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Effects of defoliation frequency on the persistence, leaf production and chloride content of Atriplex nummelaria

 

C.H. Donaldson

 

INTRODUCTION

Atriplex nummelaria (oldman saltbush) is a productive drought-tolerant halophytic shrub with a high grazing capacity (Steynberg & De Kock, 1987). Despite the presence and cultivation of this plant in the Karoo areas since the end of the last century (Alston, 1891), very little critical work on the defoliation or grazing management of the plant has been undertaken. Aucamp and Cloete (1970) mentioned, however, that oldman saltbush plants can be completely defoliated once a year without any injury to the vigour of the plants. Aucamp and Cloete (1970) found that, when utilized by sheep for four months of the year, over a six year period the grazing capacity of rainfed oldman saltbush plantations varied from 14 to 18 sheep per ha per annum for this period.

The research reported in this paper was designed to obtain preliminary data on the effect of defoliation frequencies on plant survival and dry matter (DM) leaf production of oldman saltbush. The chloride content of the leaves of the defoliation treatments was also determined.

 

PROCEDURE

The experiment was conducted on four year old plants grown under dryland conditions on two different sites. At the Grootfontein College of Agriculture (31° 29' SL and 25° 01' EL) site the plants were spaced 2 m x 2 m on a Shorrocks (-) soil series and at the Carnarvon Research Station (30° 58' SL and 22° 00' EL) the spacing was 2 m x 3 m on a Sunvalley 31 soil series. The average soil depth at these two sites was 0,5 m and 2,0 m respectively.

Leaf defoliation treatments at intervals of 6, 12, 24 and 48 weeks were applied for a 48 week period. The treatments were arranged in a randomized block design with ten replications. The leaves of the saltbush plants of all treatments were removed by hand at the onset of the experiment which was on 24 September 1987 for the Grootfontein trial and 6 October 1987 for the Carnarvon site. The leaves from the subsequent defoliation frequency treatments were oven-dried at 80 DC for 48 hours and weighed. The leaves of all plants from the same treatment were placed together and a grabsample of the pooled herbage was used for chloride determination.

 

RESULTS

The results of the Grootfontein experiment with respect to the D M leaf yields and percentage plant mortality for the frequency defoliation treatments of oldman saltbush, as well as corresponding values of the chloride concentration in the leaves of the plant, are given in Table 1. The results of the Carnarvon experiment are presented in Table 2. Rainfall figures for the period between defoliations are also submitted in these tables.

 

Defoliation-associated mortality of saltbush plants

At both the Grootfontein and Carnarvon experimental sites, complete leaf defoliation at 6-weekly intervals resulted in a 100 % mortality rate of all the bushes after the fourth consecutive defoliation treatment (Tables 1 and 2).

The 12-weekly defoliation treatment led to a 100 and 80 % mortality of plants at the Carnarvon and Grootfontein experimental sites, respectively. The 24 week's defoliation treatment lead to the mortality of 40 % of the plants at the Carnarvon experimental site (Tables 1 and 2), while the 48 week's defoliation treatment was the only treatment that led to no mortality of plants at both sites.

It is likely that the extremely dry conditions with below average rainfall experienced during the hot months of December and January could, in addition to other factors, also have had an effect on the mortality of plants. The total rainfall recorded at the Carnarvon site for the months of December 1987 and January 1988 was 0,3 mm and that for the Grootfontein site was 16,8 mm.

 

Dry matter leaf yields

The DM leaf production of the saltbush plants at the Grootfontein site (Table 1) increased steadily from a mean yield of 200,0 g/plant for the 6 week treatment to 684,6 g/plant for the 48 week treatment.

Statistically the yields of the 24 and 48 week defoliations at the Grootfontein Research Station were significantly (p = 0,05) greater than those of the 6 and 12 week treatments (Table 1). The DM leaf production of the saltbush plants at the Carnarvon Research Station revealed no significant differences in the yields of the 12, 24 and 48 week defoliation treatments. The yields of the latter treatments were however significantly (p = 0,01) greater than the yields of the 6 week treatment (Table 2). The correlation between defoliation interval and leaf production was much stronger for the Grootfontein data (r = 0,848) than was the case with the Carnarvon data (r = 0,633).

The average yield of 653,8 g/plant for plants of the Carnarvon site was much higher than the mean yield of 487,8 g/plant recorded at the Grootfontein site, despite the lower rainfall figures for the Carnarvon site. The deeper soil of the Carnarvon experiment was probably the main reason for the higher yields recorded at this site.

 

Chloride analysis of leaves

The chloride percentages of the saltbush leaves of the Carnarvon plants (Tables 2) were much higher than those of the Grootfontein plants (Table 1). This phenomenon can most probably be explained by the drier climate and heavier soils of the Carnarvon site, resulting in a higher concentration of salts in the soil. Another feature of the chloride content of the leaves of this plant at both sites is the strong tendency for a decrease in the chloride content of the leaves during the month of March for the 6-weekly and 12-weekly defoliation treatments (Tables 1 and 2).

 

DISCUSSION

From the data presented in this paper it is apparent that 4 to 5 year old bushes of oldman saltbush are unlikely to persist after complete defoliations at intervals ranging from 6 weeks to 12 weeks. Forty per cent of the plants at the Carnarvon site also succumbed after defoliation intervals of as long as 24 weeks. Less frequent defoliations also led to much higher leaf productions. It is clear from these experiments that oldman saltbush of this age category is susceptible to frequent heavy defoliations and that, if it is to be kept alive and productive, it must be rested for relatively long periods (between 6 months and 12 months) after heavy grazings.

The vulnerability of natural stands of oldman saltbush to continuous overgrazing has also been experienced in Australia. Oldman saltbush was originally the dominant perennial species over much of the south-eastern Riverine Plain of New South Wales, but as a result of continuous grazing its presence is mainly confined to areas protected from stock-grazing (Moore, 1953).

The susceptibility of other saltbush species to heavy grazing has also been recorded. Wilson, Leigh & Mulham (1969) found, for example, that with 0,6 sheep per ha there was a significant decline in Atriplex vesicaria over a three year period, but at 1,2 sheep per ha the Atriplex bushes were almost eliminated from the rangeland. There is therefore reason to believe that similar results to those obtained from the defoliation treatments reported in this paper could be obtained from oldman saltbush plants of other age categories.

Another interesting finding of the preliminary studies presented in this paper was the strong tendency for a decline in the chloride content of the leaves of this plant during the March defoliations of the relatively severe 6-weekly and 12-weekly defoliation treatments. At this stage there is no definite explanation for this phenomenon. The fact that the chloride content of the leaves occurred during the peak rainy period and at a time when mild autumn temperatures prevail is worth noting. These conditions could have led to large increments of fresh rainwater in the soil, as well as reduced transpiration rates as a result of the lower temperatures. One would expect that these factors combined with a relatively young leaf growth could have given rise to a lower chloride content of the leaves of oldman saltbush plants.

The present importance of the potential of Oldman saltbush to the small stock farming areas, and particularly under intensive utilization practices, demands that further basic studies on the management of this plant be undertaken.  Morphological and anatomical investigations are necessary regarding the behaviour and response of stem and leaf buds to various defoliation (grazing) and cutting treatments. Physiological studies into the importance of leaf, stem and root reserves, as well as the possible role of auxins in the growth processes, may also be necessary. Further experiments are also required to explain the seasonal variability of chloride concentration in saltbush leaves and its effect on palatability and for increased intake by stock. The interaction between soil moisture, temperature and defoliation frequencies on leaf production and plant survival rate should also be studied.

 

ACKNOWLEDGEMENTS

The author is grateful to Messrs E. Sykes and W. F: lmmelman for their assistance in the field work.

 

REFERENCES

ALSTON, G., 1891. Australian saltbush, results of trials. Agr. J.r. Cape Colony, 4 : 18.

AUCAMP, J.D. & CLOETE, J.G., 1970. The utilisation of Oldman saltbush. Fmg SA. 46, May: 3,7

MOORE, C. W .E., 1953. The vegetation of the South-eastern Riverina. New South Wales. 1. The climax communities. Aust. J. Bot. 1: 485-547.

STEYNBERG, H. & DE KOCK, G.C., 1987. Aangeplante weidings in die veeproduksiestelsels van die Karoo en ariede gebiede. Karoo Agric 3: 4-13.

WILSON, A.D., LEIGH, J.H. & MULHAM, W.E., 1969. A study of Merino sheep grazing a bladder saltbush (Atriplex vesicaria) - cotton-bush community on the Riverine Plain. Aust. J. Agric Res. 20, 1123-36.

 

Published

Karoo Agric, Vol. 4, No 3, 1991