Effects of spring grazing on dryland perennial ryegrass/white clover dairy pastures. 2. Botanical composition, tiller, and plant densities

Abstract

A 3-year experiment (September 1999–August 2002) in south-western Victoria investigated spring grazing impacts on botanical composition, tiller densities (perennial ryegrass, other-grasses, clover growing points, and broad-leaved weeds), and perennial ryegrass plant frequencies of a pasture of perennial ryegrass (Lolium perenne L.)–white clover (Trifolium repens L.) pasture. Spring grazing treatments, applied annually from September to November were based on ryegrass leaf development stage with high (HF), medium (MF), and low (LF) grazing frequency being 2-, 3-, and 4-leaf stage, respectively, and post-grazing height as the grazing intensity with high (HI), medium (MI), and low grazing intensity being 3, 5, and 8 cm, respectively. Five combinations were used: HFHI, LFHI, MFMI, HFLI, and LFLI. A sixth treatment, rapid grazing (RG), maintained pasture between 1500 and 1800 kg DM/ha by grazing weekly during spring and a seventh and eighth treatment, simulating forage conservation for early-cut silage (lock-up for 6–7 weeks; SIL) and late-cut hay (lock up for 11–12 weeks; HAY), were also included. Perennial ryegrass content remained stable in the RG and HFHI treatments, averaging 78 and 75% DM, respectively, and declined in all other treatments. For example, perennial ryegrass content in LFLI declined from 75% (September 1999) to 50% DM (August 2002). RG and HFHI resulted in a slight increase in other grass (e.g. Holcus lanatus and Poa annua) content over time. SIL, LFLI, LFHI, and MFMI resulted in a higher other-grass content than RG and HFHI. LFLI, LFHI, and HFLI resulted in a stable dead (litter) pasture content over time, while SIL, HAY, HFHI, and RG resulted in a decline in dead pasture content over time. For RG and HFHI spring treatments the decline in dead pasture content was greater than LFLI, LFHI, and HFLI spring grazing. Data for white clover and broad-leaved weeds (e.g. Rumex dumosus and Taraxacum officinale) were inconsistent and could not be statistically analysed. While perennial ryegrass tiller density declined over time, RG and HFHI spring grazing resulted in a higher perennial ryegrass tiller density than low and medium grazing frequency treatments, and forage conservation treatments. Over time, tiller density of other grass increased, with MFMI, SIL, and HAY resulting in a greater increase than HFHI and RG treatments. During the experiment, white clover growing point density declined, while broad-leaved weed tiller data were inconsistent and not analysed. In October 2001, perennial ryegrass plant frequencies ranged from 12 (HAY) to 27 (RG) plants/m2. RG resulted in a higher perennial ryegrass plant frequency than medium and low spring grazing frequencies, and forage conservation (HAY). At the end of the experiment (August 2002), perennial ryegrass frequencies ranged from 15 (HAY) to 45 (RG) plants/m2 with RG resulting in a higher perennial ryegrass plant density than all other treatments. HFHI grazing resulted in a higher plant frequency than LFLI, SIL, and HAY, and HFLI a higher plant frequency than SIL and HAY. RG and HFHI spring grazing favoured perennial ryegrass persistence as it maintained botanical composition and perennial ryegrass tiller and plant frequencies relative to low and medium spring grazing frequency or high spring grazing frequency coupled with low intensity grazing and pasture locked up for forage conservation.