Estimation of Genetic Parameters and Assessment of Genetic Variation for Internal Parasite Resistance Traits in Ruminants

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). 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

Genetic parameters for ewe reproductive performance and peri-parturient fecal egg counts and their genetic relationships with lamb body weights and fecal egg counts in Katahdin sheep

© The Author(s) 2018. This study estimated genetic parameters for ewe reproductive traits [number of lambs born (NLB) and weaned (NLW) per ewe lambing] and fecal egg counts (FEC) during the peri-parturient rise (PPR) for use in genetic evaluation of Katahdin sheep. Data included NLB and NLW for 23,060 lambings by 9,295 Katahdin ewes, 1,230 PPR at lambing (PPR0) for 750 ewes, 1,070 PPR at approximately 30 d postpartum (PPR30) for 611 ewes, BW at birth, weaning, and (or) post-weaning for 12,869 lambs, and FEC at weaning and (or) post-weaning for 4,676 lambs. Direct additive, permanent environmental, and residual (co)variances were estimated in univariate and bivariate animal models. Fixed effects included effects of ewe management group and ewe age for all traits, and, for PPR, a continuous effect of days between lambing and measurement. Effects of litter size on PPR0 and number of lambs suckled on PPR30 were included in univariate models but excluded from bivariate models for PPR and NLB or NLW. Heritability estimates in univariate models for NLB, NLW, PPR0, and PPR30 were 0.09 ± 0.01, 0.06 ± 0.01, 0.35 ± 0.06, and 0.24 ± 0.07, respectively. Estimates of permanent environmental variance as a proportion of total phenotypic variance were 0.02 ± 0.01 for NLB, 0.03 ± 0.01 for NLW, 0.05 ± 0.06 for PPR0, and 0.13 ± 0.07 for PPR30. Direct additive, phenotypic, permanent environmental, and residual correlations between NLB and NLW were 0.88 ± 0.03, 0.74 ± 0.004, 0.54 ± 0.15, 0.74 ± 0.003, respectively; corresponding correlations between PPR0 and PPR30 were 0.96 ± 0.07, 0.46 ± 0.03, 0.98 ± 0.50, 0.18 ± 0.05, respectively. The additive genetic correlation (rd) between ewe reproductive traits and PPR ranged from 0.12 to 0.18. Estimates of rd between lamb BW and subsequent ewe NLB and NLW ranged from 0.07 to 0.20, and those between PPR and lamb BW ranged from −0.03 to 0.29. The rd between ewe reproductive traits and lamb FEC ranged from 0.27 to 0.40, and those between PPR and lamb FEC ranged from 0.56 to 0.77. Correlations between maternal additive effects on BW and direct additive effects on PPR were low (−0.08 to 0.10), and those between maternal additive effects on BW and direct additive effects on ewe reproductive traits were variable (−0.36 to 0.11). We conclude that FEC in growing lambs and peri-parturient ewes are controlled by similar genes and that modest, but manageable, genetic antagonisms may exist between FEC and ewe productivity

Genetic parameters for ewe reproductive performance and peri-parturient fecal egg counts and their genetic relationships with lamb body weights and fecal egg counts in Katahdin sheep

© The Author(s) 2018. This study estimated genetic parameters for ewe reproductive traits [number of lambs born (NLB) and weaned (NLW) per ewe lambing] and fecal egg counts (FEC) during the peri-parturient rise (PPR) for use in genetic evaluation of Katahdin sheep. Data included NLB and NLW for 23,060 lambings by 9,295 Katahdin ewes, 1,230 PPR at lambing (PPR0) for 750 ewes, 1,070 PPR at approximately 30 d postpartum (PPR30) for 611 ewes, BW at birth, weaning, and (or) post-weaning for 12,869 lambs, and FEC at weaning and (or) post-weaning for 4,676 lambs. Direct additive, permanent environmental, and residual (co)variances were estimated in univariate and bivariate animal models. Fixed effects included effects of ewe management group and ewe age for all traits, and, for PPR, a continuous effect of days between lambing and measurement. Effects of litter size on PPR0 and number of lambs suckled on PPR30 were included in univariate models but excluded from bivariate models for PPR and NLB or NLW. Heritability estimates in univariate models for NLB, NLW, PPR0, and PPR30 were 0.09 ± 0.01, 0.06 ± 0.01, 0.35 ± 0.06, and 0.24 ± 0.07, respectively. Estimates of permanent environmental variance as a proportion of total phenotypic variance were 0.02 ± 0.01 for NLB, 0.03 ± 0.01 for NLW, 0.05 ± 0.06 for PPR0, and 0.13 ± 0.07 for PPR30. Direct additive, phenotypic, permanent environmental, and residual correlations between NLB and NLW were 0.88 ± 0.03, 0.74 ± 0.004, 0.54 ± 0.15, 0.74 ± 0.003, respectively; corresponding correlations between PPR0 and PPR30 were 0.96 ± 0.07, 0.46 ± 0.03, 0.98 ± 0.50, 0.18 ± 0.05, respectively. The additive genetic correlation (rd) between ewe reproductive traits and PPR ranged from 0.12 to 0.18. Estimates of rd between lamb BW and subsequent ewe NLB and NLW ranged from 0.07 to 0.20, and those between PPR and lamb BW ranged from −0.03 to 0.29. The rd between ewe reproductive traits and lamb FEC ranged from 0.27 to 0.40, and those between PPR and lamb FEC ranged from 0.56 to 0.77. Correlations between maternal additive effects on BW and direct additive effects on PPR were low (−0.08 to 0.10), and those between maternal additive effects on BW and direct additive effects on ewe reproductive traits were variable (−0.36 to 0.11). We conclude that FEC in growing lambs and peri-parturient ewes are controlled by similar genes and that modest, but manageable, genetic antagonisms may exist between FEC and ewe productivity