Donkey Milk Study: Selenium in Feed and Milk Fatty Acids (Meng 2025)

With 0.3 mg selenium per kg of feed dry matter, the calculated milk yield increased from 3.26 to 3.84 kg/d; the milk contents of fat, protein, and lactose remained the same. The proportion of saturated fatty acids fell from 56.74% to 54.06% (0.5 mg: 50.90%).

The study in brief

• Original title: Rectal Microbiome Reveals the Improved Effect of Dietary Selenium Levels on Lactation Performance and Milk Fatty Acid Profiles in Lactating Donkeys
• Authors: Meng F., Zhao Y., Guo Y. et al.
• Journal: Animals, 15(22), 3309 (2025)
• Country: China · Study type: Animal study

How it was investigated

Feeding trial with 24 lactating Dezhou donkeys (average 40 days in lactation, 247 ± 26 kg, 2.8 births) on a farm in Inner Mongolia, China. The animals were randomly assigned to four groups (n = 6 each): control diet without supplement (0.04 mg Se/kg DM) and supplements of 0.15, 0.3 and 0.5 mg selenium per kg of dry matter in the form of selenium yeast (≥ 98% organic selenium, predominantly selenomethionine). After 2 weeks of adaptation, 8 weeks of data collection followed with individual housing.

Key figures

• 24 – Dezhou donkeys in four groups (6 animals each)
• 0 / 0.15 / 0.3 / 0.5 mg/kg – selenium supplement per kg of dry matter (control diet: 0.04 mg/kg)
• 3.84 vs. 3.26 kg/d – calculated daily milk yield at 0.3 mg Se vs. control
• 0.39–0.41 % – milk fat content in all groups (no significant difference)
• 7.11–7.12 % – milk lactose content in all groups (no significant difference)
• 270.77 vs. 227.58 g/d – daily lactose amount at 0.3 mg Se vs. control

Results

The plasma selenium concentration increased with the supplement from 107.37 µg/L (control) to 157.20, 169.20 and 186.10 µg/L. Feed intake (7.62–8.14 kg DM/d) did not differ.

At 0.3 mg Se/kg DM, the calculated milk yield (3.84 vs. 3.26 kg/d), milk yield per kg of feed, as well as the daily amounts of lactose, fat-free dry matter and dry matter were higher than in the control. The milk fat amount was higher in all selenium groups (13.94–15.54 vs. 11.98 g/d). The percentage contents of fat, protein, lactose and dry matter remained unchanged.

In the milk fat, the proportion of saturated fatty acids fell with all supplements (56.74% to 54.84%, 54.06% and 50.90%); unsaturated and monounsaturated fatty acids as well as oleic acid increased. Polyunsaturated fatty acids, linoleic acid and the P/S ratio were higher at 0.3 and 0.5 mg; α-linolenic acid and total n-3 fatty acids only at 0.5 mg (4.28% and 4.41% respectively).

Limitations of the study

• Small groups, selenium in milk not measured: Six animals of one breed from one farm were examined per group, which the authors themselves call a limitation. The selenium content of the milk was not determined, only that in the plasma.
• Conclusion goes beyond the table: The abstract and conclusion write that the dose of 0.3 mg leads to an increase in α-linolenic acid and n-3 fatty acids; according to Table 5, these do not differ from the control at 0.3 mg (3.62 vs. 3.38% and 3.75 vs. 3.50% respectively), but only at 0.5 mg. The discussion also speaks of a higher proportion of saturated fatty acids at 0.3 mg, while the table shows a decrease (56.74 to 54.06%).
• Implausible fatty acid amounts: In Table 6 (daily fatty acid amounts), saturated and unsaturated fatty acids together result in around 318 g/d, while the milk fat amount in Table 2 is 12 to 16 g/d. The units of this table are therefore not comprehensible; in addition, the ratio and index values are identical to Table 5.
• Methodological inconsistencies: The methods section mentions Spearman correlations, while the figure legends mention Pearson. Milk yield efficiency is defined as SCM/feed intake × 100, but values of 0.22 to 0.28 are given. In the section on fecal fatty acids, an undefined "AOE" group is mentioned.
• Indices and interpretations: AI, TI and DFA are calculated metrics from the fatty acid pattern, not measured effects. The authors' statements on the nutritional value for humans and the role of individual genera of the gut microbiome are based on correlations and were not tested.

Level of evidence: Animal study – describes measured values in the examined animals and their milk, no statement on effects in humans.

Our assessment

The study describes how a selenium supplement in the feed changes the milk yield and fatty acid pattern of the milk of Dezhou donkeys; the percentage of the main components remained the same. No conclusions about health effects in humans or on commercial products can be drawn from this.

Source: Meng F., Zhao Y., Guo Y., Guo X., Zhang Q., Wang Z., Li L., Hui F., Tong M., Yan S. (2025). Rectal Microbiome Reveals the Improved Effect of Dietary Selenium Levels on Lactation Performance and Milk Fatty Acid Profiles in Lactating Donkeys. Animals. DOI: 10.3390/ani15223309 · Summarized by the DON DONKEY'S editorial team, as of October 2026.

End of the study summary. The study was conducted independently by DON DONKEY'S® and does not refer to our products. Our donkey milk is a food.