Autumn performance of rhizomatous clovers after a summer drought, under different grazing treatments

Abstract

Caucasian clover (Trifolium ambiguum; cv. Monaro and Alpine), zigzag clover (Trifolium medium; seed lines, Kentucky and Porters Pass) and white clover (Trifolium repens), were evaluated in a sheep grazing trial on a lowland medium fertility soil (Templeton silt loam) at Lincoln University, Canterbury.

Each rhizomatous clover cultivar / seed line was sown (March, 1989) in plots with four rows 0.75m apart, 80m long. There were three replicates of these plots (1.5m apart), each replicate arranged as randomised blocks. The site was oversown with Marsden high endophyte ryegrass (Lalium perenne) in the spring of 1992. Volunteer white clover Huia was present throughout the trial site.

The 80*3m plots were split into eight grazing plots 10*56m rows in August 1993 by fencing, at right angles across the original plots. This resulted in 12 clover subplots in each main grazing plot. The eight main plots were divided into two replicates of four grazing treatments: two levels of grazing intensity (Lax (L) and Hard (H)) and two levels of grazing frequency (Set stocked (S) and Rotational grazing (R)). The four grazing treatments were first applied in September 1993, and then continued in early-February 1995 after having not been grazed since late December 1994. Set stocked lax pastures were grazed for 2.5 days after an interval between grazing of 4.5 days to maintain a pasture mass of about 1700kg DM/ha. Hard set stocked pastures were maintained at between 600 to 800kg DM/ha by grazing every 4.5 days with a 2.5 day interval between grazing. Hard rotational grazed treatments aimed to have a pre-graze herbage mass of between 1500 to 2000kg DM/ha 25 days after grazing and a post herbage mass of 600 to 800kg DM/ha after 5 days grazing. Lax rotational grazed paddocks were spelled for 25 days to achieve a pre-grazed herbage mass of 2000 to 3000kg DM/ha then grazed for 3 to 4 days to a post-herbage mass of 1500 to 2000kg DM/ha.

Botanical analysis, clover morphology and grass characteristics were measured in all grazing treatments prior to grazing of the rotational grazed pastures in April, May, June and in early October after plots were grazed approximately every six weeks to a herbage mass of about 800 kg DM/ha.

The intensity of the hard set stocking treatment was indicated by reduced ryegrass tiller populations, tiller weight and length by 50% compared to the lax set stocked treatment. Infrequent grazing favoured ryegrass growth and tillering.

The hexaploid Monaro Caucasian was the superior clover in the lax rotational grazed treatment covering 14.3 to 29.3% of the subplots. Swards dominated by Monaro reduced other sward components. Hard set stocking of pasture reduced the percentage cover of all rhizomatous clover species; only white clover (8.4 to 18% cover) and weeds (16 to 24% cover) increased under the extreme grazing pressure. Hexaploid Monaro Caucasian cover was superior to the diploid Caucasian cv. Alpine in cover and all growth characteristics. Hard rotational grazing treatments increased the number of clover growing points more than the other treatments for Caucasian and white clover species. Clovers in lax grazing treatments had longer petioles, but petiole growth potential may have been reduced by declining soil and air temperatures. Zigzgag clover generally had a very low cover and growing point populations, but lax infrequent grazing favoured zigzag most.

An assessment of underground biomass in early August 1995, showed that the root weight of Monaro Caucasian (202g DM/m³) far exceeded that of white clover (9g DM/ m³). Rotationally grazed treatments had significantly (P<0.007) more Monaro underground biomass (330g DM) than set stocked treatments (78gDM/m³). This may reflect the preferential grazing of clover in reference to the high endophyte ryegrass.

A supplementary study conducted on old seed plots beside the main trial site, was grazed to a herbage mass of 450kg DM/ha on the 5th of May and used to evaluate the regrowth of clover over 50 days. Monaro Caucasian clover and white clover produced far more growing points than any other of the clovers measured (5280 and 4790 growing points/m² respectively). Petiole length of all clovers increased during the 50 day period, particularly Monaro Caucasian and stoloniferous white clover (up to 73mm). Growth of all but white clover slowed from 24 to 50 days indicating that the other rhizomatous species were becoming dormant due to low temperatures (Mean air temperature in May 9.7°C and June 6.2°C).

Results from the experiment confirm the choice of hexaploid Caucasian clover for commercial pastoral development. Zigzag clovers were very low producing and uncompetitive with ryegrass under the range of treatments used. Alpine Caucasian (2n) was less productive than Monaro (6n).

The reduced underground biomass of set stocked Monaro suggests that the rhizomatous habit may not be the ultimate response to continuous hard grazing and that some autumn spelling and or rotational grazing may be necessary to maintain root and rhizome reserves.