Longevity of a dairy herd has become a metric which has started to be investigated in terms of herd performance, due to its impact on the economics and carbon footprint of the herd. It can reflect the health and welfare status of the herd and has role in terms of public perception of the dairy industry. When defined in the dictionary, longevity means having a long existence. The natural lifespan of a dairy cow has been reported to be approximately 20 years (Nowak, 1999); however research has highlighted that it is more like 3.6 lactations (approximately 6 years) in the UK and 2.8 lactations (approximately 5 years) in the USA. This indicates that management decisions are having large impacts on the average productive lifespan of dairy cows, with the removal of cows from a herd due to old age being rare (Fetrow et al, 2006). The age at exit has been decreasing over the past 13 years, as indicated by the data from the NMR 500 herd report 2022 (Hanks and Kossaibati, 2022), where the median lactation number at exit has decreased from 3.9 lactations to 3.6 over the past 13 years. Increasing the longevity of a dairy herd has been promoted as it decreases the requirement for replacement heifers and increases the average herd production level.
Definitions around cows leaving the herd
The exact definition of longevity varies among studies, with productive life often discussed as the time a cow remains in the herd from first calving until culling or death (Brickell and Wathes, 2011; Compton et al, 2017). Other metrics used include age of culled cows (time from birth to cull/death/removal from farm), lifetime milk production (milk produced by the cow during her lifespan) and culling rate. These metrics are commonly discussed as averages, which do not consider the range of animals with the herd, which makes it hard to determine the longevity and efficiency of the herd. A better metric would be how many animals in the herd reach the target lifetime yield, with lifetime yield defined as the total production of the cow divided by the total days of life, from birth to culling. The median lifetime yield was reported to be 12.7 kg in 2022 (Hanks and Kossaibati, 2022); however, this will vary depending upon the farm system and outputs. Therefore it is preferential to look at the percentage of cows not reaching the individual farm's target.
Cows either leave the herd due to mortality on farm (deaths) or due to a decision to remove the cow from the herd (culls). Culling is a decision usually made by the dairy farmer and it is influenced by the economic interest associated with farm animals. Culls have traditionally been classified as voluntary or involuntary, which are sometimes hard to distinguish between. Another classification is ‘forced culls’ and ‘economic culls’. Forced culls are those cows for which no possible productive future exists and economic culls are those cows for which a decision has been made that replacing them with another cow is a sensible economic option (Fetrow et al, 2006). This is important in determining whether the replacement of cows with heifers is efficient, in terms of whether key management decisions are made on production, or whether the farm is being forced into making replacement decisions and thus there is an issue that needs addressing on the farm.
Rather than using averages or medians, which do not reflect the uniformity of the performance, it is preferable to look at the percentage of animals surviving to key time points (Oltenacu and Broom, 2010). Three of these metrics are highlighted in Figure 1, and provide an overview for efficiencies of different lifestages.
Rearing efficiency is an overall metric for the efficiency of the heifer rearing period, with the best herds reported to have a rearing efficiency of 92% (Bach et al, 2021). Heifer effectiveness considers the heifer rearing efficiency as well as the longevity of the herd; however, it is reported that most herds are estimated to have a heifer efficiency of 60% (Bach et al, 2021). A low heifer effectiveness but a high rearing efficiency indicates that there are issues with survival whilst a milking cow, whether that it is due to health, fertility or productivity issues. Because of this, it is prudent to determine the percentage of first lactation heifers surviving to second lactation, as national UK data shows that approximately 1 in 8 heifers calve in for the first time but fail to reach second lactation (Bach and Ahedo, 2008; Sherwin et al, 2016; Hanks and Kossaibati, 2022). A failure of first lactation heifers to survive to second lactation is a considerable economic issue for the farm, as well as having health, welfare and sustainability impacts (Archer et al, 2013; Boulton et al, 2017). An example of this is shown in Figure 2; there is an excellent rearing efficiency of 95%, but a low heifer effectiveness of 60%. The low heifer effectiveness correlates to a low first lactation survival rate, with nearly 1 out of 4 of first lactation heifers not recalving, which is a similar survival rate for second lactation (80%).
Decision making around cows leaving the herd
Decisions around when individual animals are exiting the herd are very farm-specific and commonly are multifactorial and complex to make, involving both intrinsic (health, milk yield and fertility status) and extrinsic factors (heifer replacements available, yield, housing available and market prices). Research has shown that factors such as low milk production, infertility and occurrence of health issues (eg mastitis, metabolic disease) are considered as reasons for culling cows at an individual factor (Bell et al, 2011; Pfeiffer et al, 2015); however, these intrinsic factors are not associated with culling cows at a herd level, with extrinsic economic factors playing a significant role (De Vries, 2013). A study has shown that the proportion of cows receiving veterinary treatment negatively and insignificantly correlates with herd longevity (Fall et al, 2008). This means that there is a potential for extending herd level longevity without affecting herd performance in terms of milk production, reproduction and health.
A study from the USA demonstrated that dairy cow longevity was significantly linked to changes in investments made by the farmer, primarily increasing building space and number of spaces available for dairy cows. This relates to the Asset Replacement Theory (Hartman and Tan, 2014) which is highlighted in Figure 3 (Owusu-Sekyere et al, 2023).
Risk factors for removal from the herd
The main reasons for culling in the UK are currently not reported anywhere to the author's knowledge; however a study in Canada reported that the main reasons for culling were fertility, mastitis, feet and leg problems and low milk production (1997 to 2016) in adult dairy cows (Canadian Dairy Information Center, 2020). The risk of culling has also been reported to not be constant over the life of a cow, as it varies depending on the parity, stage of lactation, milk yield and fertility status (Pinedo et al, 2010; 2014; Heise et al, 2016). This will vary between farms and over time, so it will need to be assessed on an individual basis and repeatedly over time.
How does longevity impact the economics of the herd?
Profitability of a dairy farm relies on cows reproducing regularly, maintaining high feed conversion efficiencies and remaining healthy. Increasing the productive life of the cow can improve the profitability as a result of an increase in the number of productive days in the herd, increasing the lifetime yield of the cow and theoretically increasing her efficiency and the overall efficiency of the herd. Research from SRUC has shown that the British cattle industry had a wastage of >£860 million for 2020 due to decreased DLWG (55%), mortalities (21%) and fertility (20%); calculated from data from the Cattle Tracing System with efficiency defined as a product output over the resource input (outputs = slaughter or calving) (Geragthy, 2022). In this study, there was wide variation between farms, with a significant trend for mortality and fertility having a higher importance in dairy farms compared to DLWG. Increased mortality relates to a short longevity, which indicates that animals are not expressing their maximum potential for productivity and profitability, since dairy cows become profitable at their third lactation due to high costs associated with the early life non-productive stage (Archer et al, 2013; Boulton et al, 2017). In addition, elevated culling rates result in more first and second lactation cows being culled, which decreases animal longevity and reduces the profitability of the system. Overall, the most common reason for culling of first and second lactation cows is reproduction issues (Pinedo and De Vries, 2010). During the initial third of the lactation, first calving cows have been reported to be more likely culled due to low milk production and milkability, while second lactation animals are culled due to the incidence of meta-bolic and other diseases (Heise et al, 2016).
Premature removal of animals from a herd can have a considerable economic impact on individual farms, and should be considered on an individual farm basis. For example the herd shown in Figure 4, the impact of the low first lactation survival is highlighted. The impact of a high first lactation heifer removal is the debt which is left in the herd by the premature exit of these animals, resulting in the surviving first lactation heifers needing to produce more milk to cover this debt, or the break-even point for the cohort of heifers in terms of repaying their rearing costs is shifted further into the future.
Premature removal from the herd can also relate to the stage of lactation at exit, with culling in early lactation reported to cost dairy farmers £325 to £650 per cow (De Vries, 2013).
The impact of longevity on sustainability
Dairy production is often implicated as a major emitter of green-house gases (GHG), with the three main sources identified as:
Increasing milk yield has been discussed as an option for mitigating methane emission intensity due to the dilution of maintenance; however, with increasing milk yields of cows, research has shown a decrease in the length of productive life (Knaus, 2009), resulting in a higher culling rate and a requirement for more replacement heifers. Dilution of maintenance refers to the concept of increasing the productivity of the dairy cows to reduce the amount of greenhouse gases emitted per unit of milk produced; it works as the fixed maintenance energy requirements are spread over a larger volume of milk if the yield increases and the cow is more efficient.
Increasing longevity of cows has been discussed as a method of decreasing the enteric methane emissions from replacements, as a herd with increased productive lifespan will need fewer non-milk producing replacement heifers and will have more mature, higher yielding cows. Replacement heifers have been reported to contribute between 20–33% of the whole herd enteric methane emissions (Wall et al, 2012; Knapp et al, 2014); this variation depends on age at first calving and productive lifespan. Examples of how altering the number of replacements required and the age at first calving impact the replacement contribution to the whole enteric herd CH4 are shown in Figure 5.
Increasing the productive lifespan of cows will decrease the amount of cull cow beef entering the supply chain and therefore would require an increase in beef animals to maintain the same supply chain. However, with a decrease in heifer replacement requirements, there will be an increase in dairy-beef animals entering the beef supply chain, which produce significantly less greenhouse gas per kg of meat produced, when greenhouse gas allocated to milk is accounted for, compared to meat produced by traditional non-milk producing animals (Zehetmeier et al, 2012; Vellinga and De Vries, 2018). This is an option for the industry to explore going forwards.
What is the optimum longevity of a cow?
This is a debated topic and requires multiple factors to be considered when d eciding what the optimum is. According to the research by De Vries (2020), there are five key drivers to maintaining the herd structure and determining the optimum longevity of the cow from an economic point of view. These are highlighted in Figure 6 and show the balance between the five factors.
Genetic opportunity cost is the loss from keeping older, less genetically improved cows instead of younger, more genetically improved cows. There has been a large amount of discussion around bringing in new genetic traits into the herd, with the average value of the economic selection index lifetime net merit (NM$) increasing from $30PTA (£24) of NM$ before 2010 to approximately $70 PTA (£56) of NM$ per year (De Vries, 2020). Research has highlighted that the average difference in genetic merit is increasing between heifers and older cows due to selective breeding. This has historically meant that farmers have culled cows out of the herd to replace them with heifers, limiting the productive lifespan length. However, it has been reported that the accelerated genetic gain due to genomic selection has a minor effect on optimal productive lifespan; this is because it ignores phenotypic differences between parities and herd replacement costs (De Vries, 2020). Rearing fewer heifers allows for farmers to produce more dairy-beef cross calves, which offer a calf value opportunity, through the selling of these calves, assuming there is a viable market to support the production and rearing of these calves (De Vries, 2020).
Increasing the number of replacements and culling rates results in an elevation in the lack of maturity cost relates to cows being less efficient milkers in their first to fourth lactation, with first lactation heifers producing 80% of the mature cattle equivalent. Research has estimated that the cost of a lack of maturity of $330/cow/year (£265/year) for herds with an average productive lifespan of 3 years, versus $200/year (£160/year) for herds with an average productive lifespan of 6 years. However, this is counterbalanced by the aged cost, which assumes lower milk sales minus feed costs for cows who were seventh lactation plus compared to fifth and sixth lactation cows; this is partially due to increase chance of the cow having encountered an endemic disease or health issue (the aged cow cost).
The final model suggested by De Vries (2020) was the optimal longevity for a herd is estimated to be fifth lactation, assuming the cows leaves the herd healthy; this is variable between farms and needs to be assessed for individual farms.
Conclusions
Research suggests that extending herd longevity can improve profitability by increasing productive days and lifetime yield. Additionally, increasing longevity can reduce greenhouse gas emissions by reducing the need for replacement heifers, which are significant contributors to methane emissions. The optimum longevity of a cow is a debated topic influenced by factors like genetic opportunity cost, lack of maturity cost and aged cost, which must be balanced for optimal herd structure and economic performance. The optimum longevity of a cow will depend on the individual farm but veterinary surgeons and farmers should be monitoring longevity metrics on farm, including the reasons behind premature and forced exits from the herd.