Reproductive performance of beef cattle in the tropics is affected by environmental factors usually associated with body condition at calving and their dynamics during the postpartum period¹. The interval between calving and the resumption of ovarian activity can be a major obstacle to the improvement of the reproductive efficiency, as it is responsible for the calving interval², which is influenced by breed, nutrition, milk production, presence of the calf, season of the year, presence of the sire³ and number of calving⁴.

Zebu cattle is recognized for its ability to adapt to tropical conditions, that are generally restricted for many Bos taurus breeds⁵. Based on the foregoing, since 1967 in Camagüey, province of Cuba began to forge a genetic project of the racial group Chacuba which derives from the inter se crossing of bovines 5/8 Charolais 3/8 Cuban Zebu. However, although it has generated knowledge about genetics and performance, the spread has been limited.

Although the majority of beef cattle genetic improvement programs emphasize characteristics related to the weight of animals, the inclusion of reproductive traits is important⁶. The traits used to measure reproductive efficiency have an important genetic basis, although in cattle, most of them have a low heritability⁷. One of the indicators for assessing the efficiency in cattle is the calving interval. However, this parameter makes a late fertility diagnosis; when identifying an excessive calving interval, the decrease in productivity is an accomplished fact⁴. Another measure to assess the reproductive efficiency of beef cattle is the days open, and as the proper information requires less time, its use is justified in selection to improve the reproductive performance⁸. Since the days open are measured repeatedly in the animal’s life, it is important to know their genetic relationship with other reproductive features as the number of services per conception and calving interval⁹.

Few years ago, genetic parameters and environmental factors affecting reproductive traits of the genotype Chacuba have not been studied. That problem had to be resolved to carry out parents’ selection. Therefore, the objective of this study was to estimate genetic parameters and the effect of environmental factors that affect the expression of traits used for measuring the reproductive efficiency of the Chacuba cattle.

The study was conducted taking information of 5/8 Charolais 3/8 Cebu and Chacuba (product of inter se crossbreeding of the former genotype) of the genetic rescue company of Sanguily in the Camagüey province, Cuba. The main objective of the company is the production of sires for the whole country. It has an area of 12,359.8 ha with a usable agricultural area of 11,774.7 ha, of which 8,855.8 are for pasture and forage.

The climate present two well-defined seasons: a dry season, which runs from November to April, and another rainy spanning from May to October. The rainy period is not homogeneous and is composed of two periods, in the 2-mo from May to June and September to October, separated by a minimum of precipitation in July-August; the average temperature ranges from 29.3 to 33.1 °C, the relative humidity of 76 to 84 % with rainfall ranging from 20 to 240 mm¹⁰.

The cattle is exploited under grazing conditions throughout the year, with natural pastures as Tejana (Paspalum notatum) and Camagueyana (Bothriochloa pertusa) and cultivated grasses of guinea (Panicum maximum) and Star (Cynodon nlemfuensis). They have also trees such as carob (Albizia saman), sprocket (Glyricidia sepium) and gauge (Leucaena leucocephala) with some genera of native legumes (Centrosema, Desmodium and Calopogonium).

The breeding system utilize artificial insemination and natural rearing of the calf. Weaning was at 180 d of age until 1992 and from 1993, at 210 d. After the weaning period, females go to the Development Centre and males to performance test.

Data consist of 4,957 records from cows born in nine herds between January 1980 and December 2004. The cows were daughters of 893 dams and 110 sires. The evaluated reproductive traits were the interval from calving to conception (ICC), calving interval (CI) and services per conception (SC).

To estimate the non-genetic factors that affect the ICC, CI and the SC four mathematical models using the GLM procedure of SAS were applied¹¹. The models had in common the female genotype, the number of birth and the sex of the calf. The difference among the models was combinations established for the definition of contemporary groups (herd-year-calving season), since four classification criteria were established for occurrence of the calving season (bimester, trimester, quatrimester and semester). The general model used was:

Where: y_ijklm= dependent variable (ICC, CI, SC); GC_i= group of contemporaries of herd-year-calving season (nine herds; 1980-2004; bimester, trimester, quarter, and semester); S_j= fixed effect of the sex of calf (male, female); G_k= fixed effect of the genotype of the cow (2; 5/8 Charolais 3/8 Zebu of first generation and cows product of the inter se crossbreeding of that genetic group); N_l= fixed effect of the number of calving (from the first to the eighth); e_ijklm= effect of the random error.

To obtain the variance components, analyses were performed with the ASReml program¹² and structured in the following way:

Analysis uni-character. Each character was studied as a separate feature. The mathematical model in matrix notation was:

Where: y= is the vector of observations for the trait in study; b= vector of fixed effects that contains the effect of herd-year-calving season selected previously, the genotype of the female, number of calving and the sex of the calf; a= vector of random additive effects of the animal; e= vector of random residual effects; X, Z: matrix of incidence that relates fixed effects and additive random effects of the animal, respectively.

Analysis multi-character. The model presents the following matrix notation:

Where: y_i= vector of observations for the i-th feature; b_i= vector of fixed effects that contain the effect of herd-year-calving season (previously selected), the genotype of the female, calving number and the sex of the calf to the i-th feature; a_i= vector of random effects of the animal for the i-th feature; e_i= vector of random residual effects for the i-th feature; Xi and Z are matrix designs that relate data using fixed and random effects, respectively.

It assumes That:

Where: g_ij= is the additive genetic variance for the i-th trait, when i=j, and the genetic covariance when i≠ j; A= the relationship matrix. rij= is the residual variance when i=j, and the covariance when i≠ j.

Each trait was considered independent, but genetically and environmentally correlated with the others.

The overall average for ICC, CI and SC appear in Table 1. Observed values of ICC or CI are consistent with those referred in crosses Bos taurus x Bos indicus⁴, Nellore crosses with other B. taurus or B. indicus breeds¹³, Chianina from Italy¹⁴ and in Cuban Zebu breed¹⁵. In Cuban Zebu cattle⁹ and Simmental in Mexico¹⁶ reported less prolonged ICC and CI. Calvings intervals higher than those in this work were estimated in different crosses of beef cattle in Colombia¹⁷. However, the values obtained in this study were lower than other reports referred in Zebu cows^15,18,19.

Table 1 Unadjusted mean, standard deviation (SD) and coefficient of variation (CV) for calving interval, interval calving-conception and services per conception in Chacuba cattle. 

The ICC and CI values were high, probably due to deficiencies in the reproductive management of herds, related to estrus detection and feeding of cows during the pre- and post-partum periods¹⁷.

Table 2 shows the results from the analysis of variance for the effect of the genotype of the cow and the non-genetic factors that affected the ICC, CI and SC. The genotype of the cow did not influence the evaluated reproductive traits (P>0.05). The effect of the number of calving was significant (P<0.01) for all the analyzed traits. In the first two calvings, cows presented longer CI and the ICC decreasing until the eighth calving. This coincides with other studies that found less prolonged CI and ICC intervals in animals between 5 and 9 yr of age in cows Romosimuano²⁰ and Zebu²¹. These results were associated as lactation and growth occur in parallel in early calvings, when females usually do not reach the adult size. Therefore, nutrients consumed during that period are for maintenance, growth, and lactation, which affects fertility and causes longer postpartum anestrus. The former argument agrees with a study in Brazil Nellore cattle, where nulliparous heifers showed a higher fertility than multiparous cows, and these in turn higher than those of the first calving, attributing the result to better body condition during the insemination period and the absence of stress involved in lactation²². The sex of the calf did not affect any of the dependent variables (P>0.05). The absence of a significant effect of the sex of the calf on the ICC and CI coincides with reports in Romosinuano cows²⁰, Zebu²¹ and double purpose²³.

Table 2 Effect of the genotype of the cow (G), parity (NP), sex of calf (SX) and contemporary group, on some reproductive parameters in Chacuba cattle. 

M1= Contemporary group herd-year-calving bimester; M2= Contemporary group herd-year-calving trimester M3= Contemporary group herd-year-calving quatrimester; M4= Contemporary group herd-year-calving semester. Probability values.

The contemporary group affected (P<0.001) the ICC, CI and SC. In general, the four classes of grouping (by bimester, trimester, quarter and semester of calving) have a major effect on each of the dependent variables mentioned. In another study conducted in Brazil²⁴ reproductive efficiency was affected by the season of the birth when it was divided into 20-d intervals throughout the year. The effect of period of calving on reproductive variables may be related to variation in the availability and quality of the pastures, which is associated in turn to body condition of cows giving birth and as a result the fertility in the postpartum period²⁵, affecting the CPI and the CI²⁴.

In all four models, the coefficient of determination (R²) for ICC and CI fluctuated between 31 and 38 %. The minor R² values were for SC in the four models (between 10 and 15 %). The R² value from the three dependent variables corresponds to the model that included a contemporary group of herd-year-bimester. The low R² values recorded indicate that factors other than those included in the models affect the reproductive characteristics analyzed. In correspondence with the above results, the contemporary group of herd-year-bimester was included in the uni and multi-character models for estimation of variance components and genetic parameters.

Table 3 shows heritabilities for ICC, CI and SC. The genetic parameters estimated for the reproductive traits using uni-character and multicharacter models were similar because of the low variance components for the additive genetic effect. That indicates that these features are predominantly influenced by environmental effects and the contribution of dominant genes or epistaxis, where the additive genetic variance contributes little in the total variance²⁶.

Table 3 Variance components and heritabilities for calving to conception interval (CCI), calving interval (CI) and services per conception (SC) estimated by uni-character or multi-character models in Chacuba cattle. 

Most of the literature in dairy cattle agrees that heritability for such traits is equal to or less than 0.05, although for CI reported also values of 0.19²⁶. In this sense, the literature on beef cattle is scarce. Additive variance and heritability estimates for the ICC in this study are similar to those referred to the Asturiana de los Valles breed⁸, in different crosses of B. indicus x B. taurus¹⁷ and Chianina¹⁴. However, the estimates are superior to others reported in Cuban Zebu cattle⁹ and in Santa Gertrudis²⁷, and also for Girolando cattle²⁸ and Zebu of Colombia²⁹. An estimated high heritability for the CI (0.42) was reported in Brazil Nellore cows³⁰. The value of heritability estimated for the SC in the present work coincides with some reports in Zebu cattle⁷.

Genetic and environmental correlations between the ICC and the CI were high and positive. However, SC showed a low genetic association and a moderate environmental relationship with the ICC and CI (Table 4). Genetic and environmental correlation between CI and ICC close to the unit indicates that the same genes and environmental factors affect the expression of both characteristics^8,31. Estimates of genetic correlations between CI and ICC are scarce. There are reported values of 0.98 in dairy cattle in New Zealand³² and in Colombian Holstein³¹. In the present study, the genetic and environmental correlations between ICC and CI are the same as those reported in Cuban Zebu cattle⁹; however in that work the genetic correlation between the ICC and SC turned out to be higher than in the present.

Table 4 Genetic correlations (on the diagonal) and environmental correlations (below the diagonal) among the interval calving-conception (ICC), calving interval (CI) and services per conception (SC) in Chacuba cattle. 

In conclusion, the heritability found in this study for the reproductive traits was low. This implies that the environment greatly influences the reproductive traits, so slow genetic progress is expected if it is only based on individual selection. Even so, given the genetic correlation that exists, allows the indirect selection for calving interval from the calving-conception period evaluation.