The nutritional value of Russell lupin (Lupinis polyphyllus x Lupinus arboreus) for sheep

Two field trials were conducted consecutively in Canterbury, at Lincoln University. In the first experiment, spring regrowth of Russell lupin (Lupinus polyphyllus x L. arboreus) was cut at three weekly intervals to determine changes in nutritive value with plant maturity. Six harvests were made between 5 October 1989 and 18 January 1990. Measurements included dry matter (DM) yield per plant and plant parts, nitrogen (N) and neutral detergent fibre (NDF) concentration, and in vitro cellulase DM and organic matter (OM) digestibility. Whole-plant DM yield for the six cuts increased from 40 to 160 g plant⁻¹ (or 4 to 16 t ha⁻¹). Up to pod formation the DM yield of Russell lupins was largely petioles and leaves. The N concentration in total DM decreased from 4.5 to 2.4% with maturity; corresponding values for NDF were 24.1 to 46.2%. This was due to both changes in the proportion of plant components and changes in N and NDF concentration within components. The N concentration in individual plant parts generally declined over time. The in vitro cellulase DM and OM digestibility declined from 76.5 to 56.0 % and 81.4 to 54.9%, respectively. Unlike most other pasture species, the in vitro cellulase DM and OM digestibilities of Russell lupin showed a slow, quadratic (P<0.001, R² = 0.94) decline with maturity. The high digestibility of Russell lupins together with their high DM yield gave a very high yield of digestible DM (DDM) and digestible OM (DOM). The DDM yield of Russell lupins showed two peaks; the first, at 89 g plant⁻¹, at full bloom and the second, at 91 g plant⁻¹, at the dry pod stage. Although the two DDM yields were similar, (i) the second peak had three times more DDM from dead matter, (ii) 49 and 20% of the DDM (of peak I and II respectively) consisted of plant parts with >80% digestibility, and (iii) 0 and 50% of the DDM (of peak I and II respectively) consisted of plant components with <60% digestibility. This trial showed that Russell lupins can produce highly digestible DM with a high N content over most of their growth. In the second experiment, autumn sown (March, 1990) Russell lupins were grazed (Nov., 1990 - Jan., 1991) by two-tooth Coop worth ewes (plot size 418 m², 20 sheep plot⁻¹) at full bloom, green pod and dry pod stages. The objectives were: (i) to distinguish between the two stages of peak DDM yield in terms of acceptability to sheep, per cent utilisation and amount of regrowth and determine the optimum stage to graze the lupins, and (ii) to study preference of sheep among different plant components of Russell lupins. There was no apparent difference between the three stages of growth with respect to acceptability, for average DM disappeared per sheep increased with allowance. Sheep selected against stems, but showed strong preference for leaves; defoliation of other parts increased as the proportion of leaves in total herbage decreased. As opposed to earlier reports, there was significant consumption of both green and dry pods. Per cent utilisation was 89, 80 and 75% for lupins grazed at full bloom, green pod and dry pod, respectively. Total regrowth DM (residue + current growth) yield was 6960, 3774 and 2282 kg ha⁻¹ for Russell lupins grazed at full bloom, green pod and dry pod stage, respectively. However, the difference between full bloom and dry pod in terms of estimated annual harvestable (i.e. by sheep) DM yield, which respectively was 6990, 6490 and 7410 kg ha⁻¹ for lupins grazed at full bloom, green pod and dry pod stage, was not as marked. Therefore, it was concluded that the optimum stage for grazing will depend on the feed requirement plan of the individual farmer. Farmers have the option of leaving the lupins standing till late in the season without marked loss of quality, or graze them early for better autumn regrowth.