic agricul- ture has often been reported as an option to enhance biodiversity. However, species diversity in grasslands of north-western Europe dedicated to milk production is not necessarily affected in a positive way by such conversion because of high cutting frequencies in organically managed grasslands for dairy cows (Wa- chendorf and Taube, 2001). Thus, owing to the above- mentioned difficulties and limitations in quantifying biotic resource efficiency and owing to the fact that GHG mitigation and water conservation are key issues in the global environmental protection debate, eco- efficiency, especially in dairy farming, is mainly focused on abiotic resource efficiency. During the last decade, the ecological footprint of agricultural products has increased in importance and has become a major area of research in the agronomy sector and, furthermore, an important labelling tool for agricultural products by using the advertising effect of an alleged 'environment-friendly' production. Con- sumers' awareness of environmental issues demands that there is additional product information, and thus

ecological labelling is a promising instrument in future product marketing. Within the dairy sector, Fonterra, a global-acting dairy company and the world's largest exporter of milk and dairy products, sponsored one of the first comprehensive attempts to ecologically foot- print New Zealand milk production (Basset-Mens et al., 2009). Based on the impact categories 'global warming potential', 'eutrophication', 'acidification', 'land use' and 'energy consumption', different intensi- ties in forage production were assessed (Figure 7). The study suggests that, in the case of the New Zealand dairy farming, the use of maize instead of using per- manent grassland as a feed resource reduces the demand for land because of an increased forage pro- duction while negatively affecting the categories GHG emission, eutrophication, acidification and energy con- sumption (Figure 7). Thus, in New Zealand, grassland- based, low-input forage production systems show a better ecological footprint (higher eco-efficiency) than silage (maize)-based, intensified systems. The positive effect of permanent grassland-based dairy farming on the ecological footprint of milk has, meanwhile, been highlighted by many different stud- ies worldwide, most of which have referred to the car- bon footprint of milk (e.g. Hortenhuber et al., 2010; Rotz et al., 2010; Flysjo et al., 2011b; Vellinga and Ho- ving, 2011; Sch€onbach et al., 2012). The particular value of permanent grassland on the carbon footprint of milk arises mainly from the ability of permanent grassland to sequester atmospheric carbon into the soil. Therefore, carbon sequestration is of great impor- tance for comparative carbon footprinting of different forage production systems, such as grassland-based vs. maize-based systems. A recent comparison of different dairy systems (grassland vs. confinement) in Pennsyl- vania, USA, highlights the strong impact of carbon sequestration on the carbon footprint of milk (Rotz et al., 2010). The study suggests that, if carbon seques- tration were included, grassland-based dairy farming produces less GHG emissions per unit of milk than arable cropping-based (alfalfa and silage maize) con- finement systems (Figure 8). Consequently, it has to be stated that research focusing on carbon sequestra- tion potential of grasslands, as influenced by grassland management intensities, soil properties and grassland age, is highly demanded in order to derive reliable figures for eco-efficiency of dairy products. The comparison of different grassland-based forage production systems in northern Germany revealed large variations between fertilized grasslands and non- fertilized alfalfa-grass mixtures. According to Schmeer et al. (2010), GHG emissions per unit of feed energy could be significantly reduced (up to 70%) by the use of legume-based productions systems instead of using highly fertilized grasslands. Similar results have been reported for dairy systems in the UK where organic dairy systems based on grass-clover forages resulted in significant lower carbon footprint milk than grass- land-/maize-based conventional systems (Del Prado et al., 2011), and also, Yan et al. (2012) revealed a significant reduction in GHG emissions for pasture-based milk production relying on grass-clover swards. Finally, low-input grazing systems based on grass-clover swards are associated with highest energy-use efficiency when comparing a range of forage production systems for dairy cows (Kelm et al., 2004). These results illustrate the great importance of forage production on the carbon footprint milk and, further- more, highlight the need for further research and for a support of legume-based forage systems ('home- grown proteins') to meet the required climate-friendly milk production. However, GHG emissions are determined not only by the management system but also by local environ- mental conditions, i.e. the carbon footprint milk is subjected to large spatial differences. Our own mea- surements have shown that on free-draining sandy soils in northern Germany, the cultivation of maize could eventually produce less GHG emissions per unit of feed energy than do grasslands (Rotz et al., 2005; Lampe et al., 2006; Senbayram et al., 2009), predomi- nantly due to the superior primary production of maize, the relatively low nitrous oxide emissions as a consequence of high soil aeration and the relatively low soil carbon sequestration potential of sandy soils even under grasslands (Herrmann et al., 2012). Another aspect needs to be addressed when dis- cussing the complex issue of how to sustainably inten- sify the dairy sector in general and forage production in particular. It has been suggested that the carbon footprint milk of European dairy systems is superior compared to that of dairy systems in developing coun- tries, and this is due predominantly to the relatively high individual performance of dairy cows (kg milk per cow per year) and the efficient use of resources (Gerber et al., 2010; Hagemann et al., 2012). A global sensitivity analysis of Gerber et al. (2010) has identi- fied increasing milk yields per cow in combination with increased forage digestibility as the most promis- ing measure in GHG mitigation. However, given the lack of linearity in biological systems, it has to be dis- cussed whether the maximization of individual milk production per cow generally meets the requirements of sustainable dairy farming. The cattle advisory ser- vice of the state of Schleswig-Holstein in northern Germany analysed several hundred dairy farms in the state and showed a close relationship between operat- ing farm profits and individual milk yields per cow (Thomsen, 2010); i.e. milk yields beyond 10 000 kg per cow and year as a herd average correlated linearly with superior economic performances of farms. From such a relationship, the paradigm accrues that increas- ing milk yields per cow generally corresponds with linear increasing of operating farm profits. However, high proportions of silage maize and concentrates (e.g. soybean meal) in the feed ration, as well as high- yielding breeds, are required to achieve milk yields of more than 10 000 kg per cow and year. Thus, nutrient-use efficiency in many dairy farms of north- west Europe has become limited because of increased nutrient imports. It has been recently reported that (i) nitrogen surpluses per kg of energy-corrected milk (ECM) are now in the same range, or even higher, in intensive, confinement systems, compared with mod- erate production systems and that (ii) the phosphorus balance exceeds critical thresholds (Sonneveld and Lantinga, 2011). At the same time, the number of lac- tations in intensified, high-yielding confinement sys- tems is low [23 lactations per cow (Flachowsky et al., 2011)]. The unproductive rearing period accounts for a relatively large proportion of the life cycle of these high-performing cows, i.e. only half of their lifetime is related to the production of milk. Therefore, optimiza- tion of lifetime performance is a key issue in sustain- ably intensifying milk production (Piccand et al., 2011). Furthermore, milk and beef production systems are closely interlinked as fattening of surplus calves from dairy farming and culled dairy cows plays an impor- tant role in beef production. When also accounting for other systems affected, such as beef production, it is not certain that an increase in milk yield per cow leads to a reduction in total GHG emissions per kg milk (Flysjo et al., 2012). An increase in the milk yield per cow reduces the meat output of a dairy system, which in turn requires additional meat production to compensate for the decline in beef production, e.g. with beef from suckler cow production (Flachowsky et al., 2011; Zehetmeier et al., 2012). Thus, GHG emis- sions related to compensatory beef production are basically part of the carbon footprint milk. Recent studies of Flachowsky et al. (2011) and Zehetmeier et al. (2012) have shown that compensatory beef pro- duction worsens the carbon footprint milk because beef production from suckler cow husbandry is bur- dened with relatively high GHG emissions [according to Zehetmeier et al. (2012): 212 kg CO2equivalents kg 1 meat]. Zehetmeier et al. (2012) have shown that GHG emissions per kg of milk decreased as milk yield per cow increased, but at the same time, beef produced from suckler cows to compensate for the declined co- production of beef increased with increasing milk yield per cow (Figure 9). In consequence, the carbon foot- print milk is not necessarily improved by increased milk yield per cow. In fact, maximizing milk yield per cow not only increases compensatory beef production but also requires tremendous additional system inputs (e.g. concentrate feeds, energy for confinement sys- tems) and a shift in the on-farm forage production from grass- to maize-based forage. Thus, the carbon footprint milk varies largely depending on whether or not the other systems affected (e.g. beef production, soybean cultivation) were included (Figure 10).

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However, there is still an ongoing debate about how to assess appropriately the carbon footprint milk and how to ensure comparability among different systems and studies (Flysjo et al., 2012). Therefore, homogenization of the carbon footprint method is strongly required, in particular in terms of (i) how to set system boundaries, (ii) which allocation method should be applied, and (iii) which functional unit should be chosen. In conclusion, recent studies of Flachowsky et al. (2011), Flysjo et al. (2011a,b; 2012), Zehetmeier et al. (2012) and Sch€onbach et al. (2012) have suggested that semi-intensive, grassland-based systems at low-to- moderate levels of external inputs with milk yields of about 8000 kg per year are favourable regarding their carbon footprint. A milk yield of up to 8000 kg milk per cow can be achieved by high genetic merit cows feeding a diet that is predominantly based on grass- land, preferably from obligatory grasslands, with only light-to-moderate additional concentrate feeding (Bargo et al., 2002). Thus, dairy production based on semi-intensive managed obligatory grassland does not contribute, or only slightly contributes, to global land- use conflicts. Carbon sequestration of grasslands and independence from feed imports make such systems preferable, especially in combination with home- grown proteins from forage legumes. Therefore, per- manent obligatory grasslands in north-west Europe hold a large potential for a sustainable intensification of forage production. The challenge for grassland research in Europe is nothing less than playing a key role in initiating system-related research programmes that include a wide range of expertise from animal breeding, animal nutrition, soil science and other rele- vant disciplines to identify site-specific solutions in land use, fulfilling the claim given by the new para- digm of sustainable intensification.

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