Serum immunoglobulin concentrations in young dairy calves and their relationships with weight gain, onset of puberty and pelvic area at 15 months of age

Serum immunoglobulin (Ig) concentrations in 100 young dairy calves from five herds were determined to establish whether they were related to subsequent weight gain, age at puberty and pelvic area at 15 months of age. Blood samples were collected from each calf at 6-48 h after birth and at 2 weeks of age for Ig concentrations, and monthly from 210 days of age onward for progesterone assay to indicate age at puberty. Pelvic area measurements were carried out at 15 months of age. Weight gains were expressed as percentage weight gain from birth to the age being considered. Mean serum Ig concentrations at 6-48 h of age varied between herds and decreased (P < 0.01) by 2 weeks in all herds, with one exception. The mean Ig concentrations in pooled data from the two Jersey herds were different (P < 0.001) at 6-48 h and 2 weeks of age from those in the two Friesian herds, but not between herds within these breeds. The Jersey breed had the highest serum Ig concentrations, while the Friesian breed had the lowest. Significant positive correlations were found between Ig concentrations at 6-48 h after birth and at 2 weeks of age in the Jersey breed (P < 0.05) and in the Illawarra herd (P < 0.01). Although there were positive correlations (P < 0.05) between serum Ig concentrations at 6-48 h and pelvic area at 15 months of age in one Jersey herd and between Ig concentrations (at 2 weeks of age) and pelvic area at 15 months in the Illawarra herd, there were no other correlations found between serum Ig concentrations at these ages and subsequent weight gain, age at puberty and pelvic area at 15 months. In conclusion, serum Ig concentrations during the first 2 weeks of calves' lives were generally not related to parameters of calf growth and reproduction

Rotation of treatments between spinosad and amitraz for the control of Rhipicephalus (Boophilus) microplus populations with amitraz resistance

A farmlet study was conducted over 4 years in which three treatments were applied to six groups of Holstein dairy calves. Calves in each group were infested with equal numbers of N-strain (susceptible) and Ultimo strain (amitraz and synthetic pyrethroid resistant) tick larvae to establish self-sustaining populations with an initial, measurable level of resistance to amitraz. Standard counts of all ticks between 4.5 and 8.0 mm diameter on one side of each animal were made each week and treatment was applied when tick numbers exceeded a threshold of 25 engorged adults per side. The three treatments were: 1, spinosad spray whenever tick numbers exceeded the threshold; 2, amitraz spray whenever tick numbers exceeded the threshold; 3, spinosad whenever tick numbers exceeded the threshold for the first 2 months, then amitraz for 2 months, with alternation every subsequent 2 months. Engorged adult female ticks were collected from each treatment group on 10 or 11 occasions during the study and tested using the larval packet test bioassay (LPT) for acaricide resistance. Spinosad 250 ppm provided effective control of amitraz-resistant tick populations in the field, using a similar number of treatments as in the amitraz and rotation groups. The initial infestations of all of the groups resulted in the establishment of populations with in vitro evidence of resistance to amitraz using the LPT. Treatment with spinosad or with a rotation between spinosad and amitraz every 2 months resulted in reduced levels of resistance to amitraz according to the LPT. The animals treated with amitraz alone showed increasing resistance to amitraz according to the LPT each summer and autumn with a return to full or almost full susceptibility to amitraz in early spring in all years. This pattern suggests a relative lack of fitness of amitraz-resistant ticks that might be exploited by using an acaricide rotation strategy