No abstract

Information for this study was obtained from a commercial beef enterprise located in the subhumid tropics of Yucatan, Mexico. Data from 664 females of different zebu breeds born from 1972 to 1983 were considered for analyses. Weaning and 18-month weights were adjusted to 240 (W240) and to 550 (W550) days of age, respectively. The statistical model included the fixed effects of years of birth, season of birth, age of dam at calving, breed of heifer and the random effect of sire within breed. Arithmetic mean +/- standard deviations were 32.1 +/- 3.6 kg, 208.3 +/- 29.1 kg, 678.1 +/- 94.6 g, 308.7 +/- 29.7 kg, 342.3 +/- 89.8 g and 1,060.2 +/- 118.7 days for birth weight (BW), W240, average preweaning daily gain (ADG), W550 postweaning daily gain (PDG) and age at first calving (AFC), respectively. Year and season of birth had significant (P < 0.05) effects on W240, ADG, W550, PDG and AFC. Parity number was significant (P < 0.05) for BW, W240, ADG and AFC. Breed differences were significant (P < 0.01) for all traits, except BW. Guzerat heifers were heavier at weaning and W550 than heifers from the other breeds. Guzerat heifers first calved at an average age of 995 days; Brahman and commercial zebu heifers calved about 50 to 60 days later and the Indubrazil and Gyr heifers about 2 months later still. The heritability estimates were 0.24 +/- 0.13 for BW, 0.26 +/- 0.14 for W240, 0.29 +/- 0.14 for ADG, 0.43 +/- 0.17 for W550, 0.56 +/- 0.20 for PDG and 0.46 +/- 0.15 for AFC.

The objective of this study was to evaluate the body weights up to 18 months of age of 12 breed groups of Zebu (Z), Brown Swiss (BS) and Charolais heifers (CH), and their crosses under tropical conditions. A total of 1434 data on weaning weights adjusted to 240 days (WW8), average daily gain to 240 days (ADG8) and 1025 body weights adjusted to 550 days (BW18) and average postweaning daily gain (ADG18) of heifers born from 1981 to 1995 were used. Cows and calves remained together from birth to weaning and grazed on Guinea grass (Panicum maximum). Years of birth were grouped in three periods, because of the small numbers of observations per year (1981-85, 1986-90 and 1991-95). Similarly, months of birth were grouped in three seasons: dry (February to May), rainy (June to September) and windy and rainy (October to January). Ages of dams were classified in six groups (or=8 years). Breed groups were animals with 12.5%, 25.0%, 50.0%, 62.5%, 75.0% and 100% BS genes and 25.0%, 50.0%, 75.0% and 100% CH genes and also Z and undefined crossbred animals. Data were analysed using a fixed model that included effects of period, and season of birth, age of dam and breed group of heifer. Analyses of variance showed significant (p < 0.05) effects of period, season and breed group for all traits. However, parity was significant (p < 0.05) only for WW8 and ADG8. Interactions among main factors were not significant (p > 0.05). Z heifers gained the least weight to weaning but were similar to BS at 18 months of age. CH were heavier at 18 months of age than BS heifers, but not at weaning. Body weights at 18 months of age of 1/8 and 1/4 BS crossbred heifers were similar (p > 0.05) and lower than F1 (BS x Z) (p < 0.05); however, the later and higher grades of BS had the highest body weights at 18 months of age and were similar to CH and 1/4 CH x 3/4 Z heifers. Undefined crossbred heifers had better performance than the Z breed group. Environmental factors were important sources of variation for pre- and postweaning traits. Under the conditions of this study, crossbred heifers generally performed better than Z heifers up to 18 months of age.

The objective was to evaluate the effect of cow genetic group, nutritional level and their interaction on some economically important traits of dual-purpose herds managed under field conditions. Nine herds were monitored during a production cycle in Yucatan, Mexico. Herds were grouped into four nutritional levels (NL) based on the metabolizable energy (ME) apparently available on pasture, nutritional management, and milk production. Cows were classified into three genetic groups (GG): low (< or = 25%), middle (25-75%) and high (> or = 75%) graded for Bos taurus inheritance. Total milk sold (TMS), days in milk (DIM), TMS adjusted to DIM within each NL (TMSA), body condition score (BCS) at calving, changes of BCS during lactation (CBCS), calf weaning weight (WW), age at weaning (AW), kg of calf weaned per cow (KWC) and calf mortality were studied. The statistical model included the fixed effects of NL, GG, month of calving (MC), parity number (PN) and BCS at calving and GG x NL interaction. The effects of NL, GG, MC, PN and GG x NL were significant (p < 0.05) for TMS, KWC. As expected, TMS increased with NL from 562.4 +/- 106 kg for NL1 to 2366.3 +/- 100.1 kg for NL4. KWC was greatest for NL2 (138.6 kg) followed by NL1 (135 kg); the lowest KWC corresponded to NL4 (96.0 kg) (p < 0.05). TMS values for the middle (1727 +/- 94.7 kg) and the high graded GG (1603.5 +/- 83.5 kg) were twice those for the low graded GG cows (828.5 +/- 95 kg) (p < 0.05). KWC was also higher for the middle graded group (152.8 kg) than for the low or (104 kg) or the high graded GG (118 kg) (p < 0.05). With better nutrition cows of all GG improved their milk performance but not the calf traits. CBCS was negative for all GG. The highest BCS lost was for cows in NL1 and NL2 and for cows in the high graded GG (p < 0.05).

Information on 936 birth weights (BW), adjusted weaning weights (AWW) and average daily gains (ADG) from Brahman (B), Indubrazil (I), Gyr (G) and commercial Zebu (C) cattle and their crosses were obtained from the records of a farm in the State of Yucatan, Mexico. Two statistical models were used to analyse the data: the first included the effects of year and season of birth, sex of the calf, cow's parity number and breed group. The other model included the same environmental effects as the previous model and the genetic components of breed group (breed additive, breed maternal and heterosis effects). Additive and maternal effects were expressed as deviations from those for the B breed. The means of BW. AWW and ADG for the B breed were 33.3 +/- 0.64 kg, 204.0 +/- 4.9 kg and 760.06 +/- 19.24 g/day, respectively. Both models gave similar results in terms of the variance explained. Additive effects were significant (p < 0.05) for AWW and ADG but not for BW. The minor additive effect was for the C breed (-10.75 kg and 40.77 g for WW and ADG, respectively). Maternal and heterosis effects were not significant (p > 0.05) for any of the traits. The correlation between the breed group means and the means estimated by the prediction equation was r = 0.88. Finally, there was no significant genetic effect on pre-weaning calf performance among the genotypes evaluated. The genetic model can be used to predict the genotype means with a high degree of accuracy.