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Technological developments and worldwide trade of frozen semen in dairy cattle have increased since 1970, making bulls with daughters calving in different production environments more common. At the same time, this has resulted in concerns about the predictive ability of genetic evaluations of sires obtained in different countries (
Genotype × environment interaction (G×E) studies may help in evaluating to what extent the predicted superiority of animals obtained under certain environmental conditions will be expressed in different environments (
In this respect, studies estimating genetic correlations between Canada, the US and Western European countries for milk yield traits have shown little variation in the obtained estimates with an average estimated correlation of 0.92 (
Genetic correlations between 14 and 17 countries used in international genetic evaluations of female fertility traits for Holsteins varied from 0.51 to 0.96, with an average of 0.80 for conception traits (conception rate, non-return to estrus rate) compared to 0.60 to 0.97, with an average of 0.84 for reproductive interval traits (calving interval, days open) (
Previous studies on G×E in Mexican Holstein cattle involved the estimation of genetic correlations between countries for milk yield only (
The objective of this study was to estimate genetic correlations for AFC, CI, milk, fat yield, protein yield, fat content and protein content between Mexico and Canada and Mexico and the US in Holstein cattle. An additional objective was to estimate genetic parameters for the aforementioned traits within the Mexican Holstein population.
Records for milk, fat and protein yields, fat and protein contents, CI, and AFC for Mexican Holstein cattle recorded from 1997 to 2008 were obtained from the Mexican Holstein Association (Querétaro, Mexico). Data for milk, fat, and protein yields were adjusted to 305 days mature equivalent by the Mexican Holstein Association (
In order to estimate genetic parameters, data were analyzed using single-trait repeatability models, which included calving season by parity number, and herd-year-season of calving as fixed effects, and animal, sire by herd, and permanent environment as random effects. In order to analyze AFC, an animal model that included herd-year-birth season as a fixed effect, and the random effects of animal, and sire by herd, was used. Analyses were performed using AI-REML procedure using ASReml software (
The model expressed in matrix notation was as follows:
in the case of AFC it was:
where
Expectations (
whereσ2a, σ2p, σ2s, σ2e are scalars,
Genetic evaluations for the US were obtained directly from the USDA AIPL web site (
For AFC and CI, the Canadian genetic evaluations were obtained directly from the Canadian Dairy Network (Guelph, ON, Canada), which included the genetic evaluations of 4,658 sires, of which 747 and 682 also had Mexican genetic evaluations with a reliability = 0.20 for AFC and CI, respectively. The Canadian genetic evaluations for yield and milk composite traits were obtained from the Canadian Dairy Network web site (
Genetic correlations between countries (r
g) were obtained from the observed correlations between the predicted transmitting ability (PTA) of each bull in two countries divided by the square root of the product of their average reliabilities according to the following
where r g = estimated genetic correlation, r o = estimated correlation among PTAs, arel i = average reliability of PTAs on country i, arel j = average reliability of PTAs on country j.
In order to account for a possible underestimation of the genetic correlation due to selection of the sires from the country of origin, estimates of genetic correlations between PTAs in Mexico-Canada and Mexico-US obtained by [1] were compared to genetic correlations obtained from simulated PTAs in two environments assuming no G×E interaction effects and using the same number of sires and PTA reliability from the real populations (see
Descriptive statistics for the Mexican population for the studied traits are shown in
Genetic correlations between Mexican CI and US DPR (
Genetic correlations between Canada and the US for milk (0.97 ± 0.03), fat (0.95 ± 0.03), and protein (0.96 ± 0.03) yields (
For fat and protein contents (
Average AFC for Mexican heifers was 771 days (SD = 90), similar to the 778 days (SD = 89) found in US Holsteins (
Means for milk yield, fat yield, protein yield, and fat content were lower than the corresponding means for registered US Holsteins for the same period (
Heritability for AFC (0.06 ± 0.01) was close to the estimate for US (0.03) (
Heritability for CI (0.03 ± 0.01) confirmed the low (but above zero) value previously estimated for this population (
Heritability estimates for milk, fat, and protein yields (0.18 ± 0.01, 0.20 ± 0.01, and 0.19 ± 0.01, respectively) (
In a study using three US regions, heritability for fat content was estimated varying from 0.32 to 0.41 (
Fertility traits. Although relatively low PTA reliabilities were used to estimate genetic correlations for AFC and CI, estimated genetic correlations may not be seriously underestimated, since there is no strong selection or correlated responses for these traits (
There are few estimates of the genetic correlation for AFC between countries. This correlation between Colombia and Brazil was estimated as 0.78
We can assume that the larger G×E effects for AFC between Mexico and Canada may be related to differences in heifer rearing systems or climatic factors between the two countries, even if averages for AFC were approximately similar in the two populations. Climatic and management differences may explain also the moderate G×E effects found between CI in Mexico and Canada and with daughter pregnancy rate in the US.
The genetic correlation between DPR in the US and CI in Canada estimated in our study (= 0.93), was similar to that obtained between DPR in the US and days open (multiplied by -1) in Spain (0.94) but greater than the genetic correlation between DPR in the US and first service to conception (multiplied by -1) in Canada (0.72) (
The range of estimates of genetic correlations of reproductive interval traits (CI, days open) between 14 and 17 countries used in international genetic evaluations for Holsteins (
Milk yield traits. Genetic correlations were estimated as 0.88, 0.87, and 0.87 for milk, fat and protein yields, respectively, as an average for the 27 countries members of Interbull. The average correlation for the 7 core populations with information for all traits, excluding clinical mastitis and stillbirth, was 0.92 for milk, fat, and protein yields, with a range of ± 0.07-0.08 (
Comparison with simulated values under the hypothesis of no G×E, indicates that G×E effects between Mexico-Canada and Mexico-US are probably small for these traits. Lack of G×E for milk yield traits may be related to the high production levels, which have increased over time (
Genetic correlation for milk traits between Canada or the US with the remaining portion of the dairy population of Mexico (grade Holsteins, crossbreeds) or for lower input production systems may be different and is a topic for future studies.
In conclusion, strong G×E interaction effects between Mexico and Canada were found for AFC, and moderate G×E interaction effects were found between Mexico and Canada for CI, and between CI in Mexico with DPR in the US. No G×E interaction effects between Mexico with Canada and the US for milk yield traits were found. There were small but significant G×E interaction effects for fat and protein contents. These findings need to be taken into account when selecting sires evaluated in Canada and the US to be used in Mexican Holstein herds.
HHM, HCJ and FJRL are members of the National Research System (SNI) of Mexico. Authors are thankful to Kristine Ibsen for proofreading the manuscript.