Direct and total heritabilities were higher and maternal heritability and c2 effects much lower than for weaning weight. The direct and total heritabilities were 0.16 and 0.28, somewhat higher than those found by Plasse et al. (2002), while maternal heritability (0.04) was only half the value found in that study. The direct-maternal correlation was 0.86, much higher than reported by Plasse et al. (2002), and c2 was slightly larger.
Most work with postweaning weights has been carried out with yearling weight and is therefore not comparable with our data, because in the tropics the after-weaning stress period usually extends beyond one year of age. For this reason we consider 18-month weight more appropriate for studying genetic differences in postweaning weights and for use as a selection criterion. Estimates published in the literature are generally higher than ours for direct and maternal heritability, higher or lower for total heritability and close to zero or negative for the direct-maternal correlation (Eler et al., 1995; Mercadante and Lobo, 1997).
The permanent environmental effect of the cow contributed 4% to the phenotypic variance, the same as that found in a bivariate analysis of weaning weight published by Eler et al. (1995). It was within the range obtained by Mercadante and Lobo (1997) with uni- and bivariate analyses, and was slightly higher than the estimate reported by Plasse et al. (2002) with a univariate analysis.
Correlations between 205- and 548-day weights
Estimates of the correlations from the bivariate analysis of 205- and 548-day weights are given in Table 3. The additive genetic correlation between the two weights was 0.66, similar to the estimate found by Plasse et al. (2002) in another Brahman herd and lower than the average calculated from the Bos indicus literature by Mercadante et al. (1995). It was also lower than the mean genetic correlation between weaning and yearling weight (0.81), given in the review by Koots et al. (1994b). Our value was also somewhat less than the estimate of the genetic correlation between weaning and final (16-23 months) weights (0.69) reported for crossbred Australian Zebu (Meyer, 1994). The maternal genetic correlation was 0.70, while Meyer (1994) and Plasse et al. (2002) reported estimates close to unity. The permanent environmental correlation was unity. Several within run estimates were out of the parameter space, but since REML estimates have to be within bounds, the program forced them to unity. Meyer (1994) published an estimate of 0.91 and Haile-Mariam and Kassa-Mersha (1995), as well as Plasse et al. (2002) estimated permanent environmental correlations to be unity. The high maternal genetic and permanent environmental correlations between weaning and 18-month weights suggest that important genetic and environmental effects due to the dam might still be present at 18 months in beef cattle and, supposedly, are a carry-over effect from the preweaning period produced through a part-whole relationship. Similar observations have been made by other authors (Meyer 1994; Tosh et al., 1999; Plasse et al., 2002). The estimate of the residual (temporary environmental) correlation was 0.57, lower than that found by most authors (Meyer, 1994; Eler et al., 1995; Plasse et al., 2002). The phenotypic correlation between weaning and 18-month weight was 0.64, which equals the average reported by Mercadante et al. (1995) from the world literature on zebu cattle, but is lower than the value found by Plasse et al. (2002) in another Brahman herd.
A1 = Analysis with all calves. A2 = Analysis performed on calves with known sires.
rd1d2, direct additive genetic correlation; rm1m2, maternal additive genetic correlation; rc1c2, permanent environmental correlation; re1e2, temporary enviromental correlation; rp1p2, phenotypic correlation.
Estimates of heritabilities and genetic correlations indicate that milk production could be improved by selecting for maternal genetic effects at weaning, and that this would not be antagonistic to genetic improvement of growth rate, which should be improved by selection for 18-month weight rather than using weaning weight as a criterion.
Phenotypic and genetic trends
Birth weight increased by 0.393 kg per year (Table 4) improving from an adjusted mean of 27 kg in the first year to 32 kg in the last 3 years of this study (Figure 1). The trend was quite linear and highly significant. As a consequence, birth weights under 20 kg, shown by Beltrán (1976) as a critical limit, below which early mortality is high, were recorded in only 67 (0.8%) of all calves born, with a minimum of 10 kg. Of these, only nine were born during the last 6 years. Of the 67 calves, 42% died before weaning; 46% among those died during the first 3 days after birth. On the other hand, the mean was not due to abnormally high birth weights (maximum 49 kg) and the frequency of dystocia was almost zero. The 205-day weight had a highly significant trend of 3.367 kg per year, which was quite linear. From 146 kg in the first year, the mean increased to 184 kg in the last year, with a maximum reached at 186 kg in the penultimate year. Weight at 548 days was 254 kg during the first and 280 kg during the last year, with a maximum of 283 kg one year earlier. The mean annual phenotypic trend of 548-day weight compares well with the estimates reported for Brahman in Venezuela (Hoogesteijn and Verde, 1998; Plasse et al., 2002). The annual phenotypic trend (Figure 1) for 548-day weight was quite irregular, which may be explained by the deterioration of pasture between 1990 and 1995.
bA1 = Analysis including all calves. A2 = Analysis performed on calves with known sires.
cUsing least square solutions for year of birth.
dFrom univariate analysis.
eFrom bivariate analysis.
Estimates of annual direct and maternal genetic trends for the three weights during the 14-year period are given in Table 4 and in Figure 2. Direct genetic change for birth weight calculated from the yearly means of estimated breeding values was not significant and was close to zero. This shows that selection for 548-day weight did not produce a strong correlated genetic change in birth weight. Genetic trend was irregular over the years (Figure 2) and of no practical importance. The same pattern applies to maternal genetic trend, which was not significantly different from zero. The direct genetic trend for 205- and 548-day weights was 0.142 kg (P<0.01) and 0.263 kg (P<0.01), respectively, and the maternal genetic trend was 0.115 kg (P<0.01) and 0.095 kg (P<0.01), respectively, per year. Genetic trend in milk production is reflected by the maternal genetic trend in Table 4 and Figure 2 and shows that annual lactation milk yield increased enough to support an increase in weaning weight of 0.115 kg per year. Although direct and maternal genetic trends are expressed phenotypically in different generations, over a long time period it should be acceptable to add both means in order to estimate total genetic progress achieved for each weight by direct and correlated response through selection for growth rate until 18 months in males and maternal ability in females. For 205- and 548-day weights this would amount to 0.257 kg and 0.358 kg per year. This would mean that of the phenotypic change in 14 years of 47 kg for 205-day weight and 25 kg for 548-day weight, 3.6 and 5.0 kg, respectively, would have been due to genetic selection during the 14-year period.
When comparing these results with the literature, it is necessary to keep in mind that only 36% of the calves born were produced by artificial insemination and only half of these were from proven bulls with high breeding values for 548-day weight with accuracy over 0.90. The other half of the calves produced by artificial insemination were sired by bulls preselected based on a breeding value estimated through the bulls’ own weights and those of their relatives available in the pedigree information, but with no progeny records. The accuracies were between 0.40 and 0.50. This situation is similar to that found in the entire population of the genetic cooperative to which this herd belongs. Mean direct genetic trends per year in the whole population for the years 1990-2001 were 0.07, 0.64 and 1.13 kg for birth, 205-day and 548-day weights, respectively, while for maternal genetic trends the respective changes were -0.01, 0.20 and 0.06 kg (Anonymous, 2002). This comparison shows that the herd in our study was progressing at a much slower rate than the population average, mainly because it is managed under much more extensive conditions, and with a lower than average percentage of cows assigned to the artificial insemination program. However, the direct genetic trends during the last 6 years have been much higher than for the 14-year period. This improvement is probably due mainly to the benefits of the cooperative genetic program and the use of modern genetic analysis methods. The other published data that we cited are from progeny produced by artificial insemination or in single sire herds by natural service. This means that selection pressure was much less in this herd than in those discussed by Hoogesteijn and Verde (1998) and Plasse et al. (2002) who calculated higher genetic progress in calves with known paternity. Plasse et al. (2002) reported direct genetic trends per year over a 30-year period of 0.06, 0.13 and 0.49 kg or birth, 205-day and 548-day weights, while the respective estimates for maternal genetic trend were 0.00, -0.04 and 0.25 kg per year. As a reference for what could be achieved in the future with large-scale genetic programs, the results of Crump et al. (1997) are summarized. They found the mean annual genetic trend for 400-day weight of the registered populations of Simmental, Limousin, Charolais, South Devon and Angus breeds in Great Britain to be 1.04 to 2.49 and 0.82 to 1.86 kg for the period 1980-1992 and for all years on record respectively.
Analysis of calves with known paternity (analysis 2)
Results of A1 and A2 would be expected to be different, because A1 was done with data from the whole population where pedigree relationships could be established only through the dam for 53% of the calves, whereas A2 included calves from a subpopulation, where genetic relationships could be traced through the sire and the dam for all calves. In general, slightly higher variances were found for all weights in A2 than in A1 (Table 2). However, among the three weights considerable differences were found only in the direct-maternal genetic covariance and for birth weight, also in the permanent environmental variance of the dams. For birth weight, most genetic parameters were quite different between the two analyses. For 205- and 548-day weights estimates of the direct-maternal genetic correlations differed somewhat between A1 and A2, but estimates of other parameters were similar. Genetic and phenotypic correlations between 205- and 548-day weights (Table 3) were essentially the same for both analyses. This was expected from a bivariate analysis, which uses extra information, increases accuracy of estimation and improves data structure, as reported by Ducrocq (1994).
The annual genetic trend for 205- and 548-day weights was considerably higher for offspring of artificial insemination and of single-sire herds (Table 4, A2) than for the whole population (A1). For direct genetic trend the superiorities for the two weights were 31 and 79%, and for maternal genetic trend they were 140 and 43%. The direct genetic trend for 548-day weight in calves with known sires was 0.471 kg per year, comparable to the report for another Brahman herd (Plasse et al., 2002) and slightly less than in the report for 450-day weight of 147 Nelore herds participating in a genetic cooperative in Brazil (Lobo et al., 2001).
Sire x year interaction
The small positive genetic-maternal correlation at weaning is contrary to most of the values reported in the above-cited literature and is partly explained by the results of the analysis, which included the sire x year interaction in the model. This analysis (A3) was performed with 124 sire-years and a mean of 1.6 years per sire. The interaction term was close to zero, and the results of the models with and without the random interaction term were not significantly different when a likelihood ratio test was performed. Consequently, the genetic parameters were essentially the same. Robinson (1996a), when discussing direct-maternal genetic correlations at weaning, concluded that “negative estimates were more likely to be a consequence of additional variation between sires or sire x year variation, than evidence of a true negative genetic relationship” and was able to confirm this hypothesis with the results for a simulated data set (Robinson, 1996b). Meyer (1997) suggested that lower negative genetic-maternal correlations are obtained in experimental than in field data, and that “most of the direct-maternal covariance in Herefords is environmental rather than genetic.” Lee and Pollak (1997) concluded, from the results of different analyses of simulated data sets, that a large part of the negative direct-maternal genetic covariance in weaning weight is produced by a sire x year interaction. We believe that the data used in our study, although they came from a private herd, can be considered to be “experimental data” because of the scientific design and the technical supervision of the genetic and management programs. Probably this fact and a proper definition of the contemporary groups in our model would have mitigated the reasons for the generation of an apparent negative covariance in other studies. However, the relatively low number of observations and the low mean number of years for each sire, might prevent such an interaction from becoming manifest. The effect of sire x year or herd interaction needs to be studied in the entire population of the genetic cooperative.
We found that artificial insemination used strategically in part of, rather than in the whole herd, is feasible even with extensive beef cattle production systems, if it is based on a designed management plan and accompanied by a genetic selection program. To be successful, however, artificial insemination must be restricted to the cows most likely to conceive.
While estimates of moderate genetic progress in weights and milk production were found over the whole period, genetic gains accelerated during the last years, when modern genetic-analytical methods were used for breeding value evaluation in the context of a cooperative genetic program. Reproductive efficiency apparently was not impaired by selection for 548-day weight, since pregnancy rate increased during the study from 71 to 85%. Direct heritabilities were low, and maternal heritability at weaning was higher than the direct heritability. This fact, the relatively high proportion of maternal permanent environmental variance at weaning and the high maternal permanent environmental correlation between 205- and 548-day weights confirm the importance of the preweaning maternal environment that the cows provide to their progeny for weights up to 18 months of age. Our results support the advantage of multiple over single trait genetic evaluation.
The data from this herd indicate that an adequate design of the genetic and management program might avoid sire x year interactions and the resulting negative estimates of direct-maternal genetic correlations. Comparing an analysis of the data of the whole population with that from a subpopulation in which sires of all calves were identified, generally showed considerable similarity in the estimates of variances and genetic parameters. So the choice of which type of analysis should be preferred for practical and theoretical purposes, would depend on the objective of the study.
Annual genetic trend was markedly higher in the subpopulation with identified sires and shows the definite genetic advantage of the strategic use of artificial insemination with genetically proven bulls in part of the herd.
The results suggest that the selection program used in this herd, which emphasizes reproductive efficiency in cows and in sires growth rate up to 18 months, estimated by 548-day weight, should be continued. Optimum cow efficiency should be the goal in order to continue to improve sustainable beef production on natural and improved pasture while preserving the unique ecological characteristics of floodable tropical savannas.
The authors thank Reynaldo Alvarado, Alfredo Guaicara and Garby Hernández for the supervision of data collection and processing at the ranch, and Mayanin Dagger and Florangel Quero for assistance in the preparation of data for the analyses. The authors most gratefully acknowledge the contributions of Professor Lucia Vaccaro in the preparation of this manuscript. The analyses of these data were supported by the Consejo de Desarrollo Cientifico y Humanistico (UCV) through Proyecto de Grupo No. 11-10-4148-2000 and Ayuda Institucional “A” No. 11.10-4787-2000, which is gratefully acknowledged.
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