ARCHIVE FILE
This article was published in 1994
See the original document
Internal Parasites of Sheep on Irrigated Pasture
DD Salmon, District Veterinarian, Deniliquin/Moulamein
INTRODUCTION
Irrigated pastures provide a means of dramatically increasing animal production in the semi arid areas of the Riverina. White clover dominant pastures irrigated intensively through the summer can achieve stocking rates in excess of 75 dry sheep equivalents (dse) during spring and summer and in excess of 30 dse during autumn and winter.
Under such intensive production, internal parasites become a major consideration.
With the establishment of the Drenchplan program, it became obvious that more information was required about the behaviour of internal parasites on irrigated pasture.
MATERIALS AND METHOD
An area of 64 hectares was established to Haifa white clover. It was divided into 4 paddocks of 16 ha each. This pasture was watered to demand, usually 50mm of water every 10 to 14 days during summer.
2300 lambs grazed the pasture from the beginning of November until April. The lambs were run in one mob and were moved between paddocks on a weekly basis so that each paddock was intensively grazed for one week in four.
The lambs were treated with an effective anthelmintic prior to being placed on the area, and were treated twice during the summer during December and January.
Three lambs were treated with an effective anthelmintic and placed with the mob, initially for two weeks, but later for four weeks. After the period of grazing they were place in a shed on wooden slats and fed hay for four weeks. Two of the lambs were then slaughtered and their abomasa and small intestines were submitted to the Regional Veterinary Laboratory at Wagga Wagga for total worm count.
The average larval pickup was calculated for each week that the tracer lambs were grazing.
During the autumn of the first year parasite burdens in lambs grazing the pasture became extreme. Because of this the pasture was locked up for haymaking and carried no stock between May and October.
RESULTS
The average weekly larval pickup is shown in Figure 1.
The stage of development of the Ostertagia circumcincta at the time of slaughter of the tracer lambs is shown in Figure 2.
DISCUSSION
The relatively low rate of larval pickup during the hot months indicate that short term pasture rotation as practiced (i.e. one week in four) did not result in high larval availability.
The much higher rate of larval pickup seen by the author and others (Downing pers. comm.) when sheep are set stocked on summer irrigation leads to the conclusion that under warm moist conditions, nematode larvae do hatch and become available to infect grazing animals.
The difference between set stocking and short term rotation can be explained in two ways.
Short term rotation involves crash grazing which leaves the pasture very short at the end of any grazing cycle. This very short pasture provides little by way of protection from the elements for nematode eggs and larvae.
Free living Trichostrongyle larvae do not feed (Anderson pers. comm.). If they are stimulated to activity by a combination of warmth and moisture they must live on stored energy, which has a limited availability.
The high rate of infection by Nematodirus larvae following a four month period of destocking indicates the very good rate of survival of this species in particular during cool weather.
The variable percentage of Ostertagia circumcincta larvae which had developed to adult worms after four to eight weeks of infection was interesting. This is a form of arrested development (Smeal pers. comm.). The function of this hypobiosis is unclear but it does partly explain the phenomenon observed by the author that faecal egg counts do not give as good an indication of the level of parasitism by O. circumcincta as they do for other species such as Trichostrongylus spp.
ACKNOWLEDGEMENT
I would like to acknowledge the assistance of FS Falkiner & Sons Pty Ltd who provided the test plot and sheep, the tracer sheep and funding for technical assistance at the Regional Veterinary laboratory.
Figure 1: Larval Pickup
Figure 2: Ostertagia Development
