Ngere, Lauretta (2015-04). Estimation of Genetic Parameters and Assessment of Genetic Variation for Internal Parasite Resistance Traits in Ruminants. Doctoral Dissertation.
Thesis
Internal parasites are a major concern to the livestock industry leading to huge losses. Genetic enhancement of ruminants for resistance/tolerance to internal parasites may provide for a lasting solution to the problem of internal parasite infection in livestock. The objective of this study was to estimate heritability and permanent environmental variance for internal parasite resistance traits in sheep and to apply penalties on the records of treated animals, analyzing the effect of such penalties on the genetic parameters. Records from 1008 Dorper sheep in a private South African flock comprised 17,711 FAMACHA scores, 3,758 fecal egg counts (mostly Haemonchus contortus), and 4,209 hematocrit values that were collected from 1997 – 2000. Animal models were used to conduct single trait analyses. Data were analyzed in three sets: 1) untreated records only; 2) all records; no penalties; and 3) all records with treated records penalized. Heritability estimates of Fc (FAMACHA) ranged from 0.33 ± 0.03 to 0.37 ± 0.03; FEC (Fecal egg count) from 0.04 ± 0.02 to 0.05 ± 0.03 and hematocrit from 0.19 ± 0.04 to 0.20 ± 0.05. Permanent environmental variance as a proportion of phenotypic variance was 0.02 ± 0.02 to 0.03 ± 0.02 for Fc, 0.14 ± 0.04 to 0.18 ± 0.05 for Ht and 0.07 ± 0.02 to 0.08 ± 0.03 for FEC. The Inclusion of treated animal records in the analyses, with or without penalization did not change the estimates of heritability and permanent environmental variance as a proportion of phenotypic variance. The objective of the second study was to assess genetic variation in fecal egg count and the associations of fecal egg count with other traits in growing crossbred Nelore-Angus cattle. Records of 201 F2 and F3 ½ Nelore ½ Angus steers in feedlot conditions in a genomics resource population in Central Texas were collected in 2012 and 2013. Helminth egg counts were determined from fecal samples before treatment with an anthelmintic product. The association of fecal egg count with other traits was assessed by modeling each in distinct analyses as a linear covariate. Year explained substantial variation in fecal egg count (P = 0.001). No other investigated covariate (birth weight, weaning weight, weaning temperament score, live weight, temperature, and exit velocity) was important in the different models (P > 0.2). Subsequently, sire (n= 13) was evaluated as a fixed effect (sires with less than 3 steers with records were excluded). Two sire families had significantly lower (P < 0.05) fecal egg counts (1.31 ± 0.28 and 1.57 ± 0.10) than the three sire families with the highest fecal egg counts (1.87 ± 0.10 - 2.06 ± 0.20). These results suggest the presence of additive genetic variation for fecal egg count, implying that selection can be carried out for the ability to suppress parasite worms in cattle. The objective of the second study was to assess genetic variation in fecal egg count and the associations of fecal egg count with other traits in growing crossbred Nelore-Angus cattle. Records of 201 F2 and F3 ½ Nelore ½ Angus steers in feedlot conditions in a genomics resource population in Central Texas were collected in 2012 and 2013. Helminth egg counts were determined from fecal samples before treatment with an anthelmintic product. The association of fecal egg count with other traits was assessed by modeling each in distinct analyses as a linear covariate. Year explained substantial variation in fecal egg count (P = 0.001). No other investigated covariate (birth weight, weaning weight, weaning temperament score, live weight, temperature, and exit velocity) was important in the different models (P > 0.2). Subsequently, sire (n = 13) was evaluated as a fixed effect (sires with less than 3 steers with records were excluded). Two sire families had significantly lower (P < 0.05) fecal egg counts (1.31 ± 0.28 and 1.57 ± 0.10) than the three sire families with the highest fecal egg counts (1.87 ± 0.10 - 2.06 ± 0.20).
