SPECIES COMPOSITION 1

Grazi.ng animals can change botanical composition (Milton et aI. , 1 994) and pasture

species can differ in their reaction to treading. But in these studies, treading had little effect on botanical composition. The dominant ryegrass tillers were less susceptible to treading damage compared to other grass species and could recover quickly. B ates ( 1 93 5 and 1 95 1 ) observed that ryegrass was especially prominent wherever animal treading was heavy.

Browntop development can be influenced by tillering and rhizome production (Levy, 1 924), and the build up of its large buried seed populations in soil (Harris, 1 978), aiding

CHAPTER SIX General discussion and conclusions

the invasion of surrounding pasture. Rapid colonising ability of white clover in bare soil areas may also be important in regeneration and change in botanical composition. However, the browntop-containing "other grasses" component and white clover did not develop during the regrowth period in the current studies. Grubb ( 1 977) pointed the importance of regeneration, which influences the ability of plants to take adval).tage of niches created within a pasture.

Overall, it can be concluded that grass dominant pastures, with little clover present, encourage ingress of ryegrass after treading especially in low- to medium-damaged areas. In the long-term, this may result in marked botanical change, although this was not apparent during these study periods. Both field and glass house studies (Chapter 5 ) showed that ryegrass-dominant pastures recovering from treading damage are reliant on the emergence and growth of new tillers.

6.7. MANAGEM ENT STRATEGI ES

The reduction of residual herbage mass to less than 500 kg ha-I by grazing and treading activity in high-damaged areas slowed regrowth, and these pastures took longer to recover after treading than less damaged pastures. Damaged treatments with more than 700 kg ha-I residual herbage mass, recovered within a 7 weeks regrowth period (Chapter 5). Relative to high-damaged areas, higher residual herbage mass in low- and medium-damaged areas would have caused faster plant regrowth. But total pasture production from these treatment plots would have been reduced compared to pastures with higher residual herbage mass. Bircham and Hodgson ( 1 984) showed that on a grazed area with less than 1 000 kg DM ha-I herbage mass, pasture growth was seriously affected.

CHAPTER SIX General discussion and conclusions

The data show that dairy pastures with ryegrass can withstand treading damage with a rotational grazing management system and a moderate stocking density. The relative tolerance of ryegrass under heavily trodden treatments and its recovery growth after treading in this study were similar to other field measurements (Edmond, 1 963; Vickery, 1 972).

It is clear that the effects of treading are an important factor of grazing management, since whenever hill land beef pastures or flat dairy pastures were trodden at a high stocking density, the recovery, in terms of production, was reduced. Furthermore, the combined impacts of treading and grazing proved even more detrimental to subsequent pasture recovery. It could be argued that by reducing stocking density and/or grazing time, the detrimental effect of treading will be lessened.

On a practical level, reduction in animal load per unit area of pasture to minimise the losses in tiller density due to treading by grazing cattle when the soil is wet and to reduce length of recovery would be helpful. On pastures of moist to wet soils, where a single treading occurs with no serious damage to soil structure but with the losses in tiller population density and herbage dry matter, management options of choosing right grass species such as perennial ryegrass may be effective to restore the losses caused by treading. Also to ensure sustained pasture productivity, especially to counter severe treading damage to pasture cover and to achieve faster regrowth after damage, it is important that management strategy should be focused on species which are persistent and fast growing. This will require the species to be chosen carefully for each farm, and the right ones to be selected to cope with wetter and drier than average years.

Further study is required on the effect of treading on interrelationships between soil and pasture growth in the long term, especially the root growth environment and its influence on tiller growth. Also information on what attributes cause persistence of

CHAPTER SIX General discussion and conclusions

different species and their sustainability under treading and their length of recovery in treading-sensitive areas and/or soils would be helpful in making farm management decisions. Light interception and photosynthetic activity of pasture plants between different canopy structure in treading disturbed areas will provide better knowledge in understanding pasture regrowth.

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