Culling rates and farmers´ stated reasons for culling
There is no single optimal culling rate that is applicable to all herds for all years due to variety of economic factors, farm capacities, individual cow factors, morbidity and mortality rates within the herd, availability of replacements, biosecurity considerations etc (1). Not all studies have presented the culling rates due to death/slaughter excluding sales. In the current study, the average culling rate of Estonian dairy cows was 26.24 per 100 animal-years, excluding selling from the culling definition. In selected regions of the United States the average culling rate was 31.6% in 1999 (3) but was 27.7% in Pennsylvanian herds in 2005 (28). In Canada Haine et al. (29) reported an average culling rate of 32% over the 2001–2010 decade and a dairy sell rate by 60 days in milk of 3.2%. The average culling rate of cows due to slaughter/death was 25.4% ranging between 23% (in 2007) to 28% (in 2010) in Dutch dairy herds (30). This shows that, on average, culling rates of Estonian dairy cow population are mostly comparable to that reported in other countries.
Farmers´ stated reasons for culling were analysed in the present study due to absence of more sound representative data. Care is needed when interpreting this data due to somewhat overlapping categories. Some cows might also have more that one reason for culling whereas the ELPR system allows farmers to mark only one reason for each culling event. Also, the stated culling reasons might be the consequences of the the primary disorder that might differ from what was reported by the farmers. Therefore, these results are rather indicative and further studies including necropsy, meat inspection and laboratory data together with animal and disease history could reveal more reliable results. Four most common reasons for culling due to death and slaughter were: “feet/claw disorders”, “udder disorders”, “metabolic and digestive disorders”, and “fertility problems”, the order of frequency being broadly similar to those described by other authors (31). Proportion of “feet/claw disorders” as the reason of culling was somewhat more prevalent compared to what was reported in studies performed in other European countries (6,32). As small herds with less than 20 cow-years were excluded in this study and majority of the Estonian dairy cows were housed in large herds (33), the conditions of large dairy herds and their effect on the cow hoof health were probably over-represented in the present study compared to other studies. In large Estonian dairy farms cows are mostly housed in freestalls, the latter is shown to be associated with an increased risk for lameness relative to other housing systems, including tie stalls and straw yards (34). Additionally, other factors accompanying freestalls such as prolonged standing time due to milking (35), overstocking (36) or poor stall design (35) can increase the risk of lameness.
Metabolic disorders were more a common reason for culling in multiparous compared to primiparous cows. Roberts et al. (37) concluded that primiparous cows may have a different physiological response to postcalving metabolic challenges. The need to balance between energy demands for growth and for milk production may have an effect on more effective fat mobilization before the health and productivity are compromised (37).
A minor increase in the culling hazard occurred about a year after calving. As fertility problems were the primary reason for culling in the late lactation stage (≥ 200 DAC) we might assume that at that time farmers mostly cull their non-pregnant cows. Cows sent for slaughter should not be in their last trimester of pregnancy according to a Motion for a European Parliament resolution (38). The Estonian milk recording register system allows farmers to report only one reason for each culling event. Therefore, it might be possible that some proportion of cows that were culled due to other reasons than fertility during the last third of lactation were also non-pregnant due to suffering chronic health disorders or according to farmers´ decision.
Loss of a cow at its´ first lactation is economically most devastating and therefore undesirable for the dairy farmer (1). In order to lower the culling rate of primiparous cows, measures promoting a good health of feet and udder are with utmost importance. In primiparous cows, dystocia and low milk yield both constituted roughly 9% of all culling reasons during the first 100 DAC, being nearly twice as high than that reported in multiparous cows. According to Mee (39) the feto-pelvic disproportion is the predominating risk factor for dystocia in primiparous cows, therefore bull selection as well as heifer nutrition and development might be critical factors lowering dystocia that leads to culling in primiparous cows.
We also identified some differences in culling reasons over lactations. Fertility as a reason of culling decreased in importance with each parity referring that probably more resilient cows in terms of breeding capability remained in the herds. Metabolic and digestive disorders as well as udder disorders were more frequently stated as the reason for culling in older cows. The fact that each calving event cumulatively adds the risk of suffering postpartum diseases, such as mastitis and ketosis (40,41), might also explain this identified association.
Animal-level risk factors for culling
In the current study, several common risk factors for culling in primiparous and multiparous cows were identified. Holstein breed cows had significantly higher culling hazard compared to Estonian Red and Estonian Native breed cows. Holstein breed cows have higher milk yield in Estonia (11). Concomitantly, pure Holstein breed cows are more susceptible to production-related diseases (42) and have poorer reproductive performance than crossbred cows (43), thus being more prone to culling.
We identified that higher individual milk yield breeding value was a protective factor for culling, which may be explained by farmers trying to keep cows with good genetic merit. A milk yield breeding value could be calculated for cows who have at least two test-milking result available and whose sire has obtained a milk yield breeding value. The category “missing” also included cows that were culled during the early lactation, which might be the cause of high culling hazard among primiparous cows who had no breeding value in the dataset.
In agreement with other studies, it is essential to pay attention to predisposing conditions and transmission of infectious diseases that might be associated with incidence of stillbirth or abortion as these are important risk factors for cow culling and longevity (44,45). Additionally, higher culling risk due to giving birth to a male calf compared to female indicates that birth weight of an offspring might be important factor in terms of culling via increasing the probability for dystocia. This is known to be associated with higher mortality hazard in cows (40) as well as increased risk of post-partum diseases eventually leading to culling (46). Furthermore, calving first time at a higher age was associated with a higher culling hazard. Interestingly, the association was also present in multiparous cows suggesting possible long-term impact. Heifers might calve at an older age due to management factors, health disorders, feeding management or due to herd breeding strategies (30,47), and the reason for the old age at first calving may be more important for the culling risk than the age per se.
In multiparous cows, a longer previous calving interval was associated with a higher culling probability at next lactation. Prolonged calving interval might be related with negative energy balance and diseases associated with the early post-partum period delaying conception. Due to the possible recurrent propensity of post-partum diseases at cow level, the undesired impact of a longer calving interval might manifest at the next lactation. In addition, longer calving interval allows cows to gain more weight, which may be a risk factor for developing post-partum diseases increasing the culling hazard (48).
High somatic cell count at last test-milking during the previous lactation and/or at first test-milking after calving, indicating the presence of subclinical intramammary infection, were factors associated with higher culling risk during the lactation. Presence of clinical or subclinical mastitis is a known risk factor for dairy cow culling (46). Lower milk yield at the end of the previous lactation or at first test-milking of the ongoing lactation was also associated with a higher culling probability. It has been shown that in general, farmers are more eager to cull low-producing cows (46). Still, low milk yield soon after calving might be associated with an underlying disease. The high milk fat/protein ratio is a valuable indicator of ketosis in early post-partum period (49) and it is concomitantly related to increased probabilities to develop displaced abomasum, retained placenta, metritis, clinical endometritis and clinical ketosis as well as higher culling probabilities (24,50).
Herd-level factors associated with culling
On average, the cow culling hazard was higher in larger herds. The association between herd size and health and welfare of dairy cows is complex and it includes the impact of several factors, e.g facilities, management and operational factors (20,51). According to previous studies, the positive association between the incidence of metabolic diseases and herd size have been found (52). Also, presence and dynamics of infectious diseases and different biosecurity management could explain higher culling risk in larger herds (30). According to Gieseke and co-authors (53) housing conditions and management practices have a greater effect on cow welfare than the herd size itself and more research is needed to identify factors in large farms that affect animal health. Simultaneously to European Union, the milk price dropped suddenly in autumn of 2014 in Estonia (54) resulting in a 5.2% and 11% in reduction of the number of dairy cows and herds, respectively, in Estonia during the year of 2015 (55). Due to this, change of herd size was controlled as a factor in the statistical models to account for its´ confounding effect.
Higher herd average milk yield was associated with increased culling hazard of individual cows in our study. Several previous studies have found that higher milk yield has adverse effect to the cows´ resistance to diseases as it is correlated with presence of the clinical mastitis, reproductive diseases (56), and other postpartum disorders (24). Although high milk yield and the high genetic potential for milk production are often been blamed for the short longevity of dairy cows, this does not always seem to affect cow longevity (57). On the other hand, due to uncertain causality, herds with higher milk yield might also have better reproductive performance allowing more cows to be culled.
The current study showed that herds with a longer average calving interval had a lower risk of culling. Longer calving interval may be the consequence of fertility problems in the herd or the result of a voluntary decision of the farmer to delay with breeding after calving and thus extend the lactation period (58). Although a shorter calving interval is considered as economically optimal (59), an economic benefit in extending lactations in high-yielding cows was also found (60). In the study by Allore and Erb (61), a lower risk of culling for reproductive failure was present in herds with extended voluntary waiting period. Still, due to cross-sectional study design it is impossible to draw causal inferences and the identified association might result from farm lower thresholds for culling non-pregnant cows (46).
In the current study, the cow culling hazard was also negatively associated with herd average lactation number that cumulatively aggregates the individual animal culling hazards.
Using robot milking system was a protective factor for culling compared to other milking methods. Additionally, cows that were milked three times a day at first test-milking had lower culling probability throughout the lactation compared to cows that were milked twice a day. Farms with automatic milking systems differ from those with other milking systems in many aspects, e.g. environmental conditions, feeding management, grouping policies etc. Unfortunately, it was not possible to discriminate milking method but only the milking frequency, meaning that the identified associations could be affected by other factors.
Validity and limitations of the study
The present study included lactation-level records of all cows from all herds that had at least 20 cow-years in years 2013-2015 in Estonia and participated in the milk recording system. According to the ELPR (33) 94.1% (in 2013) to 95.4% (in years 2014 and 2015) of the Estonian dairy cow population was enrolled in milk recording system probably leaving out smaller farms that produced milk for own consumption. After skipping herds with <20 cow-years, we emphasize a good external validity of this study for medium or large sized herds whereas the study results should not be extrapolated to small dairy holdings.
When analysing farmers´ reported reasons for culling, a reporting bias might be present. In Estonia, farmers are allowed to report only one reason for each animal exit to the milk recording register. Still, studies have shown that in many instances farmers report more than one reason of culling when allowed (62,63). To our knowledge, there are no studies that investigate the farmers´ behaviour in reporting culling reasons. We assume that farmers report the main and most obvious reason of culling at the time the cow is leaving the herd. However, this might not be the primary or ultimate disease or a disorder which leads to culling.
In EU, there is a harmonized mandatory registration and reporting of animal births, movements and deaths (64) making registry data reliable for research purposes. As participation in milk recording register is voluntary, the reporting in that system might not always be precise. Although the animal registry and milk recording registry makes crosschecks in their data, small discrepancies were found when comparing the cows´ exit dates of the two registries. Also, some cows had very long lactations and low number of culls occurred far away from the latest calving (Figure 2) in which a new calving date might have remained unreported to the registry. Still, due to high sample size of this study these aberrations probably have no effect on the overall results and conclusions.