| Livestock Research for Rural Development 12 (2) 2000 | Citation of this paper |
The present study was carried out to determine genetic, phenotypic and environmental parameters and genetic trend of Santa Gertrudis cattle. The traits analyzed were birth weight (WB), 120 days weight (W120), weaning weight (WW), yearling weight (WY) and 18-month weight (W18). There were records of weights of 12,737 animals and pedigree information of 29,921 animals from ten generations with records. Average inbreeding level was not high (0.0395). Heritability estimates for all traits were from moderate to low (WB = 0.16; W120 = 0.06; WW = 0.13; WY = 0.12; W18 = 0.12). The genetic trends were 0.0244 kg/year for WB, 0.0579 kg/year for W120, 0.134 kg/year for WW, 0.291 kg/year for WY and 0.406 kg/year for W18.
The new scenario of the Brazilian beef industry, inserted in the new order of a global world economy, induces the cattleman to search for more productivity in their business. To improve their herds, several tools are being used, including animal breeding and genetic improvement.
Thousands of herds of several breeds are being genetically evaluated in Brazil in the last five years and sire summaries of some breeds are currently issued.
Santa Gertrudis cattle, considered the first synthetic beef breed in the Americas was developed in the 20's in Kingsville, Texas, USA, using crosses between Zebu cattle (mainly Brahman) and European origin cattle (in this case Shorthorn), in order to improve beef production in subtropical areas. From the Zebu, Santa Gertrudis got resistance to ticks, adaptation to hot weather and rusticity and from Shorthorn the breed got precocity and weight gain. Additional information on the breed can be found in Cartwright (1978).
The breed was introduced in Brazil in 1954, through animals imported from USA and the growth of the number of animals was due to further imports but mainly from grading up mating, the way the breeds Charolais and Simmental were first established in USA (Van Vleck et al 1987). In that process, the breeders used several types of Zebu cows, mostly from Nelore breed, which led to the Santa Gertrudis animals raised in Brazil being a unique population, that needs to be studied from several approaches, including the genetic traits.
A literature review does not show too many studies in this breed. Analyzing some Santa Gertrudis herds in South Africa, Van Zyl et al (1995) found average inbreeding levels of 0.23% in 1980 and 0.78% in 1990. In 1980, 1.43% of the animals registered in the Breeders Association were inbred, a number that increased to 7.75% in 1990. In 1980, 12.5% of the herds had inbred animals, while in 1990 the number of herds with inbred animals reached 36.6%.
Aaron et al (1987) estimated genetic parameters for birth and weaning weight from 1894 Santa Gertrudis calves, born from 1978 to 1985 and found heritability estimates for birth weight of 0.38 ± 0.12 for males, 0.24±0.10 for females and 0.32±0.07 for both sexes. For weaning weight, those authors reported estimates of 0.30±0.11 (males), 0.45±0.12 (females) and 0.42±0.08 for both sexes. Phenotypic correlations between those traits were 0.31 for males, 0.27 for females and 0.29 for both sexes, while genetic correlations were 0.43±0.21 (males), 0.33±0.22 (females) and 0.40±0.08 (both sexes). Kriese et al (1991) estimated genetic parameters of datasets of Brahman and 3 other breeds derived from that Zebu beef breed: Beefmaster, Brangus and Santa Gertrudis, for birth weight (WB) and weaning weight, adjusted to 205 days (W205) and they concluded that as related to genetic parameters, those breeds were very similar to results of European breeds, reported in the literature.
The same authors suggested also that the negative genetic correlation between direct and maternal additive genetic effects should be considered in selection processes in the breeds studied. In the particular case of Santa Gertrudis, that study estimated heritability for WB of 0.34 (direct effects) and 0.26 (maternal effects), with a strong negative genetic correlation of -0.58 between direct and maternal effects. For W205, those estimates were 0.25, 0.18 and -0.43, respectively.
The objectives of the research reported in this paper were:
Data utilized in this study came from the Brazilian Santa Gertrudis Breeders Association and include growth records from 12,737 calves (6,203 males and 6,422 females), born from 1986 to 1996. The pedigree file included 29,921 animals born from 1957 to 1996.
The traits analyzed were:
Animals were arranged in contemporary groups, based on herd-year-season of birth, in a total of 700 groups. Dams were arranged in five classes, depending on their ages at the birth of the calves (class 1 = dams younger than 3 years, class 2 = dams from 3 to 5 years old, class 3 = dams from 5 to 7 years, class 4 = dams from 7 to 9 years and class 5 = dams older than 9 years). Additional information on data collection can be found in Ribeiro (1997).
Data analyses were performed at the Animal Breeding Group of the College of Animal Sciences and Food Engineering, University of São Paulo, Campus of Pirassununga, SP, Brazil. (Co)variance components were estimated using the software MTDFREML (Boldman & Van Vleck 1991; Boldman et al 1993). Inbreeding coefficients were also calculated by inverting the diagonal of the inverse relationship matrix, A-1, using the same software.
The traits were analyzed under a single trait animal model, considering:
The generic model proposed was the regular general mixed model:
y = X + Zg + Mm + Wp + e , where:
y = vector of dependent variables
= vector of fixed effects
X = incidence matrix that associates to y
g = vector of random genetic direct effects
Z = incidence matrix that associates g to y
m = vector of random genetic maternal effects
M = incidence matrix that associates m to y
p = vector of random effects of permanent environmental effects of dams
W = incidence matrix that associates p to y
e = vector of residuals
The following (co)variance components were estimated:
2a = genetic direct variance, due to the additive genetic effect of animals
2m = genetic maternal variance, due to the additive genetic effect of dams of animals
2e = variance due to residual
2p = total phenotypic variance
am = covariance between direct and maternal genetic effects
There were estimated also the following parameters:
h2a = heritability for direct genetic effects
h2m = heritability for maternal genetic effects
ram = genetic correlation between direct and maternal effects
c2 = proportion of total variance that is due to residual variance
All the figures were plotted considering the animals born before 1985 as "base animals", that means they were all joined in a unique group. That applies to figures about inbreeding trends and genetic trends. Genetic trends were plotted as average of breeding values, estimated by the solutions of the animal model equations, by year of birth and the overall trend was estimated as a regression of all the breeding values, by year of birth of cow.
Table 1 presents the descriptive statistics, informing the number of observations, mean values, coefficient of variation, minimum and maximum value for the 4 traits analyzed.
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Table 1: Number of observations (N), mean, coefficient of variation (CV) minimum and maximum for each trait birth weight (WB), weight at 120 days (W120), weaning weight (WW), yearling weight (WY) and weight at 18 months of age (W18) of Santa Gertrudis cattle in Brazil |
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| Trait | N | Mean | CV (%) | Minimum | Maximum |
| WB, kg | 12,186 | 35.78 | 11.15 | 10 | 66 |
| W120, kg | 3,810 | 124.10 | 22.60 | 48 | 235 |
| WW, kg | 7,520 | 221.11 | 24.86 | 80 | 415 |
| WY, kg | 4,942 | 288.19 | 23.40 | 100 | 559 |
| W18, kg | 3,667 | 387.76 | 23.61 | 155 | 770 |
Average inbreeding coefficient in this particular population was 3.96%, with 6,884 inbred calves out of 29,921 animals in the pedigree file. The important point in this case is that the Santa Gertrudis population in Brazil grew mainly through grading up from Zebu cows. The evolution of the inbreeding coefficient from 1985 to 1996 is presented in Figure 1, shows a small trend of reduction of inbreeding, close to 0.3%/year.
(Co)variance components and genetic parameters estimated for all the traits studied are summarized in Table 2.
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Table 2: (Co)variance components and genetic parameters for birth weight (WB), weight at 120 days (W120), weaning weight (WW), yearling weight (WY) and weight at 18 months of age (W18) of Santa Gertrudis cattle in Brazil |
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| (Co)variance components | Genetic parameters | |
| Trait | 2a | 2m | 2e | 2p | am | h2a | h2m | ram | c2 |
| WB | 1.66 | 0.94 | 7.74 | 10.30 | -0.48 | 0.16 | 0.09 | -0.39 | 0.75 |
| W120 | 26.95 | 66.53 | 305.61 | 420.2 | 21.14 | 0.06 | 0.16 | 0.50 | 0.73 |
| WW | 136.18 | 112.06 | 726.06 | 1079.6 | -65.59 | 0.13 | 0.10 | -0.53 | 0.67 |
| WY | 225.27 | 14.05 | 1237.06 | 1853.4 | 56.27 | 0.12 | 0.01 | 1.00 | 0.67 |
| W18 | 385.40 | 184.88 | 2386.58 | 2964.6 | -194.49 | 0.13 | 0.06 | -0.73 | 0.81 |
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2a = additive direct genetic variance; 2m = additive maternal genetic variance; 2ep = permanent environmental variance; 2p= phenotypic variance; am covariance between direct and maternal effects; h2a = heritability for direct effects; h2m =heritability for maternal effects; ram = correlation between direct and maternal genetic effects; c2 = proportion of total variance that is due to residual variance. |
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Figures 2 presents the trend of breeding values by year of birth of calves, observed at birth and at 120 days of age weights, while Figure 3 presents the trends for weaning, yearling and 18 months weights, including the genetic trend, expressed as regression coefficients by year of birth of calves.
The observed means were similar to those reported in the literature, which indicates that average performance of Santa Gertrudis cattle should be similar in Brazil and Southern USA or South Africa. Inbreeding level of this particular population was around 3.96%, a moderate to low level, but still higher than for other Santa Gertrudis populations, like South African, as reported by Van Zyl et al (1995). Considering that the original Santa Gertrudis population was highly inbred (Santiago 1984), this level can be considered low and the probable cause of that was the way the Brazilian population grew, by grading up from Zebu, highly unrelated, cows.
The distribution of inbreeding levels through the years, shown in Figure 1, suggests a small negative trend, close to -0.3%/year. The level of inbreeding found, but mainly the high number of inbred animals (6,884 out of 29,921 or 23%), suggest that, in the future, inbreeding levels should be included, as covariate, in animal models for (co)variance components estimation or genetic evaluation, as suggested by Kennedy et al (1988).
The genetic parameters for WB, presented in Table 2, are smaller than the ones reported in South African herds (Aaron et al 1987) or American herds (Kriese et al 1991). Heritability for direct effects (0.16) indicates that a progress in birth weight can be obtained through selection. That is important, because birth weight of the calves is rather high (average of 37.8 kg, with maximum of 60 kg). Breeders should watch the trend of that trait, presented in Figure 2, that is small, but positive as delivery problems could happen in the medium term future. The parameters estimated for that trait were smaller than reported for Nelore cattle raised in similar conditions (Eler et al 1998).
The genetic correlation between direct and maternal effects was negative, but low, and could indicate a genetic antagonism between the genes and should be considered in selection criteria. This agrees with results reported by Kriese et al (1991) in Santa Gertrudis cattle and other Brahman crosses in USA and by Eler et al (1994) in Nelore cattle in Brazil.
Weight at 120 days of age, at which point almost all the weight of the calf depends on its dam, should be a good indicator trait of milking and maternal ability of the cow. However, studies about this trait in Santa Gertrudis cattle were not found in the literature. In this particular population, the trait showed larger maternal effect than in any other trait, expressed as heritability for maternal effects (h2m =0.16), being even larger than heritability for direct effects (h2a =0.06). It is an interesting result that suggests that this trait should be submitted to further studies before defining its use as selection criterion.
Referring to weaning weight the genetic parameters estimated (h2a = 0.13 and h2m=0.10) were lower than others from the literature. Kriese et al (1991) found larger values and Aaron et al (1987) reported estimates for h2a of 0.30±0.11 for males, 0.45±0.12 for females and 0.42±0.08 for both sexes. In a similar study in Brazil, with a Nelore population, Eler et al (1994) presented estimates close to what was found in this study, which could indicate that environmental effects are very important in tropical and subtropical areas, affecting different populations in similar ways and importance, which would cause a decrease of heritability estimates in all breeds. In the study with Nelore cattle in Brazil, Eler et al (1998) found larger estimates for genetic parameters of WW.
The number of records used to estimate parameters after weaning was reduced to less the half the number of animal weaned, due to the culling policy of Brazilian herds, where a lot of animals are sold by weaning time. Only the best animals are kept, in a clear data selection. That suggests that future analysis on this dataset should consider two traits simultaneously, with weaning weight as anchor trait.
As happened with W120, there appeared to be references in the literature to studies of W12 in Santa Gertrudis cattle. In our research, the heritability estimates (h2a =0.12 and h2m =0.01) were smaller than those reported by Reyes et al (1995), using Nelore data from Brazil and Robinson et al (1992), from analyses of Brahman data from USA. This trait can be very important for selection as more precocious animals are selected, because puberty occurs closer to 12 months of age than to 18 months and selecting animals at this age can facilitate the identification of more interesting animals.
Estimates of heritability for W18 were h2a =0.13 and h2m =0.06. The relatively high maternal component, that explains 6% of total variation indicates that this effect should be kept in the model of analysis, even for W18, in such a Santa Gertrudis population. The estimate of heritability for direct effects is much lower than estimates from other breeds in similar environments. Genetic trends for all traits were positive, but can be considered small, corresponding, respectively, to 0.0, 0.0, 0.06, 0.10 and 0.10% of the means for WB, W120, WW, WY and W18. This means that genetic gain in those traits in the population studied is very small and much smaller than 1 or 2%, considered as a possible gain for direct selection for growth traits (Ribeiro 1997). The small genetic gain observed in this populations points to the low level of efficiency of selection for growth in this cattle breed in Brazil. The inclusion of sire evaluation programs, with the estimation of expected progeny differences in this breed, already ongoing in the breeders' association, should increase the level of genetic gain in this breed in Brazil.
It is concluded that:
The authors acknowledge the support of CNPq and FAPESP, funding agencies of Federal and State Government for partial financial support, and Associação Brasileira de Criadores de Santa Gertrudis for the authorization to use their data bank.
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