Cool-season management of a winter-active lucerne cultivar

Abstract

A dryland lucerne (Medicago sativa L.) grazing trial to compare cv Wairau and a winter-active cultivar, Rere, was conducted over a two and a half year period using mixed aged ewes. Four cool-season grazing treatments were imposed; no cool-season grazing, grazing in mid-June, a short spelling in spring and a combination of the mid-June grazing and short spelling in spring. During the warm season, the trial was given one management of 10 days grazing with 42 days spelling. In the first season, there was no difference in total dry matter yield of Wairau and Rere. The winter-active, Rere, out yielded Wairau during part of the cool-season, while Wairau outproduced Rere in early spring. By the third season, however, the annual dry matter yield of Wairau and Rere was 10,820 and 7,300 kg DM ha⁻¹ respectively. This difference in yield was due, in part, to the appearance of crown rot in the final season. Over 60% of Rere plants were affected by crown rot, compared to only 3% of Wairau plants. The incidence of crown rot in Rere was increased by a further 20% when grazed in mid-June, although this grazing treatment did not affect the susceptibility of Wairau to crown rot. Rere was also 90% higher in weed content than Wairau when both had been grazed in June. However, grazing either cultivar in June removed overwintering aphids and resulted in lower spring aphid numbers than in lucerne spelled over June. This difference was only significant until aphid flights in November. The short spelling in spring depressed subsequent yields of both cultivars until mid-January, although aphid numbers on lucerne subjected to this short spelling treatment remained below 2 per stem until aphid flights in November. In contrast, lucerne given a longer spelling in spring had up to 90 aphids per stem prior to aphid flights. This spring grazing treatment did not affect the incidence of crown rot in either cultivar. After two seasons, combination of the June grazing with the short spelling in spring reduced the dry matter yield of both Wairau and Rere by 40% compared to lucerne with no cool-season treatment.

Two other trials were established, with the lucerne cultivars Saranac, Wairau, Rere and Matador, which range from low to high winter-activity in that order. There was strong evidence from these trials that the depressive effect of June grazing on the yield of Rere in spring was due to a reduction in root reserves required for spring regrowth. Spring root reserves of the winter-active cultivars (Rere, Matador) were lower after the June defoliation than less winter-active cultivars (Saranac, Wairau) because of the greater use of reserves made by winter-actives in the cool-season utilisation of cool-season activity and high spring production thus proved to be incompatible.