Introduction

To meet the anticipated increase in world demand for dairy products over the next 10–12 years, it is predicted that world milk production will need to increase by 29% to over 1,000 billion litres of milk (IFCN 2013). It is forecast that this rise in production will be met by a further 47 million dairy cattle, resulting in a global population of 406 million dairy cows and buffaloes. To put this in context, the UK has 1.8 million dairy cows, and the increase in world population will therefore be approximately 25 times the current UK herd. Milk production is anticipated to increase in Europe, North America and Australia/New Zealand, but the greatest percentage increase is expected to occur in regions such as Asia that are traditionally not considered as being strong in dairying. In these regions a pastoralsystem of milk production is less common, and it is more likely that increases in production will occur via housed systems that rely on purchased feeds such as cereals, cereal by-products and protein feeds such as soyabean meal. In short, the demand for many feeds that UK dairy farmers currently consider as key in their rations will increase with consequent effects on price. Added to this is the increased demand for many traditional dairy cow feeds such as cereals for fuel production. For example, currently approximately 40% of the corn (maize) grown in the USA goes towards bioethanol production rather than human or animal feed.

Future increases in milk production will also have to be met using less resources (e.g. human edible feeds, water and fossil fuels), less land, with a lower environmental impact, using less antibiotics, drugs and pesticides, to higher welfare standards and producing a higher quality product, both in terms of safety, microbiological quality and the impact on human health (e.g. coronary heart disease and cancer). In short, we will have to produce more milk of a higher quality from less resources.

Challenges to concentrate feeding

World average milk yield currently stands at only 2100 kg/animal, a figure that is predicted to increase to 2400 kg by 2023 (IFCN 2013). This production level is considerably lower than in traditional dairy countries such as the UK, USA or New Zealand, but has implications for world feed demand. For example, a milk yield of 2100 kg/d equates to approximately 6 kg/cow/day. To increase this to 8 kg/day (an extra 33%) requires an additional 10% of purchased feed. In contrast, in the USA where daily average yields are closer to 30 kg/d, an increase in milk yield of 33% will require approximately 25% more feed. In short, it is more economic to purchase feed to increase milk yield when current yield is low, as in developing countries, than in the UK.

The principal advantage of ruminants is their ability (via the bacteria, protozoa and fungi in the rumen) to digest feeds that are high in fibre, and to convert sources of non-protein nitrogen into high quality protein for human use. This allows cattle to utilise by-product feeds from the human food and fuel industry, and to exploit poor quality pasture. However, many ruminant diets, particularly in western countries, routinely include raw materials such as cereal grains, which could be eaten directly by humans. For example, work at Nottingham University (Wilkinson, 2011) has calculated that approximately 36% of the ingredients used in dairy cow concentrates in the UK are human edible. With an increase in demand for feed for human use, it may be less economic (and ethical) to include such high levels in dairy cow rations.

Research has shown that a greater inclusion of some by-product feeds such as distillers dark grains, can support high levels of milk production. Other by-products (particularly straws) are more problematic: the use of chemicals such as alkalis to increase their energy value has long been established, but is expensive and presents certain health and welfare issues. Considerable research has therefore been focussed on areas such as feed enzymes, although to date the results have been somewhat disappointing. Greater use of metagenomics, metatranscriptomics, and proteomics however, offer the potential for identifying novel enzymes that can be used in ruminant nutrition.

Challenges to protein nutrition

There have been considerable fluctuations in the price of protein feeds such as soyabean meal, with a 3 fold increase being witnessed in the last several years. Additionally, the dairy cow is somewhat inefficient in utilising dietary protein, converting only around 25% of its dietary intake into milk protein. One of the most effective means of improving this efficiency is to reduce the dietary protein level. An added advantage is the subsequent reduction in N excretion, which is of particular relevance to dairy farmers within Nitrate Vulnerable Zones. A recent review funded by DairyCo (Sinclair et al., 2014) has shown that dietary protein levels can, through careful rationing, be reduced to 160 g/kg DM without affecting intake or milk yield. Levels below this are unlikely to have many negative effects on health and fertility, but will often result in a decrease in intake and milk yield, and research is on-going to investigate means of reducing dietary protein levels whilst maintaining milk yield.

Studies by Law et al., (2009) have reported that reducing dietary protein from 173 to 144 g/kg DM had a relatively small effect of dry matter intake, but reduced yield by 3.8 kg/d (Table 1). Reducing dietary protein levels to 114 g/kg DM had a more dramatic effect on both intake and yield, with a 2 kg/d reduction DM intake, and a milk yield reduction of 10 kg/d. Interestingly, when dietary protein concentration was 173 g/kg DM for the first 150 days of lactation and then reduced to 144 g/kg DM for the following 150 d, milk yield was similar to animals fed 173 g/kg DM throughout lactation. This resulted in a more efficient dietary N use and saving in feed costs. Other research studies are underway at Reading University to investigate the long-term effects of feeding low protein diets on animal performance and health, and at Harper Adams University and Nottingham University to examine dietary means to maintain performance at low dietary protein levels. Preliminary results indicate that dietary protein levels can be reduced to 140 g/kg DM without a major reduction in milk yield if diets can be formulated to maximise rumen microbial protein synthesis.

One option to reduce the reliance on purchased feeds such as soyabean meal is to increase the use of home grown protein forages. Legumes such as red clover, forage peas and lucerne are particularly suitable as they are high in protein, and being legumes have a low fertiliser N requirement. Recent studies at Harper Adams University funded by DairyCo (Sinclair and Birch, 2014) has shown that the inclusion of lucerne can successfully replace grass/maize silage, reducing purchased protein requirements (Table 2). Similar studies at SRUC have shown that lucerne can successfully replace grass silage only diets.

In addition to the total amount of protein in the diet, is its quality. A proportion of feed protein is broken down and available to the rumen microbes to grow: this is referred to as rumen degradable protein (RDP). The cows protein requirements are met by the flow of this microbial protein from the rumen and dietary protein that has not been broken down by the rumen microbes (referred to as undegraded protein (UDP), or by-pass protein). Most home grown forages such as grass silage or legume silages, are very high in rumen degradable and low in undegraded protein. In contrast, purchased feeds such as soyabean meal have a higher proportion of UDP. As milk yield increases, the ability of the rumen microbes to meet the cows protein requirements decreases. Therefore at higher levels of milk production, the requirement for UDP increases, resulting in a requirement for higher UDP sources such as soyabean meal. Research is investigating ways to reduce the degradability of the protein in home grown forages, by for example, using tannins. Tannins are natural compounds that bind with protein making it unavailable in the rumen, but is then subsequently released at the lower pH of the true stomach. Previous studies (e.g. Sinclair et al., 2009) have reported that tannins can reduce the proportion of rumen degradable protein in forages, and studies are under way to examine their effects in high yielding cows.

Challenges to mineral nutrition

The importance of minerals in the diet of dairy cows is well documented, and dairy cows have traditionally been supplemented with minerals to avoid deficiencies, although the effects on animal health, fertility and product quality are of increasing importance (NRC, 2001). A recent survey of mineral use during the winter on UK dairy farms and funded by DairyCo has revealed however, that the majority of dairy farms are feeding at levels well above requirements (Sinclair and Atkins, 2014; Figure 1).

There is however, little experimental evidence to support an increase in performance or animal health from feeding such high levels. For example, recent long term dairy studies in the UK using grass silage based rations have reported no adverse effects on intake, performance, bone strength or fertility from feeding P at a the recommended rather than commercial level (Ferris et al., 2010a,b; Table 3). Indeed, the only major impact of over feeding P was to increase both the cost and environmental impact, with the higher feeding level resulting in an extra 10 kg/P/ha being excreted.

For minerals such as Cu, over feeding can lead to cow deaths with an average of 26 cases of cattle deaths per year reported between 2005 and 2012 (AHVLA 2014).

Despite this, of the 50 farms sampled in the survey of Sinclair and Atkins (2014), 6 were feeding Cu above the EU maximum limit of 40 mg/kg DM, 32 feeding above the recent industry maximum guideline of 20 mg/kg DM and all were feeding substantially above the nutritional guideline of 11 mg/kg DM. Often farmers are not aware of the mineral levels being fed, or the impact on the environment or animal health, do not take into account minerals from all sources (including free access sources, boluses and water), or do not analyse their forage for minerals. Additionally, recent research has indicated that the metabolism of minerals such as Cu differ on grass compared to maize silage (Sinclair & Mackenzie, 2014), further emphasising the requirement to consider all dietary ingredients.

Challenges to forage and grazing management

Milk yield in the UK has been increasing by approximately 100 kg/cow/annum for the last 25 years, a trend that is likely to continue. In contrast, yield from forage has been at best static, with yield from grazing generally decreasing, putting greater reliance on the proportion of milk obtained from concentrates and subsequent fluctuations in commodity prices. Grazed grass is generally regarded as the lowest cost forage available to UK dairy farmers, and the public perception of dairying is often improved if cows are able to graze. However, high yielding cows are unable to consume sufficient grass to maintain yields much above 25–30 kg/d, and therefore some form of supplementation or housing is required. On-going studies at Harper Adams and SRUC have been investigating both the welfare implications of housing vs. grazing, and strategies to increase the intake of grass in high yielding dairy cows. Giving cows a choice to be inside or out can improve welfare and maintain or increase milk yield (Moutapalli et al., 2014), but may not be practical on many farms. Mufungwe et al., (2013) reported that milk yield could be maintained at around 40 kg/d and methane production per litre of milk reduced if cows had access to pasture between morning and afternoon milking if they also had access to a TMR at pasture: removal of the TMR resulted in a decline in milk yield (Table 4). Further studies funded by DairyCo to investigate the effects of cut and carrying grass to cows, time of access to pasture and pasture presentation are on-going.

Challenges to reducing the environmental impact of dairy cows

One of the primary methods of improving the efficiency of feed utilisation is to increase output. An increase in milk yield also reduces the environmental impact of dairy production, with lower N and methane emission per kg of milk. The targeted use of feed additives or oils can also be utilised to improve N efficiency and reduce methano-genesis. An added advantage of oils is their ability to enhance the polyunsaturated fatty acid content of dairy products, with subsequent benefits on human health such as cardiovascular health. Additionally, dairy products are high in ruminal biohydrogenation intermediaries such as conjugated linoleic acids that have been demonstrated to have a range of human health benefits. Reducing the saturated fat content of milk and improving the nutritionally beneficial fatty acids such as very long-chain omega-3 polyunsaturated fatty acids has and is receiving considerable research attention, and interest from milk purchasers.

Conclusions

In conclusion, the greater demand for dairy products, in association with the changing availability of feeds, may result in the use of higher quality home-grown forages supplemented with an increasing amount of by-products in association with the targeted addition of minerals and vitamins and use of additives to improve rumen function, digestion, metabolism and performance. Such advances will be driven by evidence based research, but it must be ensured that the results are translated to dairy farmers in a practical and relevant form.

References

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