Introduction
I received an email the other day from a commercial company that used data from a recent published study (Hou et al., 2026) to remind dairy producers that neonatal diarrhea may reduce future milk production in the calves that develop diarrhea. The ad used changes in median milk production to estimate the cost of diarrhea in terms of milk loss using a recent milk price.
We can quibble about whether the calculations were accurate or truly reflected the economic loss farmers experience when their calves develop diarrhea. However, the ad definitely highlighted research suggesting that diarrhea early in life can affect milk production years later. The basic message – that calf health can have consequences extending well beyond the preweaning period – is important. The difficulty is that the numbers are unusually easy to interpret too literally.
One study may report milk lost for each day a calf was sick. Another may compare calves that ever had diarrhea with calves that never had a recorded case. A third may simply show the unadjusted difference between two groups of cows. These are not interchangeable measurements. Yet, once a number is converted to pounds of milk or dollars per cow, that distinction can disappear quickly.
This Calf Note reviews several of the studies most often cited in this discussion. The objective is not to argue that calf diarrhea is unimportant – it clearly is important – but to ask a narrower question: what do the available data actually tell us about future milk production?
First, define the unit
The most tempting number is kilograms of future milk lost per sick day. There is a study that estimated exactly that. Most of the other studies did not. Before comparing results, it helps to separate four different kinds of exposure:
A sick-day variable counts the number of days on which illness was observed. An episode or disease-history variable places a calf into a category such as diarrhea: yes or no. Treatment duration counts days of treatment, which may be longer or shorter than the period of abnormal feces. Finally, an unadjusted group difference simply compares the observed milk production of two groups without attempting to account for other differences between them.
| KEY POINT: A coefficient for each sick day, a coefficient for any history of diarrhea, and an unadjusted difference between groups answer different questions. They should not be converted into one another. |
There is also a second complication. Studies have reported actual 305-day milk, mature-equivalent 305-day milk (ME305), or energy-corrected milk (ECM305). These outcomes are related, but they are not identical. The broad direction of an association can be compared across studies more comfortably than the exact kilogram values.
Heinrichs and Heinrichs: the true “sick-day” estimate
Heinrichs and Heinrichs (2011) provide the clearest basis for the idea that a sick day as a calf has a measurable long-term cost. Their prospective project followed calf management and health on 21 Pennsylvania farms and ultimately related events during the first 16 weeks of life to first-lactation and lifetime performance.
Importantly, their variable was literally “days ill”: the number of days during the first 4 months of life that a calf had scours or cough. They did not separate digestive disease from respiratory disease in this variable. Calves averaged only about 1.0 day ill, although the variation among calves was substantial.
In the first-lactation model, each additional day ill was associated with approximately 126 kg less 305-day mature-equivalent milk, along with about 4 kg less protein and 5 kg less fat. Thus, the often repeated idea that a calf sick day may be associated with roughly 100 kg of future milk has a real statistical basis.
However, a regression coefficient should not automatically be extrapolated far outside the distribution that produced it. The calves in this study averaged about one observed sick day. Multiplying 126 kg by 8 or 10 days and concluding that a prolonged illness necessarily costs more than a metric ton of first-lactation milk assumes a perfectly linear biological response that the study was not designed to prove.
Svensson and Hultgren: diarrhea as a category
A large Swedish study by Svensson and Hultgren (2008) followed 2,060 heifers in 107 dairy herds. Calves that had mild diarrhea during their first 3 months of life produced 344 kg less ECM305 in first lactation than calves without diarrhea. This is an impressive difference, and its magnitude is often consistent with the general argument that early disease can leave a long shadow.
But the exposure was not the number of days with diarrhea. It was membership in a disease category. Therefore, even if a typical bout of diarrhea on those farms lasted several days, dividing 344 kg by an assumed number of disease days does not create a valid per-day coefficient. The model never estimated that relationship.
The Swedish data also remind us that disease labels can identify calves with a broader health history. Calves experiencing one disease may be more likely to experience another, and the statistical association with later milk may reflect some combination of disease severity, repeated illness, growth setback, environment, and management rather than the isolated effect of a single loose-stool event.
Abuelo and colleagues: a remarkably similar first-lactation result
Abuelo et al. (2021) evaluated 2,272 heifers from a large U.S. dairy. A history of preweaning diarrhea was associated with 325 kg less first-lactation ME305, and approximately 50 g/d less average daily gain. The similarity between the 325-kg estimate in this study and the 344-kg Swedish estimate is certainly interesting.
Again, however, diarrhea was modeled as a history – affected or not affected – rather than as days of diarrhea. The study supports the conclusion that calves with recorded diarrhea later produced less milk in that herd. It does not demonstrate that each day of diarrhea cost a fixed amount of milk.
Not every study finds the same milk response
If the effect of calf diarrhea were a simple and universal biological constant, we would expect reasonably similar results whenever good records were analyzed. That is not what the literature shows.
Warnick et al. (1995), using records from 25 New York Holstein herds, found no association between calf morbidity and subsequent first-lactation milk production. Soberon et al. (2012) also did not find a significant first-lactation milk effect attributable simply to a reported history of diarrhea. In that Cornell dataset, however, heifers that had received antibiotics as calves produced about 493 kg less first-lactation milk than calves with no recorded antibiotic treatment. Treatment may have served as a marker for more severe disease rather than being the cause of lower milk production.
More recently, Goh et al. (2024) analyzed 9,833 calves from a large dairy in southern Australia. The adjusted difference in first-lactation ME305 between calves with and without preweaning diarrhea was -89 kg, with a 95% confidence interval from -239 to +60 kg. In other words, the overall study did not provide clear evidence of a substantial milk-yield penalty. A subgroup that developed diarrhea at 15 to 21 days produced 422 kg less ME305, but the authors specifically cautioned that this age-specific result could be spurious because the same pattern was not seen in other first-lactation outcomes.
The Australian study did find important associations with survival and later health. Calves with diarrhea had higher mortality risk before first calving, and diarrhea history was associated with slightly greater first-lactation somatic cell count and removal from the herd. Thus, a weak or uncertain milk-yield effect should not be interpreted to mean that diarrhea was harmless.
The new 2026 study: why the headline number changes
Hou et al. (2026) evaluated 1,907 Holstein cows from one large dairy in Hebei Province, China. Seven hundred cows had a recorded history of diarrhea during the calf period and 1,207 were classified as healthy. The median age at onset of diarrhea was 0.30 months – approximately 9 days – but the range extended from 0.03 to 5.53 months. The median duration of treatment was 8 days, with a range from 2 to 32 days.
That last number needs care. Eight days was treatment duration, not necessarily eight days of abnormal feces, and treatment duration was not the exposure variable in the milk-production model. Disease status was simply diarrhea history: yes or no.
The paper first presented unadjusted median comparisons among cows grouped according to the number of complete lactations available. Those descriptive differences are large and are the figures that have attracted attention in popular reports.
Table 1. Unadjusted 305-day milk-yield medians reported by Hou et al. (2026).
| Group in Table 1 | Diarrhea, kg | Healthy, kg | Difference, kg |
|---|---|---|---|
| 1 complete lactation | 10,831 | 11,492 | -661 |
| 2 complete lactations | 10,669 | 10,956 | -287 |
| 3 complete lactations | 10,343 | 11,430 | -1,087 |
Those numbers are real observations from the paper, but they are not estimates of milk caused to be lost by one bout of diarrhea. They are simple median contrasts between different groups. The groups also contained different animals: among cows with a diarrhea history, 273 had one complete lactation, 340 had two, and 87 had three; the corresponding numbers among healthy cows were 866, 269, and 72. Adding the three median differences together therefore does not estimate the lifetime milk loss of a typical individual cow.
The authors also fitted a multivariable linear mixed-effects model using 15,840 observations. After accounting for parity and selected calf factors, a history of diarrhea was associated with 132.82 kg less 305-day milk (P = 0.008). Diarrhea status explained only 0.05% of the variance in 305-day yield. The same model estimated a 0.71-kg reduction in peak milk, with diarrhea status explaining 0.17% of its variance.
| The -132.8 kg coefficient means: cows with any recorded calfhood diarrhea averaged 132.8 kg less 305-day milk after adjustment for the other variables in the model. It does NOT mean 132.8 kg per day of diarrhea, per treatment day, or per episode. |
For the same reason, dividing 132.8 kg by the median 8 days of treatment to obtain 16.6 kg per treatment day is not valid. The model did not use treatment days as a dose. Conversely, applying the Heinrichs coefficient of 126 kg per sick day to 8 treatment days and claiming a loss of approximately 1,000 kg would also go beyond the evidence.
Putting the studies side by side
Table 2. Selected studies relating calfhood disease or diarrhea to later milk production.
| Study | Disease variable | Milk association | Important interpretation |
|---|---|---|---|
| Warnick et al., 1995 | Calf morbidity / scours history | No significant first-lactation association | Older multi-herd study; illustrates that a later milk penalty is not universal. |
| Svensson & Hultgren, 2008 | Mild diarrhea before 91 d: yes/no | -344 kg ECM305 | Disease category, not days of diarrhea. |
| Heinrichs & Heinrichs, 2011 | Number of days with scours or cough in first 4 mo | -126 kg ME305 per sick day | The clearest direct estimate of a sick-day coefficient. |
| Soberon et al., 2012 | Diarrhea history; antibiotic treatment recorded separately | Diarrhea NS; antibiotic-treated calves about -493 kg first-lactation milk | Treatment may identify more severe disease; not a diarrhea-day estimate. |
| Abuelo et al., 2021 | Preweaning diarrhea: yes/no | -325 kg ME305 | Binary disease history, not disease duration. |
| Goh et al., 2024 | Preweaning diarrhea: yes/no | -89 kg ME305 (95% CI -239 to +60) | No clear overall milk-yield effect; age-specific result needs replication. |
| Hou et al., 2026 | Calf diarrhea history: yes/no | -132.8 kg 305-d milk, adjusted | Binary history; median 8 d was treatment duration, not model exposure. |
Can we say that a sick day costs about 100 kg of milk?
There is enough evidence to say that the idea has merit as a useful reminder of the potential long-term importance of calf health. Heinrichs and Heinrichs directly estimated about 126 kg less first-lactation ME305 for each recorded day of illness. That is the strongest support for the statement.
But it is too strong to treat 100 kg as a universal biological price for every day of diarrhea. Svensson and Hultgren and Abuelo et al. found losses of approximately 325 to 344 kg in calves with a history of diarrhea, but neither study measured a per-day response. Goh et al. found a much smaller and statistically uncertain overall milk difference. Hou et al. found an adjusted difference of about 133 kg for any history of diarrhea despite a median treatment duration of 8 days.
These results can coexist if the relationship is not linear. The first few days of a clinically important enteric disease may account for much of the lasting biological insult. Additional treatment days may reflect recovery rather than equivalent additional injury. Disease severity, pathogen, age, nutrient intake, dehydration, inflammation, intestinal injury, growth setback, repeated disease, and subsequent management can all influence the eventual outcome. A binary disease record collapses all of this complexity into a single yes/no variable.
Association is not the same as causation
Nearly all of the evidence discussed here is observational. Calves are not randomly assigned to develop diarrhea. A calf that develops diarrhea may differ from a healthy calf in passive immunity, pathogen exposure, birth conditions, colostrum intake, nutrition, housing, season, genetics, or later disease. Statistical adjustment can reduce some of these differences but cannot guarantee that every relevant confounder has been removed.
There is also survivor selection. Severe diarrhea can kill calves before they ever enter the milking herd. Analyses of later milk production necessarily include only animals that survive to calving. Hou et al., for example, explicitly excluded records of animals that died and included only diarrheic calves that recovered. The long-term production estimate therefore describes survivors, not the total biological or economic impact of diarrhea on the original calf population.
What can we say with confidence?
First, calf diarrhea has immediate costs that require no argument about future milk: treatment, labor, reduced feed intake and growth, mortality risk, and the possibility of subsequent disease. Preventing diarrhea and limiting its severity remain fundamental calf-management goals.
Second, several well-conducted observational studies indicate that calves with clinically recognized disease later produce less milk. The magnitude is not consistent across studies, but the repeated association is biologically plausible and economically important enough to take seriously.
Third, the research does not support attaching a fixed future milk loss to a single abnormal stool. The studies generally evaluated documented disease, days of observed illness, treatment records, or a history of diarrhea. Those exposures are substantially different from seeing one loose fecal score on one day.
Finally, the best interpretation is probably not “every sick day costs exactly X kilograms of milk.” A better interpretation is that early disease is one marker – and potentially one cause – of a calf falling off the biological trajectory that allows her to express her full productive potential later in life. The goal of excellent calf management is to keep her on that trajectory.
Summary
Research spanning several decades supports concern about the long-term consequences of calfhood disease, but the numerical estimates need to be read according to how each study defined disease. Heinrichs and Heinrichs estimated approximately 126 kg less first-lactation ME305 for each recorded day with scours or cough. Svensson and Hultgren and Abuelo et al. reported approximately 325 to 344 kg less first-lactation milk in calves classified as having had diarrhea. Other studies, including a very large Australian cohort, found little or no overall milk-yield difference.
The 2026 Hou study adds evidence that a history of calf diarrhea may remain associated with lactation performance across multiple parities. Its adjusted estimate was 132.8 kg less 305-day milk for cows with any calfhood diarrhea history. The larger parity-specific numbers reported in descriptive comparisons are unadjusted group differences and should not be summed as though they represent a three-lactation loss for one cow. Likewise, the median 8 days of treatment cannot be used to convert the 132.8-kg coefficient into a per-day loss.
So, does a sick day as a calf cost future milk? Probably – and Heinrichs and Heinrichs provide a reasonable estimate of the association in their population. But there is no single conversion factor that applies to every calf, every episode, and every farm. The stronger lesson is simpler: disease early in life can cast a long shadow, and preventing clinically important diarrhea is an investment in both the calf in front of us and the cow she may become.
References
Abuelo, A., F. Cullens, and J. L. Brester. 2021. Effect of preweaning disease on the reproductive performance and first-lactation milk production of heifers in a large dairy herd. J. Dairy Sci. 104:7008-7017. doi:10.3168/jds.2020-19791. Link
Goh, N., J. K. House, and S. M. Rowe. 2024. Retrospective cohort study investigating the relationship between diarrhea during the preweaning period and subsequent survival, health, and production in dairy cows. J. Dairy Sci. 107:9752-9761. doi:10.3168/jds.2023-24544. Link
Heinrichs, A. J., and B. S. Heinrichs. 2011. A prospective study of calf factors affecting first-lactation and lifetime milk production and age of cows when removed from the herd. J. Dairy Sci. 94:336-341. doi:10.3168/jds.2010-3170. Link
Hou, X., J. Xu, J. Ren, G. Liu, X. Wang, Q. Du, Z. Niu, Y. Cao, D. Tong, S. Wu, and J. Yao. 2026. Persistent effects of neonatal diarrhea on Holstein dairy cow lactation performance through multiple parities. J. Anim. Sci. 104:skag019. doi:10.1093/jas/skag019. Link
Soberon, F., E. Raffrenato, R. W. Everett, and M. E. Van Amburgh. 2012. Preweaning milk replacer intake and effects on long-term productivity of dairy calves. J. Dairy Sci. 95:783-793. doi:10.3168/jds.2011-4391. Link
Svensson, C., and J. Hultgren. 2008. Associations between housing, management, and morbidity during rearing and subsequent first-lactation milk production of dairy cows in southwest Sweden. J. Dairy Sci. 91:1510-1518. doi:10.3168/jds.2007-0235. Link
Warnick, L. D., H. N. Erb, and M. E. White. 1995. Lack of association between calf morbidity and subsequent first lactation milk production in 25 New York Holstein herds. J. Dairy Sci. 78:2819-2830. doi:10.3168/jds.S0022-0302(95)76912-0. Link