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On many donkey farms, the jenny and her foal are separated for a while before milking. A team from the University of São Paulo investigated whether behavior and the hormone cortisol in the saliva of 14 jennies of the Brazilian Pêga breed changed after two hours of separation. The amount of milk per milking was also recorded. The result: Significant differences were observed on only a few measurement days, with an average of 566.4 mL of milk per milking.
Observational study on the milking routine of jennies and foals at one farm – no statement about an effect on humans. Donkey milk is a food.
Study at a glance
| Feature | Information |
|---|---|
| Original title | Two Hours of Separation Prior to Milking: Is This Strategy Stressful for Jennies and Their Foals? |
| Authors | de Souza Farias S., Dierings Montechese A. C., Bernardino T. et al. |
| Journal | Animals, 11(1): 178 |
| Year | 2021 |
| Country | Brazil (Laranjal Paulista, State of São Paulo) |
| Study type | Observational study with repeated pre- and post-measurements, without a control group |
| Sample | 14 multiparous Pêga jennies and their foals, with 14 measurements per animal |
| DOI | 10.3390/ani11010178 |
What question did the researchers ask?
Is a two-hour separation of jenny and foal before milking associated with measurable changes in behavior and salivary cortisol? And how much milk can be obtained per milking afterwards? Cortisol is a hormone that can increase under stress and can be measured in saliva.
How the study was conducted
- Farm and animals: A breeding farm without commercial milk production, with 60 Pêga jennies. 14 jennies with an average body weight of 245 kg were investigated. Seven foaled in February 2018, seven in May.
- Housing: During the day on pasture with native grasses and Brachiaria grass, as well as alfalfa hay. Fixed groups of 3 jennies (Stable 1, 8 m² per animal) to 4 jennies (Stable 2, 6 m² per animal).
- Separation and milking: At around 10 a.m., the jenny and foal were separated. The foals remained in the group stables without visual contact with their mothers. After two hours, the jennies were milked by hand. Because all 14 animals were investigated at the same time, the separation lasted up to three hours for some.
- Measurements: On lactation days 45, 60, 75, 90, 105, 120, and 135, saliva samples were taken before and after separation, and behavior was recorded every six minutes per jenny. The amount of milk was recorded per milking.
The results
Across all measurement days, only individual significant pre- and post-differences were observed. The paper provides the p-values, but not the direction of change for every day.
| Measurement | Result |
|---|---|
| Milk per milking | 566.4 ± 205.2 mL per jenny |
| Salivary cortisol in jennies (mean) | before 790 pg/µL, after 840 pg/µL |
| Cortisol in jennies, day 135 | significant difference (p = 0.02) |
| Cortisol in foals, day 75 | significant difference (p = 0.03) |
| Vocalizations of jennies, day 120 | significant difference (p = 0.03) |
| Other days and behaviors | no significant differences |
The authors convert the milk volume using a density of 1.03 g/mL to approximately 0.6 kg per milking, around 0.25% of the body weight. With up to eight milkings per day, they estimate up to 4.8 kg per day – this is a calculation, not a measurement. They did not find a correlation between milk volume and salivary cortisol. Their conclusion: Based on the data collected, the separation is "possibly" not a stressful experience.
In this group, behavior and salivary cortisol changed significantly after two hours of separation on only a few measurement days. An average of 566.4 mL of milk was obtained per hand milking.
Critical assessment
- No control group: There were no animals without separation. Before-and-after comparisons on the same day also capture differences due to the time of day.
- Limitations according to the authors: True baseline values for salivary cortisol, behavioral observations of the foals, and more milk measurements are missing. Long-term effects were not investigated.
- One herd, one breed: 14 purebred Pêga jennies at a farm without commercial milk production – other breeds, farms, and routines may react differently.
- Inconsistencies: The text and Figure 2 are not clear regarding the conspicuous behavior on day 120. The stated date of the ethics approval (November 17, 2019) is after the data collection in 2018.
- Fluctuating separation duration: Depending on the order, individual animals were separated for up to three hours.
Funding, conflicts of interest, and level of evidence
The work was funded by the Brazilian Research Council CNPq. The authors declare no conflicts of interest. They work at the University of São Paulo and the Federal University of Bahia.
Level of evidence: Observational study on livestock animals without a control group – describes behavior, salivary cortisol, and milk volume, no statement about effects on humans.
What does this mean for donkey milk as a food?
The study says nothing about the composition of the milk. It describes a milking routine: separation in the morning, hand milking afterwards, with an average of about half a liter per milking. It also shows how much the amounts vary – the standard deviation was 205.2 mL. Whether such a routine is stressful for the animals cannot be conclusively assessed from this single study.
Our research question: What information about the milking routine – separation duration, milking times, volume per milking – is useful and verifiable when we report transparently about the origin of our milk?
Frequently asked questions
Why are foals separated before milking?
During the time without the foal, milk accumulates, which can then be milked. According to the authors, jennies are milked two to eight times a day depending on the farm.
How much milk did a jenny give per milking?
An average of 566.4 mL, with a standard deviation of 205.2 mL. The paper does not report values for individual measurement days.
Was the milk analyzed in this study?
No. Only the volume was measured. Behavior and cortisol in the saliva of jennies and foals were primarily investigated.
You can find all measurement values, interactive graphics, and the full critical assessment on our research page: Study 87: Two Hours of Separation Prior to Milking. Further studies in the research archive.
de Souza Farias S., Dierings Montechese A. C., Bernardino T., Mazza Rodrigues P. H., Albano de Araujo Oliveira C., Zanella A. J. (2021). Two Hours of Separation Prior to Milking: Is This Strategy Stressful for Jennies and Their Foals? Animals, 11(1), 178. DOI: 10.3390/ani11010178
Summarized and reviewed by the DON DONKEY'S editorial team based on the original publication · As of: October 2026. This summary does not replace the original work.
End of the study summary. The study was conducted independently of DON DONKEY'S® and does not refer to our products. Our donkey milk is a food.
