Resistance to naturally acquired gastro-intestinal (GI) nematode parasite infections (predominantly Haemonchus contortus) was studied in 166 Red Maasai, 230 Dorper and 294 crossbred (Red Maasai Х Dorper) ewes in the sub-humid coastal region of Kenya. Live weights (LWT), blood packed-cell volume (PCV) and faecal egg counts (PEC) were recorded at mating, 3 months post mating, 1 week before lambing and 1, 2 and 3 months post lambing for four separate lambings that took place between 1993 and 1996. The Red Maasai ewes were more resistant to GI nematode infections than Dorper ewes as shown by their significantly lower FEC and significantly higher PCV at most of the sampling times over the reproductive cycle. The breed difference for FEC was significant in the lactating ewes but not in the non-lactating ewes. At most sampling times, the crossbred ewes were as susceptible as the Dorper ewes in terms of both PCV and FEC, particularly at the 1 and 2 month post-lambing samplings. Resistance was also manifested by a lower proportion of ewes having to be treated with an anthelmintic and a lower mortality rate in the Red Maasai than the Dorper. The Red Maasai ewes were significantly lighter by about 1 to 2 kg than the Dorper ewes at all sampling times. There was a significant increase in FEC and decrease in PCV over the first 2 months of lactation in lactating ewes compared with non-lactating ewes. This peri-parturient increase in FEC occurred in both breeds and the crossbreds but was more marked in the susceptible Dorper ewes.
A study was carried out from 1992 to 1996 to compare the resistance to naturally acquired gastro-intestinal (GI) nematode parasite infections (predominantly Haemonchus contortus) of Galla and Small East African goats in the sub-humid coastal region of Kenya. A total of 204 Galla and 349 Small East African (SEA) kids were born from five kiddings. These were the progeny of 18 Galla and 17 SEA bucks. Live weights (LWT), blood packed-cell volume (PCV) and faecal egg count (FEC) were recorded at 1- to 2-month intervals from birth to about 14 months of age. The SEA kids were more resistant to GI nematode parasites than Galla kids as shown by their significantly lower FEC (P< 0·001) in the post-weaning period (8- to 14-month-old kids) and lower mortality from birth to 14 months of age (P< 0·05). There was no significant (P> 0·05) breed effect on PCV, but Galla kids were significantly heavier (P< 0·001) at all measurement times between birth and 14 months of age. Heritability estimates for LWT, PCV and FEC at the different sampling times were characterized by high standard errors. Heritability estimates for records taken at 4·5 and 8 months of age from a repeated measures analysis were 0·18 (s.e.0·08) for PCV and 0·13 (s.e.0·07) for logarithm-transformed FEC. The phenotypic and genetic correlation estimates between PCV and LFEC were moderately to highly negative and averaged –0·36 and –0·53, respectively. The results are discussed in relation to the limited evidence for resistance to GI nematode infections in goats and compared with the much stronger evidence for resistance in sheep.
Resistance and resilience to naturally acquired gastro-intestinal (GI) nematode parasite infections (predominantly Haemonchus contortus) were studied in 1785 lambs born over six lambings (1991 to 1996) consisting of 212 Red Maasai, 311 Dorper and 1262 crossbred (Red Maasai-Dorper) lambs in the sub-humid coastal region of Kenya. These lambs were the progeny of 41 Dorper and 35 Red Maasai rams. Live weights (LWT), blood packed cell volume (PCV) and faecal egg counts (FEC) were recorded at 1- to 2-monthly intervals from birth until the lambs were about one year of age. Red Maasai were more resistant and resilient post weaning to infections with GI nematodes than Dorper lambs as shown by their significantly lower FEC and their significantly higher PCV, respectively. An increasing proportion of Red Maasai genes in the crossbred lambs was associated with decreased FEC and higher PCV, but there was no heterosis for logarithm-transformed FEC (LFEC) or PCV. From one month of age Red Maasai lambs were significantly lighter than Dorper lambs by about 1 kg, but Red Maasai lambs had significantly lower lamb mortality rate from birth to 12 months of age (proportionately 0·30 and 0·66, respectively). Heritability estimates from a repeated measures analysis for records taken at 6 and 8 months of age were 0·14 (s.e. 0·05) for PCV from an animal model and 0·12 (s.e. 0·05) for LFEC from a sire model. The heritability estimate for LFEC from a repeated measures analysis including the four measurements recorded between 6 and 12 months of age was significantly higher (P < 0·05) for Dorper-sired lambs (0·15, s.e. 0·05 for an animal model and 0·19, s.e. 0·07 for a sire model) than for Red Maasai-sired lambs (0·00 and 0·01, s.e. 0·02). The phenotypic and genetic correlations between PCV and LFEC were moderately to highly negative and averaged –0·34 and –0·81, respectively. None of the genetic correlation estimates between LWT and PCV and LWT and LFEC for lambs post weaning were significantly different from zero. The heritability estimates for PCV and LFEC have important implications for within-breed genetic improvement programmes: for the Red Maasai, improvement should concentrate on resilience (e.g. selection for high PCV); for the Dorper, selection should be feasible for both improved resistance (low FEC) and resilience (high PCV).
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