Technical note: Comparison of techniques for evaluating the relative preference by sheep among saltbush clones

This research compared 4 field methods of evaluating the relative preference by sheep of 28 clones of saltbush (Atriplex halimus L.). The methods were as follows. 1) Leaf dots (LD): 8 leaves per shrub were marked on the lower surface with a small dot using a water-resistant, nontoxic ink. 2) Twig marks (TM): 2 current-year twigs per shrub were marked with 3 lines using the same ink approximately in the middle of the basal, median, and apical thirds. 3) Branch length (BL): 2 branches per shrub were marked with ink at the base of the current year's growth. The twigs were measured from the marked point to the top, before and after sheep browsing. 4) Ocular estimation (OE): the percentage of the total number of current-year twigs browsed was visually estimated for each shrub. The percentage of use was calculated by counting the residual dots (LD) or marks (TM) after browsing and by calculating the difference between the 2 measures in the BL method. The trial was conducted in August and repeated in November and in both grazing periods 4 observations were made. Highly significant differences among clones were observed. The different methods generally gave similar results for the ranking of the clones, but each method showed a different discriminatory capacity. On the basis of the F ratio, the OE method seemed more efficient, although results were subjective and mainly dependent on the experience and skill of the observer. Among the methods based on the counting or measurement of markers, the discriminatory capacity decreased from LD to BL and TM, but the opposite order was observed for the ease of setting and counting the markers.