Longevity in dairy cows is very important, as it affects production economy (Grandl et al., Reference Grandl, Furger, Kreuzer and Zehetmeier2019; Dallago et al., Reference Dallago, Wade, Cue, McClure, Lacroix, Pellerin and Vasseur2021), the environmental impact of milk production (Grandl et al., Reference Grandl, Furger, Kreuzer and Zehetmeier2019), and the welfare of cows (Bruijnis et al., Reference Bruijnis, Meijboom and Stassen2013). Additionally, many consumers likely expect dairy cows to live long lives (De Vries and Marcondes, Reference De Vries and Marcondes2020). Dallago et al. (Reference Dallago, Wade, Cue, McClure, Lacroix, Pellerin and Vasseur2021) reviewed the longevity of dairy cows and concluded that dairy cow longevity has decreased in most high milk-producing countries during the last five decades.
The ability of a cow to stay in a dairy herd for a long time is influenced by many factors, some of which interact in complicated ways (Roche et al., Reference Roche, Renaud, Genore, Shock, Bauman, Croyle, Barkema, Dubuc, Keefe and Kelton2020; Rostellato et al., Reference Rostellato, Promp, Leclerc, Mattalia, Friggens, Boichard and Ducrocq2021). These risk factors primarily relate to the life of the cow after her first calving. Very few studies have focused on risk factors for cow longevity associated with the early life of cows – the period before the cow calves for the first time. Previously, the association between longevity and season of birth (Bobadilla et al., Reference Bobadilla, López-Villalobos, Sotelo and Damián2024), age at first calving (Froidmont et al., Reference Froidmont, Mayeres, Picron, Turlot, Planchon and Stilmant2013; Sherwin et al., Reference Sherwin, Hudson, Henderson and Green2016), growth rates, early-life diseases, and other rearing conditions (Hultgren and Svensson, Reference Hultgren and Svensson2009; Bach, Reference Bach2011; Heinrichs and Heinrichs, Reference Heinrichs and Heinrichs2011) have been studied. Dallago et al. (Reference Dallago, Cue, Wade, Lacroix and Vasseur2022) evaluated the association between longevity, measured as length of life and length of productive life (LPL), and calving ease, calf size, and twinning in a large sample of Holstein cows from Quebec, Canada. However, almost half of the cows included in their study did not complete the study but were censored before they were culled.
The present study is the first study to evaluate the association between early-life risk factors and cow longevity measured as both LPL and lifetime milk yield in a large sample of cows without censoring of animals. Additionally, this study is one of very few studies including cows of breeds other than Holstein. The objective was to evaluate the relationship between calf size, twinning, age at first calving, breed and month of birth, and LPL and lifetime milk yield, respectively, among Danish dairy cows.
Materials and methods
Danish dairy farmers are required to report certain information to the Danish Cattle Database, which is managed by the Danish Cattle Federation. This includes dates of calvings, slaughter, deaths and all movements of animals between herds. Likewise, farmers must report the sex and number of calves born. Farmers can also report the size of the calf, which is not mandatory but is done in approximately 80% of cases. Among many other additional things, the Danish Cattle Database also includes information about the cow's breed. In June 2024, a dataset from the Danish Cattle Database including information on all Danish dairy cows culled during the five-year period 2019–2023 was generated. ‘Culled’ included slaughtered, euthanised, and unassisted dead cows, but not cows sold to other Danish herds or exported for milk production. Only cows of the dairy breeds Holstein, Jersey, or Danish Red Dairy, or crosses between these breeds were included in the dataset. Only cows culled from milk-producing herds were included. Approximately 93% of all Danish dairy cows are part of the official milk recording scheme. For these cows, monthly milk recordings at cow level were available.
Longevity was measured in two ways: LPL and lifetime milk yield (ECMlife). LPL was defined as days from first calving to culling, and ECMlife was defined as lifetime milk yield of the cow measured in kg energy-corrected milk (Compton et al., Reference Compton, Heuer, Thomsen, Carpenter, Phyn and McDougall2017; Schuster et al., Reference Schuster, Barkema, De Vries, Kelton and Orsel2020). For each cow, milk yield per day of life was calculated as ECMlife divided by the number of days from birth to culling.
Explanatory variables included in the statistical analyses are listed in Table 1. All explanatory variables related to birth refer to the birth of the culled cow, not to her own subsequent calving(s). As an example, twinning thus refers to the culled cow being born as a twin or not, and not to whether she has had twins or not at one or more of her own calvings. Calvings with three or more calves were recorded as ‘yes’ for twins. Farmers recorded the size of calves as ‘small’, ‘medium’, or ‘large’ (compared to other calves of the same breed). Age at first calving was grouped into quartiles and differed between breeds, with median ages at first calving being 751 days (1st quartile: 714; 3rd quartile: 801) for Holstein, 714 days (677; 763) for Jersey, 761 days (724; 811) for Danish Red Dairy and 753 days (714; 806) for crossbred cows. Therefore, groups were based on quartiles within breed. Heifers calving for the first time at an age of less than 500 days or more than 1,500 days were excluded from the dataset.
Table 1. Descriptive statistics (distribution, lifetime milk yield (ECMlife) and length of productive life (LPL)) for each level of the explanatory variables included in an evaluation of longevity among Danish dairy cows culled during 2019 to 2023
The association between the outcomes LPL and ECMlife and the explanatory variables were analysed using linear mixed models (PROC MIXED, SAS 9.4). LPL and ECMlife were evaluated in separate models. To take differences between herds into account, herd was included in the models as a random effect. The approximate normal distributions of LPL and ECMlife were confirmed using Kolmogorov-Smirnov tests. Models were reduced using backward elimination with P < 0.05 defined as significant. To take a possible correlation between twins and calf size into account, interactions between twin and calf size were included in the models. The final models thus included the outcomes LPL and ECMlife, respectively, and the explanatory variables age at first calving, month of birth, breed, and the interaction between twin and calf size.
Results
The dataset included information from 931,437 cows. Of these, 767,305 cows had recordings of calf size, and 716,120 additionally were from herds participating in the official milk recording scheme, and thus also had information on ECMlife. Overall, mean LPL was 1,074 days (minimum: 0; first quartile: 582; median: 997; third quartile: 1,483; maximum: 6,787). Mean LPL was highest in Jersey cows (1,139 days), 2 months longer than Holstein cows, and more than 4 months longer than crossbred cows (Table 1). Mean ECMlife was 32,088 kg energy-corrected milk (minimum: 0; first quartile: 16,007; median: 28,925; third quartile: 44,782; maximum: 214,775). ECMlife was highest for Holstein cows (33,195 kg), followed by Jersey, Danish Red Dairy, and crossbred cows (Table 1). Fig. 1 presents the distribution of LPL and ECMlife. Of the culled cows, 83.65% were slaughtered, 10.10% died unassisted, and 6.25% were euthanised. Differences in LPL and ECMlife between slaughtered, dead, and euthanised cows were minor, with mean LPL of 1077, 1075, and 1034, and mean ECMlife of 32,027, 32,620, and 32,056, respectively. Descriptive statistics of LPL and ECMlife stratified by the explanatory variables are given in Table 1. Fig. 2 presents the mean energy-corrected milk yield per day of life for cows culled during different lactations, showing increasing milk yields per day of life from approximately 7 kg milk per day of life in cows culled during the first lactation to more than 20 kg per day in cows culled in their fifth or later lactations. Table 2 presents results from the two analyses of the association between LPL and ECMlife and the explanatory variables. For both LPL and ECMlife, type III tests of fixed effects were significant (P < 0.0001) for all explanatory variables, and the interactions between twin and calf size were significant (P < 0.0001). The combination of twins and large calves was uncommon: only 0.6% of all twins (116 calves) were recorded as ‘large’. For singletons, the estimated LPL was almost 2 months longer for cows born as large calves than for small calves, and more than 1 month longer than for medium-sized calves. Cows born as twins had 2-to-3-month shorter estimated LPL compared to cows born as singletons. For cows born as twins the effect of calf size was small with less than one month difference in estimated LPL across calf sizes, with the highest LPL in medium-sized calves. For singletons, ECMlife was approximately 1,200 kg higher for large calves than for medium-sized calves, and 2,100 kg higher than for small calves. Compared to singletons, ECMlife was approximately 1,500 to 3,500 kg lower in cows born as twins, again with only minor differences between calf sizes within the twin group. Age at first calving was significantly associated with both LPL and ECMlife: Cows calving for the first time among the third quartile of age within breed had an estimated LPL of 2.5 months longer than cows calving among the first quartile of age. ECMlife was more than 2,600 kg higher in cows calving for the first time in the third quartile compared to the first quartile. The month of birth was distributed approximately equally throughout the year with 7.3 to 9.1 percent of cows born each month. Longevity, measured as both LPL and ECMlife, was significantly associated with month of birth. Cows born in April to September had an estimated LPL of approximately 10–20 days less than cows born in November to March, and ECMlife was approximately 400–800 kg lower in cows born in April to September compared to November to January.
Figure 1. Distribution of length of productive life (LPL, days) and lifetime milk yield (ECMlife, kg energy corrected milk) for 767,305 and 716,120 Danish dairy cows, respectively, culled during 2019–2023.
Figure 2. Box plot illustrating mean milk yield (kg energy-corrected milk) per day of life for 716,120 Danish dairy cows culled during different lactations.
Table 2. Results from two linear mixed models evaluating the association between length of productive life (LPL) and lifetime milk yield (ECMlife) and early-life risk factors in Danish dairy cows culled during 2019 to 2023
Discussion
The results from this study clearly demonstrate the importance of ‘a good start’: Cows with a difficult start – born as a twin or of small size – generally had decreased longevity, irrespective of whether longevity was measured as LPL or ECMlife. Previous research has documented several problems associated with small calves, calves born at difficult calvings, or calves born as twins. These problems include an increased risk of failure of passive transfer, increased morbidity and decreased average daily weight gain (Lombard et al., Reference Lombard, Garry, Tomlinson and Garber2007; Waldner and Rosengren, Reference Waldner and Rosengren2009; Shivley et al., Reference Shivley, Lombard, Urie, Kopral, Santin, Earleywine, Olson and Garry2018). Heinrichs and Heinrichs (Reference Heinrichs and Heinrichs2011) found that dystocia was negatively associated with lifetime milk yield. Bach (Reference Bach2011) found that calves with a higher growth rate 12 to 65 days old had a higher probability of reaching the second lactation. Dallago et al. (Reference Dallago, Cue, Wade, Lacroix and Vasseur2022) studied the association between birth conditions and longevity in Holstein cows. Using conditional inference survival trees, they found the longest life in large or medium-sized calves born at an unassisted calving. Small calves born as twins had the shortest life. Large or medium-sized calves born at an unassisted or surgery calving had the longest productive life, whereas twin calves from an assisted calving had the shortest productive life. In the present study, calf size was associated with longevity only in singletons, whereas being born as a twin was associated with decreased longevity, irrespective of the size of the calf.
As illustrated in Fig. 2, longevity influenced milk yield per day of life to a very high degree. Cows culled in their fifth or later lactations had a milk yield per day of life which was approximately three times higher than cows culled during the first lactation. This clearly illustrates the benefits of cows living long lives on both production economy and climate impact. Overall, the explanation behind this is fewer productive days after first calving compared to unproductive days before first calving in cows culled at a younger age. Additionally, the low milk yield per day of life among cows culled during their first lactation – compared to cows culled during their second or later lactations – is likely also influenced by some first lactation cows culled after very few days in milk and the fact that culling for low milk production is more likely during the first lactation compared to later lactations (Thomsen and Houe, Reference Thomsen and Houe2023).
In the present study, the best longevity, measured as both LPL and ECMlife, was found among cows belonging to the third quartile of age at first calving. This is in contrast with findings from some previous studies. Hultgren and Svensson (Reference Hultgren and Svensson2009) found the lowest risk of culling in cows calving for the first time less than 771 days old. However, differences between this age group and older age groups were relatively minor and often not significant. Bach (Reference Bach2011) found a small difference in the mean age at first calving of cows completing the first lactation (724 days) or not (737 days). Froidmont et al. (Reference Froidmont, Mayeres, Picron, Turlot, Planchon and Stilmant2013) found the highest lifetime milk production and the highest number of productive days in cows calving for the first time 22–26 months old. Sherwin et al. (Reference Sherwin, Hudson, Henderson and Green2016) found increasing odds of culling during the first lactation with increasing age at first calving. It is noteworthy that most of these studies are much smaller than the present study and typically focus on stayability during the first lactation only. Age at first calving is heavily influenced by management decisions made by the farmer. Age at first calving may thus be seen as a manageable risk factor, defined as a risk factor which a farmer can relatively easily (in terms of labour and investments) change, eliminate or mitigate (Thomsen and Houe, Reference Thomsen and Houe2024). Risk factors related to the birth of a cow (size of calf and twinning) are not manageable in the same way. However, knowledge about the association between such risk factors and longevity may be used as a decision support tool when farmers need to make an early selection between two otherwise ‘equal’ replacement heifers: which one to keep and which one to sell. Age at first calving has been shown to be associated with numerous other outcomes, for example, risk of dystocia, production economy, fertility, and risk of sole ulcers (Ettema and Santos, Reference Ettema and Santos2004; Eastham et al., Reference Eastham, Coates, Cripps, Richardson, Smith and Oikonomou2018; Atashi et al., Reference Atashi, Asaadi and Hostens2021; Thomsen and Houe, Reference Thomsen and Houe2024). The optimal age at first calving varies between these outcomes, and it is therefore important not only to focus on one outcome when deciding the target age at first calving in a herd.
The findings of lower LPL and ECMlife in cows born in April to September may be due to heat stress. Heat stress is relatively common in Denmark during the summer months. Bobadilla et al. (Reference Bobadilla, López-Villalobos, Sotelo and Damián2024) studied longevity in a pasture-based milk production system in Uruguay and found that season of birth had a small impact on the longevity of dairy cows, with the longest LPL in cows born in the spring. Heat stress may affect calves both in utero and during early life. Ahmed et al. (Reference Ahmed, Younas, Asar, Monteiro, Hayen, Tao and Dahl2021) found that late-gestation heat stress of dairy cows reduced foetal growth and influenced postnatal performance and immune status of the offspring. Broucek et al. (Reference Broucek, Kisac and Uhrincat2009) found a reduced intake of concentrates and reduced growth in pre-weaned Holstein calves subjected to heat stress. Even though the effect of season found in the present study was relatively small compared to the other risk factors identified, a reduced longevity in cows subjected to heat stress in early life adds to the existing list of negative effects of heat stress in dairy cows and calves (Cartwright et al., Reference Cartwright, Schmied, Karrow and Mallard2023). Season of birth is often correlated with season of both first insemination and first calving. As Danish dairy cattle are typically only at pasture during the period May to November, the likelihood of calves being at pasture at a young age also differs with season of birth. These factors may also partly explain the observed association between season of birth and longevity.
This study included more than 80% of the approximately 931,000 Danish dairy cows culled during the five-year period studied and is thus considered representative of the population of dairy cows in Denmark. The retrospective study design eliminated the need for censoring of cows still alive, hereby removing uncertainty associated with the future faith of cows. The negative effects of ‘a bad start’ may be even bigger than estimated here, as this study only included animals calving at least once. Calves experiencing ‘a bad start’ are likely to have a higher risk of culling before their first calving. Thus, seen in a broader perspective, the effect of early-life risk factors for cow longevity may be even larger than estimated in the present study. Generally, the quality of the included data was considered very high. LPL was calculated based on information about dates of birth and death which farmers are required to report with 7 days after the event (EU legislation: EC 1760/2000). The risk of recall bias is thus very small, and the farmer-reported dates were double-checked, e.g., with dates of slaughter or death reported by slaughterhouses and incineration plants. ECMlife was calculated based on information about milk yield recorded by the Danish milk recording association, certified by the International Committee for Animal Recording (ICAR). Farmers’ reports of twinning are also considered very reliable, whereas calf size is more subjective and might be influenced by the assessment of individual farmers.
In conclusion, the milk yield per day of life was three times higher in cows culled during their fifth or later lactations, compared to cows culled during the first lactation. This fact underlines the importance of longevity in dairy cows, both in relation to production economy and climate impact. A good start impacts cow longevity. Cows born as singletons and being large at birth generally had longer productive lives and produced more milk during their life. Age at first calving also impacted longevity, with longer productive lifespan and more milk produced by cows belonging to the third quartile of age at first calving.
Acknowledgements
This work was supported by Foreningen PlanDanmark (grant no. 23054). The study sponsor had no influence on the selection of data, analyses, presentation of results, interpretations or conclusions.
Competing interests
The author has no competing interests.
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