2.46 b


F, Female; S, steer; N, normal; D, drought; s, supplement feed;abc, differing letters in a row are significantly different (P< 0.05); Cont., control; DOR, doramectin; IVE, ivermectin; i, implant.

Programme use of MLs is an uncommon practice in North America unless one calls two treatments per year, one in spring and one in autumn, a programme. However, use of programmes for stockers in the southeastern USA has been investigated. Williams et al. (1990) compared three or eight treatments of ivermectin over an 11-month period with untreated controls and a positive control. There was no difference in gain from November to March among the three groups. From March to October, there was a significant increase in gain of 180 lb for the eight treatments of ivermectin programme versus untreated controls. Also, Williams et al. (1997) showed a significant increase in productivity of 0.68 and 0.84 lb day-1 versus untreated controls with two doses of ivermectin or doramectin, respectively, to grazing stockers at an 84-day interval over a 161-day grazing period.

Sustained-release bolus of ivermectin is another form of programme treatment. Sixteen 6-month-old stocker calves (407 lb) were grazed for 135 days on eight separate Arkansas pastures (two per pasture) along with one tracer per pasture. At the end of the winter/spring grazing, the principal treated calves gained 110 lb more than their control principal counterparts (Yazwinski et al., 1995).

Most of the 35 milllion acres (14 million ha) of tall fescue in the USA are infested with an endophytic fungus that is associated with a toxic syndrome including poor productivity. Bransby (1999) demonstrated that treatment of cattle with ivermectin at turnout and 56 days later grazed on highly infected endophyte pastures enhanced productivity by 0.66 lb day-1 over 140 days compared with untreated cattle, whereas ivermectin treatment on low endophyte infected pastures enhanced productivity by 0.22 lb day-1 (Alabama). This impact may be related to the short-term reduction observed by pre-treatment of rats with ivermectin in the thermoregulatory response to ergovaline, the primary toxin in endophyte-infested tall fescue (Spiers et al., 1997).

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