Milking performance and plant specialized metabolites in the milk of goats fed silage from willow (Salix acmophylla) irrigated with saline water

Abstract

Willow trees irrigated with recycled water yields fodder of good nutritional value that can be ensiled to provide feed year-round for dairy goats. We investigated the nutritional value of silage from willow irrigated with saline water (Electrical Conductivity 4.0) in a cross-over experiment (twenty eight goats, two periods). We compared total dry matter intake (TDMI), milking performance and milk composition, with a focus on plant phenolic compounds, of Damascus goats fed oats and vetch hays as the sole source of roughage (all-hay) or oats hay with willow silage, with 470 g/kg DM concentrates. Willow silage represented 175 and 205 g/kg of TDMI in the last five days of periods 1 and 2, respectively. Individual TDMI tended (P<0.06) to be 3.7% lower in the willow-fed goats. Milk yield, and the milk contents and daily yields of fat, protein and lactose did not differ between treatments. However, willow consumption was associated with lower contents of milk urea (P<0.02) and 59% higher anti-oxidative capacity (P<0.04). Consumption of willow silage was associated with higher levels of plasma malondialdehyde, but did not affect leucocyte clustering. We assessed the contents of plant specialized metabolites (PSMs) in the feeds and in the milk of five goats of each treatment at the end of period 2 by liquid chromatography / time-of-flight / mass spectrometry (LC-TOF-MS), with exact or tentative compound identification, depending on availability of appropriate standards. Although many flavonoids were present in feeds, they were not found in the milk of either treatment. Major phenolic compounds, including the salicinoids salicin, esculetin and chaenomeloidin found in feeds, were not recovered in milk but salicylic acid, found in all feeds, was present equally in the milk of both control and treated goats. Only two compounds directly related to the metabolism of salicin, i.e., salicylic acid and saligenin, were found in milk. Overall, milk from willow silage-fed goats contained more phenolic compounds, including benzyl alcohol, hydroquinone, methyl-syringate (P<0.001), and a saligenin derivative (P<0.005), and tended to contain more 4-hydroxybenzoic acid and hippuric acid (P<0.10) than milk from all-hay fed goats. Volcano plot-to-target changes in milk metabolites related to willow (untargeted LC-TOF-MS) showed intense upregulation of sulfated derivatives of phenolics. Growing willow for silage for dairy goats is an appropriate way to produce feed where saline water irrigation is available.