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Spring Means Peak Parasite Loads on Pasture

Dairy Pipeline: April 1998

by Tom L. Bailey
Dairy Production Medicine
Virginia-Maryland Regional College of Vet Medicine
Virginia Tech

The arrival of spring signals green grass, warm days, moisture, and the emergence of peak parasite (worms) loads on pasture. Regular deworming and treatment programs for internal and external parasites are essential to improve heifer performance. At the Virginia Tech dairy facility, we use a strategic deworming protocol. This protocol not only deworms the heifers, but decreases or minimizes the parasite (worm) egg population on the pasture. All heifers are dewormed first in early May, using a product with residual activity. Residual activity means it has killing activity for 2 to 3 weeks or longer (See table below). You can obtain advice from your local veterinarian for the product to use. This first rids the heifers of the initial worm burden obtained during the early warm days of spring when parasite loads are beginning to peak in the heifers and on the pastures. A second deworming is administered 3 weeks plus the residual activity. The reason for this second injection is two-fold. First, the product has internal parasite killing activity or "residual activity" for an extended period. Therefore, any parasites developing in the intestines during this time will be eliminated. After this residual time period any parasite eggs ingested through grass or pasture contamination will develop into adults. This development period is approximately 3 weeks (egg to adult parasite). The second injection, will eliminate or kill these adults. These two injections give adequate protection during the peak period of parasite contamination. This strategic deworming program not only eliminates parasites in the heifers, but also decreases parasite load on pasture. Typically by early July pasture parasite loads will begin to decrease due to hot weather and decreased moisture. If good parasite management has been implemented, parasite burdens on the pasture will be negligible. Producers should be cautioned against introducing heifers which have not been dewormed, as they can be a source of parasite contamination for the pasture and the original dewormed heifers. A third deworming of a single dose of product may be beneficial in late October. This would eliminate parasites in the intestines and external parasites (grubs and lice) obtained during the summer and decrease the parasite load prior to winter feeding. Also, if the summer season has been mild and moisture has been high, parasites will survive in higher numbers on the pasture, causing a higher heifer parasite load going into the winter feeding season .

The following products have label claims of residual effect:
ProductTrade NameRouteImportant Virginia WormsGet Inhibited WormsResidual Activity
IvermectinIvomec CattleInjectableYesYes2 Weeks
IvermectinIvomec Pour-onPour-onYesYes3 Weeks
IvermectinSustain Release BolusOral BolusYesYes135 Days
DoramectinDoramectin Pour-onPour-onYesYes4 Weeks
DoramectinDectomaxInjectableYesYes3 Weeks
EprinomectinEprinexPour-onYesYes? Weeks
MoxidectinCydectinPour-onYesYes4 Weeks

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