Alnus viridis is a shrub species that has been increasingly spreading in many European mountain chains. This nitrogen-fixing species has triggered negative agro-environmental impacts, such as a loss of agricultural land and biodiversity, as well as an increase in nitrogen (N) leaching and soil acidification. Targeted livestock management systems have been recently investigated to reduce its encroachment, since some robust goat, sheep and cattle breeds are able to feed on A. viridis leaves, thus reducing its cover and creating a mosaic of pasture and shrub patches. Among cattle breeds, Highland cattle can feed on a higher proportion of woody species compared to production-oriented breeds. Moreover, other studies have shown that N balance across pastures can be actively affected by livestock grazing through N ingestion and excretion. The aims of this study were to assess: 1) the temporal variation of A. viridis leaf feed value along the grazing season and 2) the pasture use by Highland cattle and their N translocation from A. viridis-encroached areas to adjacent open pastures. A. viridis leaves were collected three times during the grazing seasons of 2020 and 2021 (in June, July and August), in four highly encroached sites across Italy and Switzerland, at similar elevations and with varying pedo-climatic conditions. Based on the data collected by meteorological stations, the sum of temperatures was calculated for each site. We measured the functional traits (specific leaf area and leaf dry matter content), nutritive value (macro- and micro-elements and fibre content) and phenolic content of the leaves. The in vitro organic matter digestibility (IVOMD) and related gas emissions (CO2 and CH4) were also investigated using a ruminal incubation system, with a diet composed of A. viridis leaves and hay in 1:4 ratio in the DM. In 2019 and 2020, two Highland cattle herds equipped with GPS collars were placed in four A. viridis-encroached paddocks, which were representative of an A. viridis cover gradient. Using cattle GPS locations and collar activity sensors, livestock activity phases (grazing and resting) were discriminated. From 11 to 24 herbaceous samples for each paddock were collected in multiple homogeneous vegetation patches before grazing. Cattle dung pats were also sampled throughout both seasons every 10 days. The N content of all vegetation and dung pat samples were then measured. Direct observations were carried out to explore the diet composition of Highland cattle. Based on these data, N ingested by cattle was estimated considering a diet characterized at 88% by herbaceous vegetation and 12% by A. viridis leaves from vegetation patches that were visited and grazed by cattle during 24 hours before dung sampling (N24H). Finally, the N translocation produced by cattle was estimated as a N flux in each vegetation patch through the estimations of N ingestion and N excretion (urine and dung) during the season. The estimation of urinary N excretion was made using literature data from studies carried out in similar environmental conditions and with similar cattle type. Leaf functional traits and all macro- and micro-elements significantly varied during the season, with similar decreasing patterns in leaf N, P, and K values. Contrarily, leaf Ca and Mn significantly increased through the summer. Leaf N and P were on average of 32.1 ± 0.36 g.kg-1 DM and 2.3 ± 0.07 g.kg-1 DM, respectively. On the other hand, leaf neutral detergent fiber, acid detergent fiber, and acid detergent lignin significantly increased during the season. Phenols did not show a significant seasonal trend except for condensed tannins, which slightly increased along the season. Including A. viridis leaves in cattle diet reduced OM digestibility and CH4/digested organic matter (dOM) emissions compared to a control diet of 100% hay. The N content of herbaceous vegetation was significantly higher in A. viridis understory areas compared to adjacent open pastures, due to the nitrophilous habitat created by soil N accumulation. The average N content in Highland cattle dung pats (31.2 ± 3.4 g kg-1 DM) was higher than average values from literature on grazing cattle under crude protein rich diets. The N dung content was marginally affected by N24H, with higher dung N values when Highland cattle grazed in the most N rich areas. Finally, most of this N (29.5 ± 10.3 kg ha-1 yr-1) was redistributed across the pastures, and actively translocated from A. viridis-encroached and steep areas towards resting areas, which generally occurred on flat open pastures. These results highlighted 1) the potential of A. viridis leaves as a valuable forage resource and identified the beginning of the summer season as the ideal grazing period and showed that 2) Highland cattle can effectively translocate part of the ingested N from A. viridis-encroached towards open areas. Therefore, they could help defining targeted management strategies for silvopastoral systems to increase beef cattle productivity, while reducing greenhouse gas emissions and controlling A. viridis-encroachment.
