Dietary options to reduce the environmental impact of milk production

SUMMARY A range of options was explored to test the hypothesis that diets for dairy cows could be formulated to reduce the carbon footprint (CFP) of feed, increase efficiency of conversion of potentially human-edible feed into milk, increase nitrogen use efficiency (NUE) and reduce methane (CH4) emissions per kg milk. Diets based on grazed grass, grass silage, maize silage or straw, supplemented with raw material feeds, were formulated to meet requirements for metabolizable energy and metabolizable protein for a range of daily milk yields. At similar levels of milk yield, NUE, predicted CH4 emissions and diet CFP were generally higher for diets based on maize silage than for those based on grazed grass, grass silage or straw. Predicted CH4 emissions and human-edible proportion decreased, while NUE increased with the increasing level of milk yield. It is concluded that there is potential to reduce the environmental impact of milk production by altering diet formulation, but the extent to which this might occur is likely to depend on availability of raw material feeds with low CFPs.


INTRODUCTION
The feeding of dairy cows involves formulating and delivering diets to meet nutritional requirements for specified levels of daily milk output in relation to stage of lactation, availability and cost of raw materials, and season of year. The environmental impact of milk production systems has to date received relatively little attention from legislators in Europe, except for inclusion in general restrictions on manure and waste disposal. In contrast, there has been legislation in the USA since 2003 to control the environmental impact of concentrated animal feeding operations (CAFO) where the main emphasis is on control of point-source pollution of watercourses. Current CAFO regulations include dairy units of 200 cows or more where the animals are housed for more than 45 days per annum and where crops are not grown on the unit (US Environmental Protection Agency 2012). European pig and poultry systems are controlled under the European Union Integrated Pollution Prevention and Control (IPPC) Directive 2010/75/EU, which requires agricultural activities with a high pollution potential to have a permit (Anonymous 2015). The emphasis is on controlling pollution of water by components of manure (such as nitrates and phosphates) and pollution of air, mainly by ammonia. Dairy units are not included in the current IPPC regulations (Eurostat 2013), but the situation is under review.
Concern over rising concentrations of greenhouse gases (GHG) in the atmosphere led to publication by the United Nations Food and Agriculture Organisation (FAO) of 'Livestock's Long Shadow' (Steinfeld et al. 2006), which drew attention internationally to the environmental impact of livestock production. In 2008, the UK Government published the Climate Change Act (Office of Public Sector Information 2014), which set ambitious targets for reduction of GHG emissions, including those from agriculture. The FAO published subsequently a more detailed life-cycle assessment of GHG from the dairy sector (Gerber et al. 2010) and a detailed assessment of global ruminant supply chains (Opio et al. 2013). These reports highlighted the significance of enteric emissions of methane (CH 4 ) from dairy cattle together with emissions of carbon dioxide (CO 2 ) and nitrous oxide (N 2 O) from feed production and of N 2 O and CH 4 from manure and manure management systems. Global cattle emissions were estimated to amount to 4256 Mt CO 2 equivalents (CO 2 e), of which 1419 Mt were from milk production and 2837 Mt from beef production. Average emission intensity was 2·8 kg CO 2 e/kg fat and protein-corrected milk. However, there was a large range in emission intensity between regions and systems of production, with arid regions having the highest emission intensity, humid regions intermediate and temperate regions the lowest (Opio et al. 2013).
Average emission intensity for temperate milk production was 1·9 kg CO 2 e/kg product for grassland systems and 1·6 kg CO 2 e/kg product for mixed systems, with relatively little difference between temperate grassland and mixed systems in proportions of emissions from enteric fermentation, manure and fertilizer, feed and energy use. Overall, enteric CH 4 accounted for 0·47 of total dairy emissions, feed production and processing (including fertilizer, crop residues and land use change) accounted for 0·24 of total emissions, and manure (including manure management systems) accounted for 0·26 of total emissions (Opio et al. 2013).
Another important consideration for livestock production is competition for arable land to grow crops for human food v. animal feed. This will be central to global food security in the future because arable land is limited. Ruminant production potentially has a distinct advantage over pig and poultry production because ruminants can utilize grazed grass, forage and co-products that are unsuitable for human consumption, whereas pigs and poultry compete directly with humans for the majority of their dietary ingredients .
The objective of the work reported in the present paper was to test the hypothesis that diets for dairy cows could be formulated to reduce the total diet carbon footprint (CFP) and also reduce the proportion of human-edible feed in the total diet. Effects on feed nitrogen use efficiency (NUE) and enteric CH 4 emissions per kilogram milk of implementing a range of nutritional strategies were also explored for cows differing in daily milk yield specified in diet formulation. The dietary strategies considered here are relevant to conventional systems of milk production operated on farms in Northern Europe and America. Organic options have been explored elsewhere (e.g. Olesen et al. 2006;Williams et al. 2006;Weiske & Michel 2007), as have feed supplements and feeding management in terms of their potential to reduce greenhouse gas emissions and increase efficiency of animal performance (e.g. Blaxter & Czerkawski 1966;Tamminga et al. 2007;Bodas et al. 2008;Hristov et al. 2013;White & Capper 2014).

MATERIALS AND METHODS
Diets were formulated using the Ultramix diet formulation programme (AGM Systems Ltd., Romsey, UK). Ultramix incorporates a database containing the composition of feed raw materials, a modelling package containing equations to calculate nutrient requirements, a linear programming package to calculate least-cost formulations and a report-writing module to present results. Equations of the Feed into Milk (FiM) feeding system for dairy cows (Thomas 2004) were used in the modelling package to calculate maximum dry matter (DM) intake (DMI), metabolizable energy (ME) and metabolizable protein (MP) requirements and supply, extended to calculate nitrogen (N) excretion. Factors affecting predicted CH 4 emissions were also considered, together with interactions between CH 4 and N emissions. The CFP of each diet was calculated and the effect on diet CFP of varying raw material feed ingredients was explored. Point feed conversion efficiencies, i.e. not including the rearing or dry periods, defined as kg milk yield per kg DMI were explored together with the effect of diet formulation on the proportion of potentially human-edible DM in the total diet DM.

Diet specifications
Across all diets the following animal specifications were kept constant: Live weight = 650 kg; milk fat concentration = 39 g/kg; milk protein concentration = 31 g/kg. Diets were formulated at three levels of milk yield: 20, 30 and 40 kg/d, for which maximum live-weight changes were 0, -0·5 and -0·6 kg/d, respectively. These levels of milk yield and liveweight change were chosen to represent the range of formulation targets that might be encountered on dairy farms.
A maximum constraint was imposed for DMI and minimum constraints were imposed for ME and MP requirements. To ensure that N supply did not limit microbial protein synthesis, the ratio of effective rumen degradable protein (ERDP) to microbial crude protein (MCP) was constrained to a minimum of 1·0.
To minimize the risk of acidosis, and to encourage rumination and butterfat synthesis, rumen stability value (RSV) balance (Thomas 2004) was constrained to a minimum of +20 and the proportion of total DMI derived from forage DM was constrained to a minimum of 0·4.
Intake of forage was not constrained. Maximum intake of individual non-forage raw materials was constrained to 4 kg/d, except sugar beet pulp, which was constrained to a maximum of 6 kg/d, and protected fat, which was constrained to a maximum of 0·6 kg/d. Diets were not formulated for mineral and vitamin requirements; a fixed quantity of mineral and vitamin supplement was included at 0·2 kg/d.

Feeds
A database of feeds was constructed from raw materials in common use in diets for dairy cows, i.e. grazed grass, grass and maize silages, cereal grains, oilseeds, co-products and rumen-protected fat (Appendix 1). In addition, a set of diets was formulated with chopped wheat straw as the only forage, and some additional co-product feeds (i.e. biscuit meal, breakfast cereal and moist distillers' grains) to emulate the experimental diet used by Roberts & March (2013).
Values for the CFP of diet ingredients were obtained from the Dutch FeedPrint database (Vellinga et al. 2012;Appendix 1). Diet CFP was calculated as the sum of CFPs of individual ingredients. Each ingredient CFP included CO 2 e released during crop growth (e.g. seed, pesticides, green manure, crop residues, organic manure, fertilizer, fuel for cultivation), storing the crop (including crop losses during storage), transporting the crop (to processing, feed mill and farm) and processing the crop (e.g. drying, grinding). Each ingredient CFP also included allowances for land use (changes in management) allocated on the basis of long-term equilibrium (e.g. 200 years for permanent grassland) and land-use change (e.g. deforestation) allocated on a global basis, so as not to penalize unduly individual crops or land that has been in cultivation for many years. Values for the CFP of raw materials used in the diets (with the exception of grazed grass and grass silage) were Dutch averages that allowed for potentially different CFP of imported and home-grown commodities according to the balance of trade and countries of origin. For co-products, such as soya bean meal and sugar beet pulp, CFP components were allocated within FeedPrint to primary and secondary products on the basis of economic value.
To provide a metric to evaluate competition for land use between human food and animal feed,  allocated proportional values to different categories of raw materials according to their estimated potential use for human food. Grass and forages were allocated a value of zero; cereals, pulses and soya bean products were allocated a value of 0·8; other oilseeds, cereal and food co-products, such as sugar beet pulp, were allocated a value of 0·2. Thus, a potential human-edible proportion was calculated for each ingredient and for the formulated diets.
Nitrogen supply and utilization In the FiM system (Thomas 2004), MP supply is calculated from flows of digestible microbial true protein and digestible undegradable protein at the small intestine. Requirements for MP are calculated from milk protein yield, pregnancy, live-weight change and endogenous N losses in urine, faeces, hair and scurf. By definition, protein that is not digested or metabolized is excreted in faeces or urine. Nitrogen excretion was, therefore, calculated from the FiM equations by summing the indigestible and non-metabolizable fractions at each step ( Fig. 1 and Appendix 2). Urinary N losses included endogenous urinary protein, excess ERDP, microbial non-protein N, and the difference between net protein and MP required for milk protein synthesis. An adjustment was made for the non-protein N (urea) content of milk to allow for this alternative route of N excretion. Faecal N losses included endogenous faecal protein, acid-detergent insoluble N, indigestible undegradable protein, and indigestible microbial true protein. An adjustment was made for endogenous protein absorbed from the hind-gut, as indicated in FiM. Nitrogen excretion was the sum of urinary and faecal N losses. Nitrogen use efficiency was calculated as total N output in milk protein divided by total N intake.

Methane emissions
Methane emissions were calculated from the composition and predicted intake of the formulated diets using the equation of Yates et al. (2000): where CDMI is concentrate intake (kg DM/day) and NDF is neutral detergent fibre concentration (kg/kg total diet DM).

Feed conversion efficiency
An indirect indicator of GHG emissions is feed conversion efficiency (FCE), defined conventionally as average daily output of energy-corrected milk per annum divided by average daily total DMI. Feed conversion efficiency is inversely related to CH 4 per unit of milk output (Colman et al. 2011) and varies widely between dairy systems in different regions of the world, reflecting the wide range in diet quality (FAO, IDF & IFCN 2014). In the present study, point FCE (pFCE) values were calculated to allow comparisons between levels of milk yield and between diets. Values for pFCE are higher than typical values for FCE in the literature because they are calculated from output and input on a single day in lactation rather than as annual averages, which include the dry period when feed is consumed but milk yield is zero.

Diet formulations
For each daily milk yield level (20, 30 or 40 kg/d per cow), diets were formulated to represent a range of contrasting forage-feeding systems: (i) grazed grass, (ii) grass silage, (iii) maize silage and (iv) straw.
To test the scope for lowering diet CFP, the diet based on maize silage at a milk yield level of 40 kg/day was taken as a baseline diet (Base) and diet CFP was imposed as a constraint with progressively lower values until no feasible solution could be found to the formulation. The feasible diet with the lowest possible CFP was designated Low-C. Another diet was formulated (Low-C2) in which proportions of forages were constrained to the same as the baseline diet, but sources and proportions of other raw materials could vary with decreasing CFP, thereby testing the scope for lowering CFP of diets by adjusting only ingredients of concentrates.

Grazed grass
Diet formulations for grazed grass are shown in Table 1 for each level of daily milk yield. For all three milk yield levels, DMI and ME requirement were active nutrient constraints; for milk yields of 30 and 40 kg/day, MP requirement was also an active nutrient constraint; ERDP to MCP ratio and RSV were not active constraints in grazed grass diets. There was little difference between diets formulated for milk yields of 20 and 30 kg/day, but when level of milk output was increased from 30 to 40 kg/day, greater quantities of concentrate supplements were required to meet total ME requirement. Nitrogen excretion and NUE increased with increasing milk yield, but N excretion decreased when expressed per unit of milk yield (Fig. 2). Predicted CH 4 emissions per kg of milk decreased with increasing milk yield (Fig. 3). Diet CFP per kg of milk showed little change with level of milk yield, reflecting substitution of grazed grass with feeds of higher CFP at higher levels of milk yield. Proportion of human-edible raw materials increased with increasing milk yield level (Fig. 3), reflecting higher levels of cereal grain in diets.

Grass silage
Formulations for diets based on grass silage are presented in Table 1. As with diets based on grazed grass, quantities of raw materials in each diet depended on raw materials offered, raw material composition and constraints imposed. For all three milk yield levels, DMI and ME requirement were active nutrient constraints; for milk yields of 30 and 40 kg/day, MP requirement was also an active nutrient constraint; for a milk yield of 30 kg/day, ERDP to MCP ratio was an active constraint; for a milk yield of 40 kg/day, RSV was an active constraint. Total N excretion (g/day) increased with increasing milk yield (Fig. 2). As with the diets based on grazed pasture, predicted CH 4 emissions per kg of milk decreased with increasing milk yield. However, diet CFP per kg of milk produced varied little with milk yield (Fig. 3).

Maize silage
Diets based on maize silage are also shown in Table 1. For all three milk yield levels, DMI and ME requirement were active nutrient constraints; for milk yields of 30 and 40 kg/day, MP requirement was also an active nutrient constraint; for all three milk yield levels, ERDP to MCP ratio was an active constraint. Total N excretion increased with increasing level of milk yield, but decreased per unit of product with increasing level of milk yield. Nitrogen use efficiency increased with increasing level of milk yield (Fig. 2). As with the diets based on grass silage, predicted CH 4 emissions per unit of product decreased with increasing level of milk output (Fig. 3).

Straw
The final columns of Table 1 show diets formulated with chopped wheat straw and a range of co-product feeds. For all three milk yield levels, DMI, ME requirement and MP requirement were active nutrient constraints; for milk yields of 20 and 30 kg/day, ERDP to MCP ratio was an active constraint; RSV was not an active constraint in straw-based diets. Straw contributed 0·32 of total diet DM for 20 kg milk/day, 0·26 of total diet DM for 30 kg milk/day and 0·24 of total diet DM for 40 kg milk/day. Total N excretion (g/day) increased with level of milk output, but there was no consistent trend in N excretion per unit of product or in NUE (Fig. 2). Predicted CH 4 emissions decreased with increasing level of milk yield, but diet CFP per unit of milk was similar for the three levels of milk yield (Fig. 3). The human-edible proportion was low for all levels of milk output, reflecting the relatively large contribution of co-product feeds to the total diet.

Diets with low carbon footprint
The diet with the lowest CFP (Low-C; Table 2) had a diet CFP that was 40% lower than that of the baseline diet and contained maize silage together with a high proportion of co-product feeds with low CFP. Active nutrient constraints for this diet were DMI, MP requirement, ERDP to MCP ratio and RSV; ME requirement was no longer an active constraint, so ME was oversupplied by 21 MJ/d (0·08 of ME requirement). The diet with the lowest CFP formulated by changing concentrate ingredients (Low-C2; Table 2) had a diet CFP 30% lower than that of the baseline diet and relied on raw materials with high nutrient density (e.g. soya bean meal) to achieve the reduction in diet CFP. Active nutrient constraints for this diet were DMI, MP requirement, and ERDP to MCP ratio; ME requirement was no longer an active constraint, but ME was oversupplied by only 1 MJ/d.

Nitrogen use efficiency
Comparison of the diets based on grazed grass, grass silage or maize silage revealed that the lowest levels of N excretion and the highest NUE were obtained with the diets based on maize silage, in agreement with Tamminga et al. (2007) and Reynolds et al. (2010). This is because maize silage has a relatively low concentration of N, and the N in maize silage is less degradable than N in grass and grass silage. Consequently, diets based on maize silage were associated with a better match between ERDP supply and ERDP requirement. In the present study, urea was included in the maize-silage diets for milk yields of 20 and 30 kg/d, so ERDP requirement was met exactly. Diets based on grass silage had lower N excretion and greater NUE than diets based on grazed grass. This can be attributed to the higher N content of grazed grass, which consists of more leafy material harvested at an earlier stage of growth than ensiled grass. Although rumen degradability of N can vary with different silage additives, total N concentration of grass silage is usually similar to total N concentration of the fresh grass from which it is made (Grenet 1983). The main environmental consequence of grazing pasture of high N concentration is low NUE (<0·20 for some pasture-based systems with high inputs of artificial fertilizer N) because NUE is inversely related to total N intake (Dewhurst 2006;Ledgard et al. 2009). In view of the importance of grazed grass in milk production systems (Gerber et al. 2010;Opio et al. 2013), the significant contribution of fertilizer N, manure N and grazing returns of N to N 2 O emissions from soils under grassland (Opio et al. 2013), and the relatively high concentration of CP in grazed grass, ways of increasing NUE of grazing dairy cows as a potential GHG mitigation strategy are essential. For intensive grazing systems, high-sugar grasses potentially offer a better balance between ERDP and fermentable carbohydrates within the grass, resulting in greater NUE (Miller et al. 2001). Alternatively, as with any diet that supplies excess ERDP, supplementation with fermentable carbohydrates is an effective strategy to increase capture of excess ERDP and increase MCP generation; this strategy is more effective at increasing NUE than altering the CP of the overall diet (Broderick 2003;Sinclair et al. 2014).
Diets based on straw and co-products were included in the present study to explore the potential for formulation of diets that do not require any land for primary production of animal feeds. These unusual diets were based on an experimental diet in use at a UK research centre where cows yielded in excess of 10 500 kg milk in the 12 months to April 2013 (Roberts & March 2013). Across all milk yield levels, NUE for straw-based diets was intermediate between NUE for grass-and silage-based diets. The low ME and CP concentrations of straw resulted in much lower proportions of forage in straw-based diets (0·24-0·31 of total DM) than in silage-based diets (0·53-0·70 of total DM), so NUE depended more on composition of supplementary co-products offered rather than the basal forage.
In general, NUE increased with increasing levels of milk yield. This was to be expected because as milk yield increased the protein required for maintenance, i.e. non-productive protein, became a decreasing proportion of total protein requirement. Also, as milk yield increased the proportion of forage in diets tended to decrease as more constraints became active in formulations, and the selected supplementary ingredients tended to have higher energy to protein ratios in order to meet increased energy requirements. In typical diet formulation scenarios, energy is the most expensive constraint, so at lower levels of production least-cost solutions will over-supply protein. Another factor that affects the relationship between NUE and milk yield in the present study is live-weight loss. Live-weight loss was set at zero for milk yield of 20 kg/d, 0·5 kg/d for milk yield of 30 kg/d and 0·6 kg/ d for milk yield of 40 kg/d. These are typical allowances encountered in practice, but live-weight loss inflates NUE when calculated on a daily basis because the dietary N originally used to generate the protein reserves is not taken into account. Live-weight loss provides the equivalent of 138 g MP per kg loss (Thomas 2004) which, in the present study, corresponds to 0·035 of daily MP requirement for milk yields of 30 and 40 kg/d. Adjusting to zero live-weight loss for milk yields of 30 and 40 kg/d reduces mean NUE from 0·33 to 0·32, but does not alter the underlying principle that NUE is positively related to milk yield.
The present study was designed to explore dietary options for reducing environmental impact of dairy systems. The boundaries of the analysis, therefore, are from feeds consumed to release of potential pollutants by the cow. Additional options are available to improve NUE of the whole farm, including crop husbandry and manure management, but these are beyond the scope of the present study. Proportions of N lost after excretion can vary between 0·01 and 0·99 of N excreted, depending on housing, manure handling and storage methods (Rotz 2004). The less N excreted, however, the less is available to cause emissions of N 2 O, since under the Tier 1 methodology of the Intergovernmental Panel on Climate Change (IPCC) the emission factor for N 2 O is a fixed proportion (0·01) of total N applied (De Klein et al. 2006).

Methane emissions
Predicted CH 4 emissions per kg of milk decreased with increasing milk yield, as observed on commercial farms (Garnsworthy et al. 2012a, b;Bell et al. 2014). Many studies have shown that maize silage reduces CH 4 emissions compared with grass silage (e.g. Tamminga et al. 2007;Garnsworthy et al. 2012a), although mitigation of CH 4 emissions may be offset by soil carbon loss following the ploughing of grassland for maize cultivation (Vellinga & Hoving 2011). Methane emissions are related positively to dietary NDF concentration and inversely related to concentrate proportion (Yates et al. 2000), so predicted CH 4 per litre of milk decreased as mean NDF decreased from 431 to 358 g/kg DM, and mean proportion of concentrates increased from 0·30 to 0·35 of total DMI when comparing grazed grass and maize silage. Substitution of grass silage by legume silage may reduce CH 4 emissions per unit of silage DM consumed (Waghorn et al. 2002;Kasuya & Takahashi 2010), but it is not known if the reduction in methanogenesis would be evident in the mixed diets simulated in the present study. Other methods of predicting CH 4 production may give different results; for example, those described in Gibbs et al. (2002) and Dong et al. (2006) do not take account of differences in diet composition other than its overall effect on digestibility and/or gross energy intake.

Human edible feed use
Human-edible feeds may potentially be consumed by the human population (e.g. cereal grains, pulse grains, soya bean meal) whereas inedible feeds (e.g. rapeseed meal, distillers' dried grains with solubles) cannot.  and Ertl et al. (2015) discussed the basis of allocation of feeds and their respective human-edible proportions. The application of the concept of human-edible and inedible feeds to feed efficiency was considered by a task force of the Council for Agricultural Science and Technology (CAST 1999). The task force concluded that measures of efficiency of whole-diet feed use did not take into account the considerable proportion of inedible feeds in ruminant rations. The concept was taken further by Ertl et al. (2015), who concluded that milk could make a net contribution to human food supply since more human-edible energy and protein was produced in milk than was consumed by cows in feed. Ertl et al. (2015) found that human-edible feed efficiency (kg milk/kg feed) at the whole-farm scale was negatively correlated with amount of concentrates per kg milk and positively correlated with the area of grassland utilized per tonne of milk produced. In contrast, in the present study where diet formulation was altered within constraints of animal requirements to meet specified levels of daily milk yield, the highest human-edible proportions at each level of daily milk yield were in the diets based on grazed grass and grass silage, reflecting the need to supplement these forages with cereal grain to meet ME requirements and, at the two higher levels of milk yield, soya bean meal to meet MP requirements. Where the forage source was maize silage, the human-edible proportion of the diet was reduced compared with the diets based on grazed grass and grass silage due to reduced input of supplementary cereal grain, and was reduced further in the diets based on straw and co-products that contained no cereal grain.

Feed conversion efficiency
The main factor influencing pFCE was level of milk yield. Mean pFCE (kg milk/kg DMI) were 1·3 at 20 kg milk/day, 1·6 at 30 kg and 1·9 at 40 kg milk/ day; pFCE increased by 0·03 kg/kg, or 3·4 kg milk, per 0·1 FCE unit. Estimated global average response, including feed consumed in the dry period, was 2·5 kg milk per 0·1 unit increase in FCE (FAO, IDF & IFCN 2014). Differences between forage sources in pFCE were generally small at the same milk yield, reflecting similar total daily DMI and diet formulations that balanced variations in forage quality with concentrates to meet requirements for ME within DMI constraints. Across the range in levels of milk yield, pFCE values tended to be highest for diets based on grazed grass (1·7 kg milk/kg DMI) and lowest for straw-based diets (1·5 kg/kg) with diets based on silage being intermediate (1·6 kg/kg).

Feed carbon footprint
Globally, emissions from dairy feed production and processing, account for about 0·2 of the total CFP of milk production systems (Opio et al. 2013). O'Brien et al. (2014) estimated that purchased concentrate feeds accounted for 0·12 of total emissions in a housed mixed forage/concentrate system of milk production in which diets were similar to those based on maize silage in the present study and with feed emission burdens allocated on the basis of relative economic value, as in the present study. Although O'Brien et al.
(2014) used a range of forages and raw material feeds, they did not study the effect of changing diet formulation on diet CFP at different daily milk yields.
The allocation of emission burden on the basis of relative economic value of primary products and co-products (Vellinga et al. 2012) may be criticized on the basis that the choice of relative values may be inappropriate and can change over time. Nevertheless, although co-product raw materials are important components of animal feeds (Wilkinson 2013), they are by definition produced as a consequence of the production of the primary product, usually a human food or drink, and are thus generally considered of lower economic value.
Imposing diet CFP as a constraint demonstrated that diet CFP could be reduced by up to 40% compared with the baseline diet. As expected, the diet contained maize silage together with a high proportion of coproduct feeds with low CFP. However, ME was oversupplied by 8% of requirement. Oversupply of ME is unusual in least-cost diet formulations because the marginal cost for energy is usually greater than for other nutrient constraints. With CFP as the main constraint, however, raw materials with the lowest CFP are preferred, and energy is no longer the most expensive constraint. There was less scope for lowering diet CFP by altering ingredients of concentrates, but Diet Low-C2 had a diet CFP 30% lower than that of the baseline diet.
The value for the CFP of soya bean meal used in the present study was 1056 g CO 2 e/kg DM, which is comprised of 625 g CO 2 e/kg DM derived from growing, processing and transporting the crop, and 431 g CO 2 e/kg DM derived from land use and land-use change. This CFP is considerably lower than the value used in some studies (e.g. 7690 g CO 2 e/kg DM for Brazilian soya bean meal in Gerber et al. 2010) due to a difference in allocation of land-use change, and acknowledges that most soya bean production is on land that has been in arable cropping for more than 20 years and is now in carbon equilibrium. Soya bean production in America has lower GHG associated with its production than UK winter oilseed rape (Wilkinson & Audsley 2013). Transporting soya bean meal overseas has only a marginal effect on its CFP compared with GHG from crop production, and Lehuger et al. (2009) found that a dairy cow diet containing Brazilian soya bean was more environmentally efficient than one containing European rapeseed meal when land use change was excluded from the analysis. For a detailed review of land-use change in soya bean production see Opio et al. (2013).
In formulating the diet for the lowest feasible concentrate CFP (Diet Low C-2), soya bean meal was included at 1·6 kg DM/day, which might seem counter-intuitive given the relatively high CFP of soya bean meal (1056 g CO 2 e/kg DM) compared with alternatives such as wheat distillers' dried grains with solubles (DDGS, 797 g CO 2 e/kg DM), rapeseed meal (714 g CO 2 e/kg DM) and wheatfeed (359 g CO 2 e/kg DM). Replacing soya bean meal by rapeseed meal, wheat DDGS and wheatfeed increased the CFP of the whole diet and decreased NUE. This is because soya bean meal has a more favourable ratio of DUP to CFP (0·18) than the other materials (mean 0·14). In other words, of the protein-rich raw materials, soya bean meal is competitive environmentally with other raw materials. The trend to more soya bean meal being produced from land that has been in arable cultivation for more than 20 years will help to sustain soya bean meal as a suitable raw material for inclusion in low CFP diets because of its high concentration of both CP and ME in addition to its superior amino acid profile.
As well as reducing diet CFP, diets Low-C and Low-C2 also had lower human-edible proportions compared with the base diet. Furthermore, the Low-C and Low-C2 diets had similar diet CFP to the diets based on straw and co-products.
A common aspiration of milk producers has been to increase annual milk output per cow. In the UK, for example, average milk yield per cow increased progressively over the period 1990-2013 (Department for Environment, Food and Rural Affairs (DEFRA) 2014). Higher milk output gave environmental gains in terms of reductions in CH 4 per litre of milk, in agreement with practice on commercial farms (DairyCo 2012; Bell et al. 2014), and also in terms of increased NUE. In the present study, however, there was no benefit at the highest level of milk yield in terms of lower diet CFP per kg of milk or in terms of reduced human-edible proportion, due to greater quantities of cereal grain and soya bean meal in the diet formulations. A life-cycle assessment comparing high-yielding dairy systems based on either grazing in Ireland or feeding silages in UK and USA found that unless carbon sequestration is considered significant for grassland, grass-based and continuously housed dairy systems have similar CFPs per unit of milk, but silage-based systems have greater feed and N efficiencies (O'Brien et al. 2014).

CONCLUSIONS
The hypothesis is accepted that diets for dairy cows can be formulated to reduce the CFP of the diet and also increase efficiency of conversion of potentially human-edible feed. However, the extent to which the environmental impact of feed use by dairy cows may be reduced via diet formulation depends on choice and availability of raw material concentrate feeds, level of milk output and whether or not the desired environmental outcome is reduced CH 4 emissions, reduced diet CFP, increased NUE, reduced human-edible feed use, or some combination of these objectives.
Diets formulated to include high proportions of coproduct feeds are capable of supporting high levels of milk output and are environmentally attractive compared with those based on grazed pasture or silage with concentrates formulated specifically for reduced diet CFP.