Introduction
The UK Dairy Industry, like other livestock sectors, faces pressures from increasing global demand, negative publicity about animal production, competition for land to grow animal feed versus human food, rising input costs, and concerns about environmental emissions. To address these issues, producers should aim to make more efficient use of resources, which calls for an increase in production efficiency whilst reducing environmental impact.
This paper will examine measures of production efficiency and show how they are influenced by fertility of dairy cows. A central feature is that fertility affects efficiency through premature culling, which lowers lifetime performance and increases the number of replacement heifers that need to be reared.
Production efficiency
Production efficiency is the ratio of milk output to feed input, and can be increased by increasing milk yield, reducing feed intake or a combination of the two. A cow with a higher milk yield is biologically more efficient than a cow with a lower milk yield when fed on the same diet and with the same live weight. This is because, although the high-yielding cow has a greater dietary energy requirement for milk production, maintenance requirements are the same for both cows, so the high-yielding cow partitions a greater proportion of her energy intake to milk production. For example, a cow yielding 10,000 litres of milk per annum partitions 69% of her energy intake to milk production, whereas a cow yielding 4,000 litres of milk per annum partitions 46% of her energy intake to milk production. A different way of stating this is that the higher-yielding cow spreads her maintenance requirements over more litres of milk, so is more efficient.
For a group of cows, milk yield per cow determines the number of cows required for a given level of milk production. To produce one million litres of milk per annum, for example, requires 250 cows averaging 4,000 litres per annum or 100 cows averaging 10,000 litres per annum. In this comparison, one million litres of milk involves maintenance requirements for either 250 or 100 cows. Again, higher-yielding cows spread maintenance requirements over more litres of milk, so production efficiency is greater.
Production efficiency, based on milk yield, is not always the same as resource efficiency (e.g. land use) or economic efficiency (i.e. profit). There are clear benefits from using lower-yielding cows in grazing systems, especially where they produce higher milk solids yield per hectare of grassland. In this case, production efficiency would be the ratio of milk solids output to feed input.
Milk yield and fertility
The strong relationship between milk yield and production efficiency encouraged cattle breeders to select mainly on milk yield for many decades of the last century. This was particularly seen in North American Holsteins, where elite bull mothers were often produced by embryo transfer from the very highest yielding cows, and then superovulated to produce the next generation of cows and bulls. Globalisation of cattle genetics enabled widespread use of semen from these bulls to provide rapid gains in genetic merit for milk yield. According to Defra statistics, average annual milk yield per cow in the UK increased from 4,000 litres in 1973 to 6,000 litres in 2000.
Unfortunately, focus on milk yield as a single breeding goal led to a progressive decline in genetic merit for fertility in Holstein cows. The problem was exacerbated in the UK by challenges in meeting increased nutrient demands of high-yielding cows from traditional feed resources, leading to metabolic stress and poor reproductive performance. The fertility decline was highlighted by a study at the University of Nottingham, which showed that pregnancy rate to first service had declined from 56% in the 1970s to less than 40% in the 1990s, accompanied by an increase in atypical ovarian hormone patterns (Royal et al., 2000).
Fertility and replacement rate
Failure to get in calf is the main reason for involuntary culling of dairy cows worldwide (Dallago et al., 2022), and can account for up to 50% of culls (Esslemont and Kossaibati, 2002; Brickell and Wathes, 2011). In a survey of 18 dairy farms, Brickell and Wathes (2011) found that 19% of cows were culled in Lactation 1, 24% were culled in Lactation 2, and only 55% of replacement heifers calved for a third time.
Poor fertility is a major determinant of replacement rate because cows culled prematurely have to be replaced to maintain herd size and milk output. Oestrous detection rate and conception rate affect the number of cows that get pregnant and avoid culling for fertility. These factors were modelled in the study of Garnsworthy (2004). An oestrous detection rate of 50% and conception rate of 38% resulted in a replacement rate of 35%, which concurred with national statistics. Improvement of oestrous detection to 70%, or conception rate to 55%, resulted in a replacement rate of 25%; improvement in both factors reduced replacement rate to 20%.
Replacement rate is the inverse of survival, so herds with a replacement rate between 25 and 33% cull cows in their third lactation on average. Recent data from NMR statistics for 2021 found average replacement rate was 28% and cows exited at 3.5 lactations (Hanks and Kossaibati, 2021).
Fertility and efficiency
Annual milk yield per cow increases over the first three lactations and reaches a plateau. In their first lactation, cows typically yield only 70 to 80% of their mature milk yield because they divert some nutrients towards growth rather than milk. Furthermore, economic costs of rearing are not repaid from milk income minus fed costs until approximately half way through the second lactation (Boulton et al., 2017). Premature culling due to infertility means that cows do not reach their potential milk yield and profitability, so efficiency is low.
Even if a cow is not culled for infertility, low fertility can affect efficiency. A cow with good fertility can have four lactations in the first four years of life in the milking herd. A cow with average (low) fertility will only have three lactations in four years. The cow with good fertility will produce 27% more milk in her lifetime for an increase in energy intake of only 12%, so feed efficiency is increased by 8%.
Furthermore, methane emissions per litre of milk are reduced by 13%. Basically, the cow with good fertility spreads feed inputs and methane outputs incurred during the rearing period over more litres of milk in her lifetime.
At the herd level, a higher replacement rate means that more heifers have to be reared, so more feed is required. Feed required for replacement heifers also increases if age at first calving is greater than the target of 24 months. Over a range of 20 to 45% replacement rate and 24 to 36 months age at first calving, feed energy required for replacement heifers varies from 16 to 44% of total feed energy required for the whole herd. Fertility, therefore, affects feed efficiency of the whole farm.
Under the Research Partnership between Nottingham University and AHDB Dairy, a project was conducted to quantify whole farm feed efficiency (WFFE) over the range of UK dairy systems (Garnsworthy et al., 2019). Whole farm feed efficiency was defined as annual milk production divided total feed dry matter produced or purchased for all dairy animals, including milking cows, dry cows and youngstock. Farms were classified into five systems according to number of months cows spent grazing each year. System 1 farms grazed cows for >9 months; System 2, 6-9 months; System 3, 3-6 months; System 4, 0-3 months; and System 5 farms housed cows all year. Detailed data were gathered by visiting 21 dairy farms quarterly for one year, and more widespread data on feed use and economics were gathered by adding questions to the National Farm Business Survey for 300 dairy farms.
Average WFFE increased as grazing time decreased across systems, ranging from 0.99 for System 1 farms to 1.13 for System 5 farms. This was expected because there is less control over feed supply when cows are grazing than when they are housed. Within each farming system, however, there was considerable variation. Ranges in WFFE for individual farms were 0.5 to 1.3 in the detailed survey, and 0.2 to 1.5 for the National survey. There was a strong relationship between WFFE and gross margin per hectare.
The main driver of WFFE in both surveys was milk output per hectare, which was a function of stocking rate, milk yield per cow and grass/forage quality. Feed wastage or feed underutilisation was another important driver. Of the animal management and health factors, the biggest single driver was proportion of cows culled for fertility, followed by age at first calving. Reducing fertility culls from 40% to 20% would improve WFFE by 15%.
Sustainability
Although most people think of sustainability only in the context of environmental impact, sustainability has three pillars – environment, society and economics. An enterprise is only sustainable if it does not unduly affect the environment, is socially acceptable, and returns a profit. Improving fertility has positive effects on all three pillars.
According to the UK greenhouse gas (GHG) Emissions Inventory, agriculture accounts for 9% of total emissions, and dairy accounts for 27% of agricultural emissions or 2.4% of total emissions. The substantial carbon sequestration by grassland, trees and hedges on dairy farms is not credited to agriculture, but is in the category of land use, land use change and forestry. Carbon sequestration within animals, which can be for up to ten years in dairy cows, does not feature in emissions calculations. Overall, therefore, dairy makes a small contribution to UK GHG emissions compared with emissions by the energy sector. Nevertheless, all sectors have to aim for reductions in GHG emissions.
According to FAO, dairy emissions comprise feed carbon footprint (46%), enteric methane (39%), manure management (10%), and farm energy use (5%). By improving fertility, and thereby reducing replacement rate and increasing feed efficiency, GHG emissions can be reduced from all sources, with the possible exception of farm energy use (e.g. electricity for milking and refrigeration, fuel for machinery).
Improving WFFE by reducing replacement rate will have obvious benefits on feed carbon footprint because less feed will be required per litre of milk. Lower methane emissions from a cow with good fertility (13%) are discussed above. Reducing replacement rate through improved fertility will have an even greater effect on methane emissions by a whole herd because fewer replacement heifers are required. Herd replacements emit up to 27% of herd methane emissions, and improvements in fertility could reduce methane emissions by up to 24% (Garnsworthy, 2004). Similar improvements can be expected for reduced ammonia, nitrogen and phosphorus excretion.
An indirect route by which the dairy industry benefits national GHG emissions is through dairy beef. It is estimated that approximately 50% of UK beef originates in the dairy herd, either from cull cows or from dairy cross beef calves that are raised for beef production. Carcasses from calves originating in the dairy herd have one third of the carbon footprint of calves originating in the beef herd (Opio et al., 2013). This is because in the beef suckler herd impact of breeding animals is allocated to beef, whereas in the dairy herd impact of breeding animals is allocated to milk. Reducing replacement rate through improved fertility will lower the number of cull cows available for beef, but this will be more than offset by increased numbers of dairy cross beef calves available for fattening into prime beef.
Genetic improvement
Identification of the decline in cow fertility up to the 1990s provided a stimulus for developing a genetic index for fertility. The Fertility Index was introduced in 2005 and has led to steady improvement in genetic merit for fertility whilst genetic merit for milk production continues to improve (Winters, 2022).
Heifer replacements should have the highest genetic merit in the herd. Theoretically, culling cows in Lactation 3 provides faster genetic gains than keeping them until Lactation 4. As discussed above, however, cows culled in Lactation 4 have greater lifetime efficiency than cows culled in Lactation 3. Furthermore, it is often the highest yielding cows that are culled early, and there may be little opportunity to cull low yielding cows. These conflicting drivers were modelled by De Vries (2021) who concluded that the optimum lactation to balance culling trade-offs was the Lactation 4.
Another consideration related to genetic improvement, is how many heifer replacements are produced by each cow. Under normal circumstances, with a three-lactation herd life, a cow would produce 1.5 bull calves and 1.5 heifer calves. There will be some losses among heifers, and we might expect 80% pregnancy rate, 5% calf mortality, and 5% other losses. Given these losses, the cow produces a total of 1.08 heifer replacements – she only just replaces herself. This leaves little scope for genetic selection apart from which bull to use.
Development of sexed semen has been a game changer for producing female replacements. Early attempts were disappointing, but the latest technology produces about 90% female offspring, although sexed semen may reduce conception rate by 10%. Instead of producing 1.08 heifer replacements in three lactations, sexed semen would result in 1.95 heifers per cow. Sexed semen allows producers to breed replacements from the best heifers and cows, and to produce dairy cross beef calves from inferior cows. According to AHDB statistics, sexed semen was used for 70% of dairy inseminations in 2021/22.
Sexed semen does not overcome effects of fertility on replacement rate, WFFE and methane emissions from cows and dairy replacements. In fact, reduced conception rate with sexed semen
might increase replacement rate and reduce WFFE. However, overall sustainability of dairy plus beef production is likely to be improved by using sexed semen.
Conclusions
Poor fertility in dairy herds results in premature culling of cows, lower lifetime performance, less opportunity for genetic selection, and increased replacement rate. Increased replacement rate means that more heifers have to be kept on the farm, consuming feed and contributing to environmental impacts.
The optimum lifetime of cows is four lactations, which provides the best balance between lifetime milk yield, spreading rearing costs, and genetic improvement. The majority of cows, however, are culled before their third lactation, and failure to conceive is the main reason for culling.
Improving fertility is a win-win strategy that increases resource efficiency, feed efficiency, and all aspects of sustainability.
Acknowledgements
Background research that informed this paper was funded in several projects by MAFF, DEFRA, LINK, DairyCo, AHDB Dairy and industry partners.
References
Brickell JS, Wathes DC (2011). A descriptive study of the survival of Holstein-Friesian heifers through to third calving on English dairy farms. J Dairy Sci, 94, 1831.
Dallago GM, Wade KM, Cue RI, McClure JT, Lacroix R, Pellerin D, Vasseur E (2021). Keeping dairy cows for longer: a critical literature review on dairy cow longevity in high milk-producing countries. Animals, 11, 808.
De Vries A (2021). Profitability and efficiency of the five lactation average dairy cow. British Cattle Breeders Club Digest 76, 45-48.
Esslemont RJ, Kossaibati M (2002) DAISY Research Report No. 5: The costs of poor fertility and disease in UK dairy herds - Trends in DAISY herds over 10 seasons. UK: University of Reading.
Garnsworthy PC (2004). The environmental impact of fertility in dairy cows: a modelling approach to predict methane and ammonia emissions. Animal Feed Science and Technology 112, 211–223.
Garnsworthy PC, Gregson E, Margerison JK, Wilson P, Goodman JR, Gibbons J, Dorigo M, Topliff M (2019). Whole farm feed efficiency on British dairy farms. Proceedings of the British Society of Animal Science, 2019, 193.
Hanks J, Kossaibati M (2021). Key Performance Indicators for the UK national dairy herd. A study of herd performance in 500 Holstein/Friesian herds for the year ending 31st August 2021. UK: University of Reading.
Opio C, Gerber P, Mottet A, Falcucci A, Tempio G, MacLeod M, Vellinga T, Henderson B, Steinfeld H (2013). Greenhouse gas emissions from ruminant supply chains – A global life cycle assessment. Food and Agriculture Organization of the United Nations (FAO), Rome.
Royal MD, Darwash AO, Flint APF, Webb R, Woolliams JA, Lamming GE (2000). Declining fertility in dairy cattle: changes in traditional and endocrine parameters of fertility. Animal Science 70, 487–501.
Winters M (2022). Breeding to achieve net zero – the power of genetics. British Cattle Breeders Club Digest 77, 22-24.
Photography Credit @Jenny Wood Photography