Herbage intake and N excretion by sheep grazing monocultures or a mixture of grass and white clover

In 1988 and 1989, swards of grass (G0), while clover (C0) and grass/white clover (GC0) receiving no N fertilizer, and a grass sward supplied with 420 kg N ha^?1 (G420), were grazed by non-lactating sheep to maintain a sward surface height of 6 cm. Herbage organic matter (OM) intakes averaged between 1200 and 1700 g OM ewe^?1 d^?1. For treatments G0, C0, GC0 and G420 respectively, the ewes' live weight gain was 102, 112, 100 and 110 g d^?1 and changes in body condition scores were +0·28, +0·52, +0·36 and +0·44 units season^?1. However, the effect of treatment was not significant for either variable. There were similar levels of output of faecal N ewe^?1 but significantly more urinary N ewe^?1 was excreted on treatments C0 and G420, where the concentrations of N in herbage laminae were also higher. For example, in 1989, total daily N excreted was 39·7, 64·4, 44·0 and 63·3 g N ewe^?1 for G0, C0, GC0 and G420 respectively. Taking into account the mean daily stocking rates, which were 19·4, 26·6, 27·2 and 36·5 ewe ha^?1, the total faeces and urine returns over the season were 161, 358, 249 and 484 kg N ha^?1 for each treatment respectively. The herbage OM intakes ewes^?1 d^?1 measured in September and October were similar for C0 and G420, and so the intake of herbage OM ha^?1 d^?1 was related to stocking rate, i. e. the estimated herbage intake ha^?1 over the growing season for the white clover monoculture was 73% of that for N-fertilized grass. Excretal nitrogen returns to the pasture from grazed mono-cultures of clover were high, and similar to those from a grass sward receiving 420 kg fertilizer N ha^?1. Consequently potential losses of N to the environment are high under these management systems.     

Changes in the structure and physiology of a perennial ryegrass sward when released from a continuous stocking management: implications for the use of exclusion cages in continuously stocked swards

A perennial ryegrass sward was managed by continuous stocking with sheep (April–September) for 4 successive years after sowing. The sward was grazed to maintain a leaf (lamina) area index (LAI) close to 1.0. Areas of the sward were released from grazing on three occasions: once during summer in the third year after sowing, and twice during spring and summer in the fourth year after sowing. There were marked changes in the structure and physiology of the continuously stocked sward following release from grazing. After several successive years of continuous stocking, the sward comprised a large population of small tillers and the small LAI resulted in consistently low rates of photosynthesis. Following release from grazing, photosynthesis increased markedly as the LAI increased but this change was associated with the loss of a large proportion of the population of tillers. There were seasonal differences in the pattern of changes in photosynthesis and tiller numbers following release from grazing which were not apparent under continuous stocking. The changes in the structure and physiology of the sward following release from grazing suggest that the net accumulation of herbage in areas of sward from which the animals are excluded, for instance using cages, may be an unreliable estimate of production under continuous stocking.     

Use of a model to analyse the effects of continuous grazing managements on seasonal patterns of grass production

The effects of management on the seasonal patterns of production of grass swards under continuous grazing are examined using a previously described mathematical model. For continuous set-stocking it is observed that, if the leaf area index of the sward at turn-out is too low, there may be a sustained reduction in animal intake throughout the season. Under continuous variable stocking, where the sward is maintained at a constant leaf area index, animal intake is observed to follow broadly a pattern determined by the environmental conditions. It is concluded that grass production, in terms of both gross tissue production and animal intake, is not independent of management. Furthermore, no evidence is found to suggest that the seasonal patterns of production undercutting and grazing are the same.     

Intake and behaviour responses by sheep to changes in sward characteristics under rotational grazing

A 10-week grazing experiment was conducted on a perennial ryegrass sward with lactating ewes and their twin lambs. Three paddocks were rotationally grazed with rest periods of from 4 to 5 weeks. Sward surface heights at the start of each grazing were 145, 259 and 250mni for treatments RG1, RG2 and RG3. A further four paddocks were maintained by continuous variable stocking (CS) at sward surface heights (SSHs) of about 30, 60, 90 and 120mm. Sward and animal measurements were made on the two different grazing managements as the RG swards were grazed down, giving measurements at similar sward heights for treatments RG and CS.
There was less green leaf and the total herbage mass present under RG was less than on CS swards at the same sward heights, demonstrating the differences in structure between rotationally and continuously grazed swards.
Regression analysis of animal factors on sward factors showed that grazing behaviour was more highly correlated with green leaf mass than SSH or any of the other sward measurements. On the RG swards, maximum intake per animal was reached at about 1500 kg green leaf mass ha⁻¹. A SSH of 60mm allowed the CS ewes to achieve the highest intake rate, but at this height the ewes on treatments RG2 and RG3 were restricted to approximately half this rate. The results suggest that green leaf mass or leaf area index, rather than sward surface height, could be used as a rational basis to relate intake of herbage to sward state for swards changing rapidly in leaf to stem ratio.     

The incidence, cost and factors associated with foot lameness in dairy cattle in south-western Victoria

A survey of 73 dairy farms in south-western Victoria was conducted to assess the cost and mean herd incidence of foot lameness for the period from calving to the end of November, 1985, and to identify the herd, management and environmental factors associated with foot lameness. The mean herd size was 125 cows (range 82 to 220). Lameness occurred in 64 (88%) herds, and the mean herd incidence was 7.0% (range 0.0 to 30.9%). The main clinical signs associated with lameness were the presence of overworn and/or bruised soles, or stones lodged in the interdigital cleft. Factors associated with lameness were: property and herd size, age of cow, bail feeding, voluntary entry into the bails, and features of the farm track including its length, the presence of steep slopes, the type of surface material, presence and treatment of broken sections and maintenance including rolling history. The association of these factors with specific clinical signs was examined. The mean cost was estimated to be $42.90 per lame cow due to loss of production, treatment, the culling or death of lame cows, and extra man hours spent managing lame cows. It was concluded that the site, construction, maintenance and use of the farm track were of major importance to the incidence of lameness in herds in this area and recommendations for reducing lameness are made.     

Leaf age structure and canopy photosynthesis in rotationally and continuously grazed swards

The leafage structure of ryegrass canopies and its role in canopy photosynthesis were compared under continuous and rotational grazing by sheep. Under continuous grazing, an increase in the intensity of grazing increased the proportion (by leaf area) of young leaves in the sward. A mechanistic mathematical model was used to demonstrate how this may have arisen, even though it would largely have been the young leaves that were eaten.
However, the observations do not confirm the hypothesis that continuously grazed swards have a characteristically greater proportion of young leaves, and so a greater photosynthetic potential, than rotationally grazed ones. The proportion of young leaves increased during regrowth following severe rotational grazing (residual LAI < 05) and the photosynthetic potential of the canopy became greater than under continuous grazing.
A model of canopy photosynthesis was used to demonstrate that the observed difference in the proportion of young leaves alone was unlikely to account for all the differences in canopy photosynthesis between managements, and further differences in canopy structure were evaluated. Despite the delay in the restoration of leaf area following severe grazing in a rotation, the total photosynthetic uptake of a system involving some 12–13 days regrowth and 3 days grazing exceeded that of a well-utilized continuously grazed sward. Re growths of longer duration led to progressively greater total photosynthetic uptake, though this was not considered synonymous with greater yield.     

A cross-sectional study of risk factors affecting the outcome of rabbit haemorrhagic disease virus releases in New South Wales

OBJECTIVE: To determine what factors governed the extent of outbreaks of rabbit haemorrhagic disease (RHD) following releases in New South Wales. DESIGN: Retrospective cross-sectional study. PROCEDURE: Information from the data set of official releases was subjected to two preliminary analyses. More comprehensive information on a subsample of official RHD releases, sites and animals was gathered by telephone survey of Rural Lands Protection Board staff and farmers. Data were analysed using multivariate techniques to determine which factors were associated with rabbit mortality within one month of RHDV release, within several months of release and in affecting the proportion of the population killed. RESULTS: A strong association was found between the presence of heavy flea infestation (odds ratio 2.7), breeding in rabbits and outbreaks of RHD. For each week following breeding there was an 8% decline in the odds of an outbreak. Low temperatures also promoted outbreaks. Less important effects included the prior presence of RHD at the release site, which reduced the likelihood and severity of outbreaks. The presence of cattle and proximity to the nearest water body were associated with increased severity and likelihood of outbreaks respectively. CONCLUSION: Both breeding of rabbits and associated high flea numbers may act together or independently in promoting outbreaks of RHD. Stresses involved with rabbit reproduction and low environmental temperatures also appear to influence the likelihood of outbreaks. The effects of proximity to cattle and water suggests that both flies and mosquitoes may have a minor role in local transmission.