An evaluation of the yield and quality of oat - legume and ryegrass - legume mixtures and legume monocultures harvested at three stages of growth for silage

Abstract

The effect of harvest time on the dry matter (DM) yield and quality of 10 annual temperate legumes [Trifolium subterraneum, var. brachcalycinum and var. subterraneum L. (subterranean clover), T. michelianum Savi (balansa clover), T. alexandrinum L. (berseem clover), Medicago murex Willd. (murex medic), M. truncatula Gaertn. (barrel medic), T. vesiculosum Savi (arrowleaf clover), Vicia sativa L. (common vetch), V. benghalensis L. (purple vetch), Pisum sativum L. (peas) grown in monoculture or in mixtures with oats (Avena sativa L.)] was examined in a Mediterranean environment in southern New South Wales. Three of the legumes (subterranean clover, berseem and peas) were also grown in mixtures with ryegrass (Lolium multiflorum Lam.). The forages were sown on 29 May and harvested at three times (2 October, 23 October and 6 November), corresponding to the boot, anthesis and dough stage of the cereal oat variety, Kalgan. Peas were the highest yielding of the legume monocultures at each of the three harvest times (6.9, 11.6, 12.3 t DM/ha), followed by purple vetch (6.2, 9.9, 8.7 t DM/ha). Of the small-seeded pasture legumes, berseem, murex and arrowleaf clover were the highest yielding at the second harvest (7.5–8.8 t DM/ha) when most reached their peak biomass. The inclusion of oats with the legumes increased total DM yield at the first, second and third harvests by 10–54, 7–99 and 3–102%, respectively. Oat monocultures were high yielding (up to 17.7 t DM/ha), but had low N contents (6.3–12.5 g N/kg DM). Oat–legume forage mixtures had nitrogen (N) contents of 7–15 g N/kg compared with 17–40 g N/kg in the legume monocultures. The N content and digestibility of most species and mixtures decreased as harvest time was delayed, whereas total DM peaked at the second harvest for all species except Cooba oats. The legume content of the mixtures was negatively correlated with total DM yield, DM content and water soluble carbohydrates, but positively correlated with N content, in vitro digestibility and buffering capacity. The digestibility of oats decreased more rapidly than the legumes with advancing maturity, and it would need to be harvested at the boot stage to produce a silage of sufficiently high metabolisable energy for the production feeding of cattle. A later harvest at anthesis when yield is higher would produce a silage of sufficient quality for production feeding from an oat–large seeded legume mixture with a legume content of 0.50. The oats and ryegrass had high water soluble carbohydrate and low to medium buffering capacity and would, therefore, be expected to produce well-preserved silages. The legume monocultures had low water soluble carbohydrate contents and high buffering capacity values and there would be a high risk of a poor silage fermentation in these forages without field wilting or the use of a silage additive.