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Growth, management and nutritive value of willows (Salix spp.) and other browse species in Manawatu, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Ph.D.) in Plant Science at Massey University, Palmerston North, New Zealand
Pastoral farming in New Zealand depends mainly on the grazing of ryegrass (Lolium perenne) and white clover (Trifolium repens). These pastures yield less during dry summers and farmers are then faced with feed shortages. This study revealed the potential of deciduous willows, Salix matsudana x alba and Salix kinuyanagi as useful supplementary fodder during dry summers. The species can be managed under cut and carry or in situ browsing systems. The use of the Salix spp. as drought fodder is a viable option for pastoral farmers to assist them to overcome pasture feed shortages while maintaining their role in conserving soil. Three experiments were conducted to determine (1) the effects of cutting height and frequency on browse yield and quality of deciduous Salix spp., compared with Dorycnium rectum, a small leguminous shrub and (2) the effects of planting stocks and fertiliser use on the yield and quality of the Salix species. Experiment 1, conducted at HortResearch, Aokautere (10 km from Palmerston North) demonstrated that edible dry matter (DM) yield of the Salix spp. and D. rectum was uninfluenced by stump height and frequency of harvest. S. matsudana x alba outyielded S. kinuyanagi and D. rectum. The in vitro organic matter digestibility (OMD) and nitrogen (N) concentration of the three species were: S. matsudana x alba (670 g kgDM-1 and 21 g kgDM-1), S. kinuyanagi (613 g kgDM-1 and 18 g kgDM-1) and D. rectum (665 g kgDM-1 and 22 g kgDM-1). Experiments 2 and 3 were conducted at AgResearch Grasslands, Ballantrae (25 km from Palmerston North). Experiment 2 showed that unrooted stem cuttings produced as much foliage as rooted stem cuttings, and the former are recommended due to their cheaper establishment. S. matsudana x alba consistently outyielded S. kinuyanagi even though DM yields were lower than at Aokautere. Experiment 3 revealed that DM yields of the Salix species were unaffected by fertiliser application. The low DM yield at Ballantrae may be due to the effects of strong winds and low temperatures. At both Aokautere and Ballantrae, S. kinuyanagi (255 vs. 289 g kgDM-1) had higher total condensed tannin concentrations than S. matsudana x alba (60 vs. 154 g kg DM-1). Two experiments were conducted at AgResearch Grasslands, Palmerston North to determine (1) the leaf lifespan and effect of leaf maturation on leaf quality of the Salix spp. and the legume Chamaecytisus palmensis, (2) the appropriate time to cut the Salix spp. to optimise regrowth and browse quality for summer use, and (3) the changes in non-structural carbohydrate reserves associated with defoliation. The long leaf lifespan of the Salix spp. (6.5 months) and Chamaecytisus palmensis (5.5 months) indicated that the green standing biomass could be retained until needed in summer. The leaves of the Salix species and Chamaecytisus palmensis declined in N concentration and other nutrients at 3 months, which was longer than for those of most herbaceous species (1-2 months) during most times of the year. Leaf N concentration in all three species at each harvest met the recommended level (17 g kgDM-1) for a diet adequate for a lactating ewe rearing a lamb. S. matsudana x alba yielded 70% more DM than S. kinuyanagi when cut in mid-spring. Sucrose comprised over 90% of the total soluble carbohydrate concentration in the roots of S. matsudana x alba defoliated once and three times in the growing season. Two experiments were also conducted at Massey University to determine (1) the effect of browsing intensity and frequency on the regrowth of the Salix spp. and C. palmensis, and (2) sheep preference for these browse species at different times of the growing season. Regrowth of S. matsudana x alba and C. palmensis was similar under heavy or light browsing whereas heavy browsing in S. kinuyanagi increased regrowth in woody stem and total DM yields. S. matsudana x alba was the preferred species. Sheep biting rate, percent time spent browsing species and preference rating of species were higher in summer than autumn for S. matsudana x alba and similar for C. palmensis in both seasons. For S. kinuyanagi, these variables were higher for autumn and almost zero for summer browsing. The two Salix spp. will complement C. palmensis, when grown in moist sites, where Chamaecytisus trees have low survival. The Salix species under proper management will provide useful supplementary feed for livestock during summer.