Genetic factors affecting milk production, growth and reproduction traits in Bos indicus x Bos taurus crosses in Ethiopia

[摘要] English: Five separate studies were conducted to investigate the genetic factors affectinggrowth, milk production and reproduction traits in Bos indicus x Bos tauruscrosses in Ethiopia. The first three studies were based on early growth datacollected from the purebred Boran (Bo), Barca (Ba), Horror (Ho) (Bos indicus)and their crosses with the Friesian (F), Jersey (1) and Simmental (S) breeds (Bostaurus). Early growth data were for calves that were representing three purebredBos indicus and 38 crossbred genotypes and were separated from their dams oneday after birth and fed milk from a bucket. The fourth and fifth studies werebased on milk and reproduction data collected from cows of two purebreds(Boran and Friesian) and eight crossbred (crosses of Friesian and Jersey withBoran) genotypes.In the first study, five genetic models were evaluated for goodness of fit andestimation of crossbreeding parameters. These models were: 1) Dominancemodel, 2) Dickerson's model (recombination loss), 3) Additive x dominanceinteraction model, 4) Dominance x dominance interaction model and 5)Kinghom's model (x). Models 2 to 5 are epistatic models that included all effectsin model one plus one type of epistatic interaction based on a two-locus genemodel. The models were evaluated using data for birth, weaning, yearlingweights and preweaning average daily gain. All five models tested provided highlevels of fit, with adjusted R2 values averaging 93% over traits. All the epistaticmodels fit the data significantly (P<0.05) better than the dominance model for allthe traits. Among the epistatic models, Dickerson's model (Model 2) gavesignificantly (P<0.05) higher R2 values compared to the other epistatic models.Crossbreeding parameters estimated from this model has relatively lowersampling correlations and correspondingly lower standard errors. This modelcould, therefore, be considered as the most appropriate one for parameterestimation and prediction of performances of untested genotypes for futurecrossbreeding decisions for the breeds involved in this study.In the second study, breed difference, heterosis and recombination loss wereestimated for birth weight (BWT), weaning weight (WWT), preweaning averagedaily gain (ADG) and yearling weight (YWT), fitting an animal model.Differences between genotypes were significant (P<0.01) for all traits. Geneticgroup means adjusted for environmental effects ranged from 20 to 30 kg forBWT, 88 to 114kg for WWT, 122 to 157kg for YWT and 358 to 492g for preweaningaverage daily gain (ADG). The breed additive effect of the F as adeviation from the Bo was significant (P<0.01) and positive for all traits.Relative to the Bo mean, the additional breed additive contribution of the F breedon BWT, WWT, ADG and YWT was 39.3, 16.5, 9.3 and 10.3%, respectively.On the other hand, the breed additive effects of the Ho and J breeds weresignificantly (P<0.01) negative for all traits. The heterotic effects weresignificantly negative (P<0.01) for BWT for all F and S crosses, but positive(P<0.01) for all other traits for all types of crosses. The average heterosisestimated within the F, J and S breeds were: -2.1±0.6, 0.2±0.6 (P>0.05) and-2.3±0.6 kg for BWT, 8.8±2.1, 11.8±2.3 and 13.7±2.4 kg for WWT, 60.4±11.3,64.8±12.3 and 90.6±12.7 g for ADG and 19.8±2.6, 19.5±2.8 and 20.8±2.9kg forYWT, respectively. The recombination effects were significant (P<0.01) for themajority of crosses for all traits. The estimates for the recombination loss werenegative for all traits, except for BWT. The average recombination effectsestimated within the F, J and S breeds were: 2.6±0.8, 2.9±0.9 and 2.4±1.0 kg forBWT, -13.6±3.2, -4.2±3.4 (P>0.05) and -16.0±4.0 kg for WWT, -88.0±17.1,-39.4±18.4 and -102.2±21.3 g for ADG and -14.4±4.0, -0.1±4.4 (P>0.05) and-17.5±4.9 kg for YWT, respectively.In the third study, variance components and direct and maternal heritabilitieswere estimated for weight at birth, weaning and yearling and preweaning averagedaily gain. Data were analysed using six alternative animal models (direct andincluding or excluding maternal effects). The direct heritability estimates fromthe best model for each trait were: 0.14±0.03 for birth weight, 0.08±0.03 forweaning weight, 0.06±0.02 for preweaning average daily gain and 0.13±0.03 foryearling weight. The direct maternal heritability estimates were small, butsignificantly different from zero for only birth weight (0.07±0.02), weaningweight (0.04±0.02) and preweaning average daily gain (0.04±0.02). Directgenetic correlations between birth weight and the other three traits were:0.66±0.08, 0.55±0.19 and 0.50±0.12 with weaning weight, preweaning averagedaily gain and yearling weight, respectively. The genetic correlation betweenweaning weight, preweaning average daily gain and yearling weight was highand ranged from 0.82±0.11 to 0.97±0.O1. Small, but non-zero maternalheritabilities estimated for weaning and preweaning average daily gain forartificially reared calves in this study should be interpreted cautiously because ofpotential bias from unaccounted breed additive and non-additive effects of thedam. Results of this study also showed that estimates of variance componentsand genetic parameters suitable for general use can be obtained from mixedpurebred and crossbred data after appropriately accounting for breed additive andnon-additive effects.In the fourth study, breed additive and non-additive effects plus heritabilities andrepeatabilities for milk yield per lactation (LMY), milk yield per day (DMY),lactation length (LL), annual milk yield (AMY), annual milk yield per metabolicbody weight (AMYBW) and cow weight at calving (BW) were estimated. Inaddition, genetic, phenotypic and permanent environmental correlations wereestimated between AMY and LL, AMY and BW and LL and BW. Data for eachtrait were analysed, using two equivalent repeatability animal models: first,fitting genotype as a fixed group effect and in the second model substitutinggenotype with breed additive, heterotic and recombination effects as fixedcovariates. Among the genotypes the Bo had the lowest and the F the highestperformance for all traits. The least-squares means for the Bo breed were 529±65kg for LMY, 2.8±0.1 kg for DMY, 193±6 d for LL, 514±61 kg for AMY,7.8±0.7 for AMYBW and 304±3 kg for BW. Both F and J breed additive effects,measured as a deviation from the Bo breed were significant (P<0.01) for alltraits, except for BW of the J. The F and J additive contributions were 2774±89and 1473±362 kg for LMY, 7.1±0.2 and 4.8±0.8 kg for DMY, 146±8 and 81±7 dfor LL, 2345±71 and 1238±319 kg for AMY, 20.6±0.9 and 18.9±4.3 kg forAMYBWand 140±4 and -21±22 kg (P>0.5) for BW, respectively. The heteroticcontributions to the crossbred performance were also positive and significant(P<0.01) for all traits, except for BW in the F x Bo crosses. The Fl heterosisexpressed as a deviation from the mid-parent values were 22 and 66 % for LMY,11 and 20% for DMY, 29 and 29% for LL, 21 and 64 % for AMY, 42% (P>0.05)and 42 % for AMYBW and 2% (P>0.5) and 11% for BW for the F x Bo and J xBo crosses, respectively. The recombination effect estimated for the F x Bocrosses was negative and significant for LMY (-526±192 kg, P<0.01), DMY (-3.0±0.4 kg, P<0.01)AMY (-349±174, P<0.05) and BW (-68±11 kg, P<0.001).For the J x Bo crosses the recombination loss was only significant and negativefor DMY (-2.2±0.7 kg, P<0.05) and BW (-33±17, P<0.05). The directheritabilities (h2) and repeatabilities (r2) estimated for each trait after correctingfor the fixed environmental and breed additive and non-additive effects were0.24±0.04 and 0.39±0.02 for LMY, 0.19±0.03 and 0.30±0.02 for DMY,0.13±0.03 and 0.19±0.02 for LL, 0.23±0.04 and 0.37±0.02 for AMY, 0.17±0.05and 0.39±0.02 for AMYBW and 0.10±0.03 and 0.34±0.02 for BW, respectively.The estimated genetic correlations between AMY and LL, AMY and BW, LLand BW were 0.71±0.08, 0.17±0.18 and 0.23±0.20, respectively.In the fifth study, estimates of breed additive differences, heterosis andrecombination loss, as well as heritabilities were obtained for age at first calving(AFC), calving interval (Cl), days open (DO) and number of services perconception (SPC). The genetic parameters were estimated using a repeatabilityanimal model for Cl, DO and SPC and a unitrait animal model for AFC. Theoverall least-squares means estimated were: 38.3±0.26 months, 435±4 days,145±10 days and 1.58±0.03 (number) for AFC, Cl, DO and SPC, respectively.The breed additive effects of F and J were only significant (P<0.01) for AFC.Relative to the Bo, both the F and the J additive contributions for AFC were-5.4±0.5 and -5.5±1.9 months, respectively. Crossing the F and J breeds with theBo breed also resulted in significant heterosis (P<0.05) ranging from I0ta 21%in all traits. The estimated recombination loss was only significant for AFC(2.8±1.0 months) for the F x Bo crosses. Heritability estimates were high forAFC (0.44±0.05) and low for Cl (0.08±0.03), DO (0.04±0.03) and SPC(0.08±0.02). The corresponding estimates for the repeatability (r2) were0.14±0.02 and 0.14±.0.02 for Cl and DO, respectively. The repeatability estimatefor SPC was zero.

[发布机构] University of the Free State