Ficha técnica

The area of pasture influenced by animal excreta was investigated by Petersen et _{al., ( 1 956) ,}

Maclusky ( 1 960), MacDiarmid and Watkin (1 971 , 1 972a, 1 972b), Kennedy and Till ( 1 98 1 b) and Morton and Baird (1 990). The area influenced by 35s labelled sheep urine was studied by During and Martin ( 1 968) and Kennedy and Till ( 1 981 b) , respectively. Maclusky ( 1 960) showed that dung affected the growth over an area about six times that actually covered. This agreed with the findings of MacDiarmid and Watkin ( 1 971 , 1 972a). MacDiarmid and Watkin (1 971 , 1 972a) concluded that grasses growing up to 1 5 cm from the edge of a cow dung patch can derive N from the region under the patch owing to the lateral spread of grass roots. They found an increase in available soil N at 6 inches beyond the edge of the dung. Assuming a mean area of dung patch of 0.05 m2' this was equivalent to an affected area of about 0.25 m2. Further, it was shown that the percentage area of paddock covered by cow dung patch after 24 hour's grazing ranged from 0.31 to 0.68%, depending principally on stocking rate (MacDiarmid and Watkin, 1 972a, 1 972b). At stocking rates of approximately 20 stock unit per ha, it was calculated that at any grazing time approximately 5% of the pasture could be affected by cow dung-patch nutrients and it would take 2.5 years for an area equal to that of the paddock to be affected by dung patches and ten years for the same area to be covered by dung patches. This was similar to an empirical calculation performed by Petersen et _{al., ( 1 956) who estimated}

that it would take 1 3 animal-years of grazing (4745 cow-days) to cover 1 00% of the pasture with dung.

However, there appears to be no existing similar i nformation on the area of soil that is i nfluenced by sheep dung return.

Lotero et al. ( 1 966) observed that the greatest growth response occurred in the centre, and decreased towards the periphery of the roughly circular area affected by urine. According to Kennedy and Till ( 1 98 1 b) , it was found that the area i nfluenced by a urine patch extended as far as 20 cm; with time and with more rain may be further. There was both lateral and vertical

movement of S from the urine patch and this was in agreement with the result reported by During and Martin (1 968). On average, recovery by pastures of 35s from urine was less than 5%. Initially (after 7 days), the percentage of plant S derived from 35s urine (%SD FU) was highest (21%) in the innermost zone and decreased toward the outer zone. After 1 0 days, there was not much difference in %SDFU among zones (average 5-7%).

Since the animal in the pasture system is mobile, cycling of nutrients through animals will be a function of this mobility (Wilkinson and Lowrey, 1 973) . Factors affecting the time-space distribution of excreta include stocking rate, camping, grazing pattern, type of animal and the amount and frequency of excretion (Wilkinson and Lowrey, 1 973) . The problem of u neven excreta return has been discussed by Sears ( 1 950), Petersen et al. _{(1 956), Hilder (1 964),}

Gillingham and During, (1 973), Till ( 1 975), Gillingham (1 978), Boswell ( 1 983) , Rowarth (1 987), Williams ( 1 988), Morton and Baird (1 990) and Saggar et al., (1 990a, 1 990b). S ingested over a large area is voided to a small area, thus concentrating the S at that site, such as a campsite . Losses by leaching can therefore be accentuated ( Petersen et al. , 1 95 6 ; Gillingham, 1 978; Boswell, 1 983 ; Williams e t al., 1 988; Hedley e t al., 1 990 ; Saggar e t al. ,

1 990a, 1 990b; Goh and Nguyen, 1 990). Hence the cycling of S through animals is spatially less effective than through plant root and shoot litter, o r in other words, nutrients are transferred to an area where the nutrients are unproductive (an area already rich in nutrients or raceways and yards).

The unevenness of dung distribution on a flat paddock was shown by Hilder ( 1 964), in a study of grazing sheep in Australia. lt was found that the campsite which comprised 3% of the total area, contained about 22% of the total dung and that 1 0% of the total area contained 34% of the total dung. Similar conclusions were also reported in New Zealand by Gillingham ( 1 980) studying hill country pasture land where the added effects of land slope, aspect to sun and prevailing climatic conditions influence animal grazing and camping behaviour (Saggar, 1 990a, 1 990b). Gillingham ( 1 980) found about 1 0.3, 1 .2 and 0.1 5 t ha-1 yeaf1 of sheep dung were deposited on campsite (0-25° slope), moderate (25-45° slope) and steep slopes (>45° slope), respectively. Boswell ( 1 983) showed that at the campsites total soil S throughout the soil profile and leaching loss of S from soil were generally higher than that in the non-campsites. However, more than 50% of urine S was retained in soil by immobilization processes rather than redistributed by plants grown on campsites.

The spatial distribution of urine is unknown but it is expected to follow dung distribution (Till, 1 975).

According to Till (1 975)

"An estimate of the likely importance of redistribution can be made by considering an improved pasture stocked at 10 sheep ha-1 and fertilized with S at 25 kg ha- 1 year-1 . In

such a system the return of S in excreta would be about 4 kg ha -1 year- 1 in dung and 6 kg ha-1 year-1 in urine. If the dung distribution is 34% on 10% of area (Hilder, 1964), this would give deposition (and perhaps effective loss) of 1 .4 kg dung S or 5.6 percent of the annual application"

If dung and u rine are similarly distributed the loss would be 2.4 kg S and equivalent to 9.6% of the annual S application.

2.6.4.3 Excreta decomposition

With the exception of pioneering work by Barrow ( 1 961 b) and recent work by Boswell ( 1 983), there has been little published information on the rate of release of S from dung. Barrow (1 961 b) showed that the amount of S mineralized was closely related to the S content of the dung and the proportion of S mineralized was less than with plant material. Dung was more resistant to decomposition than was plant material (Barrow, 1 961 b; Boswell, 1 983). Barrow ( 1 961 b) also reported that there was likely to be net immobilization below a dung S content of 0.22% while similar results were likely to occur with plant litter with S content below about 0 . 1 2%.

Dung mineralization rates are more rapid in crushed samples than pad samples (Bromfield and Jones, 1 970; Rowarth, 1 987) and in the presence of arthropods such as earthworms ( Barley, 1 964) and dung beetles (Bomemissza and Williams, 1 970) which can be expected to enhance the mineralization rate through the action of communition of material and transporting it to the vicinity of microorganisms (MacFadyen, 1 978). Similarly dung decomposition rates are slow in dry conditions as compared to cold moist conditions. For example, Hilder ( 1 966) reported sheep pellets remaining for 1 0 weeks in cold moist conditions in Armidale, Australia. However in New Zealand, Rowarth ( 1 987) reported that dung samples (by physical disappearance) decomposed within 20 days in cool moist winter periods, while longer periods (up to 60 days) were observed during summer.

The fate of 35s labelled urine and dung and decomposition of unlabelled dung was studied by Boswell (1 983) in controlled conditions in the field by the litter bag technique. Results can be summarized as follows:

1 . Uptake of S from urine in camp site soil was initially higher than from non-camp site soil.

2. The mean rate of release of S from dung (0-37 days) was about 4.5 mg S g-1 S day-1 which was much lower than from litter.

3. There was i mmobilization of soil S after S had been released from dung. This agreed with the results reported by Barrow (1 961 b).

4. Movement of S from dung and u rine to soil organic S was as rapid as from plant litter. I n particular, about 68% of u rine S was in organic S within 6 d ays. In general, more than 80% of the S released was converted into organic forms. Unfortunately, there was no partitioning of organic S fractions in the study.

The uptake of 35s labelled sheep excreta was also investigated by Kennedy and Till ( 1 981 b) under field conditions. Results showed that the percentage of plant S derived form u rine and dung (Excreta, %SDFE) were as high as 94% and 54% respectively. About 20% and 1 0% of the S from the urine and dung were taken up by plants. Recovery of S by pasture plants from both sources dropped rapidly with time , especially from urine. lt was calculated that the excreta from 20 sheep ha-1 could provide 20% of the S require ment of the above ground plants. Furthermore, it was found less than 40% of total activity were recovered by pastures after 384 days. Leaching loss and lateral spread were suggested as responsible for the low recovery.

Both dung and urine have also been shown to affect soil properties where they were returned, particularly at campsites. They have been shown to increase cation exchange capacity, organic matter, N , P, exchangeable potassium, calciu m and magnesium (MacDiarmid and Watkin, 1 972a; During and Weeda, 1 973) and soil pH (Doak, 1 952; Watson and Lapins, 1 969; During and Weeda, 1 973). As the soil pH increases, greater desorption of sulphate may occur (Ensminger, 1 954; Kamprath et al. , 1 956; During and Weeda, 1 973; Bolan et al. , 1 988) and mineralization of soil organic S may increase (White, 1 959; Barrow, 1 960b, 1 960c; Williams, 1 967). This favours accelerated local losses of soil S by leaching (Boswell, 1 983; Sinclair and Saunders, 1 984; Saggar et al. , _{1 990a, 1 990b) and leads many researchers to consider that}

nutrient cycling through grazing animals is an inefficient process resulting in considerable loss of effective nutrients from productive areas of paddocks (Hedley et al. , _{1 990; Saggar} et al.,

1 990a, 1 990b; Goh and Nguyen, 1 990)

To summarize, the general fate of S in a paddock being grazed by animals can be represented by litter, dung, urine and amounts retained in animal tissue. Of the total S in pasture plants, about 1 0-30% are in litter, 25-30% as dung, 30-40% as urine and about 1 0-15% held In animal

products. Of the total S in l itter, dung and u rine returned to the soil, it appears that approximately 5-1 0%, less than 5% and 20-30%, respectively, will be re-utilized in the short term by pasture plants. The remainder (about 60-75% of total S returned to the soil) is mostly immobilized into soil organic matter. From this review, the urine deposition is expected to stimulate the greatest increase in plant S uptake or leaching in the short term. Deposition of S in carbon rich, plant litter and particularly dung is expected to have low plant recoveries and in the short term may be responsible for promoting immobilization of soil sulphate.