Transfer of fatty acids from blood to milk in dairy asses: effect of different fibre sources

Scientific references on ass’s milk composition has been rare for a long time. An important gap regards the donkey nutrition which is one of the main sources of variations in equid’s milk production. Donkeys as well as horses, being monogastric herbivorous animals, have specific mechanisms for regulating milk secretion, since nutrients absorption in equids precedes the ceco-colic fermentation. In this direction, few data to establish relationship between endproducts of digestion and milk secretion in dairy asses are reported. As a part of a larger study on the effects of the administration of different fibre sources on productive and nutritional response of dairy asses, this study focussed the attention on the transfer of fatty acids from blood to milk in dairy asses following the substitution of the wheat bran with dehydrated beet pulp in concentrates. In an organic farming system, sixteen pluriparous Martina Franca asses, homogeneous for body weight (260 ± 40 kg) and day of lactation (125 ± 48 d), were divided into two groups, fed twice a day a diet made of meadow hay (8 kg) and two isoenergetic and isonitrogenous concentrate (2.5 kg), including either wheat bran (20% - group A) or dehydrated beet pulp (20% - group B). Dry matter consumption, about 3.2% of body weight, was on average characterised by 10% of crude protein and 8.5 MJ DE/kg. The trial lasted 84 days; every 21 days, feeds, individual blood and milk samples were sampled and analysed by GC-FID, to determine the fatty acid composition; each fatty acid was expressed as a percentage of the total fatty acids identified. Data were subjected to ANOVA to evaluate differences for the acidic profile in the blood and in the milk of each group (group B vs. group A); the relation between the fatty acid content of blood and milk was estimated through correlation analysis. With regard to the essential fatty acid percentages in the feedstuff (B: dehydrated beet pulp vs. A: wheat bran), significant differences were observed for the linolenic (B: 0.58% vs. A: 0.34%; P = 0.041) and linoleic (B: 14.77% vs. A: 11.53%; P = 0.018) acids. No differences between groups were observed for blood linolenic and linoleic percentages, which were similar for the amount (group B = 2.72% vs. group A = 2.82%). Similar values were measured for the milk linolenic acid (group B = 8.45% vs. group A = 8.78%), whereas a significant decrease was observed for the linoleic acid (group B = 10.79% vs. group A = 12.57%; P = 0.0007). Correlation analysis found a positive relationship between fatty acids in blood and in milk for most of the fatty acids. In particular, a strong correlation of the linolenic acid (0.60; P²0.001), sum of the n3 series PUFA (0.66; P²0.001) and a weak correlation of the linoleic (0.31; P²0.05) and the arachidonic (0.30; P²0.05) acids were observed between the plasma and milk. The results showed a more efficient transfer of the n3 series PUFAs than that of the n6 series PUFAs from blood to milk; this could probably be related to the other destinations of the plasmatic n6 series PUFAs during the metabolic cycle (constituents of adipose tissue, of tryglicerides as well as of membranes phospholipids). From a dietetic point of view, this transfer enhanced the nutritional characteristics of the milk.