Data from a selection experiment for growth carried out in Brazil were analyzed in order to evaluate the direct responses on yearling weight (YW) and the correlated responses on the size and reproduction traits of cows. The experiment was started in 1976, and in 1980 three lines of Nelore cattle were established: selection (NeS), traditional (NeT), both selected for higher YW, and control (NeC), selected for mean YW. The NeT was an open line that eventually received bulls from other herds. Yearling weight records for animals born from 1978 to 1998 and yearling hip height (H550) offemales born from 1985 to 1998 were analyzed by fitting an animal model in order to obtain the genetic trends. The means for weight, height, and body condition score at the start of the breeding season, days to calving, and calving success of cows born from 1993 to 1996 (pertaining to the third to fourth generations of selection) were compared between the selected (NeS and NeT) and control lines. The genetic trends obtained after 16 yr for YW were 1.7 +/- 0.2, 2.3 +/- 0.2, and -0.1 +/- 0.1 kg/yr for males and 1.9 +/- 0.2, 2.4 +/- 0.2, and -0.1 +/- 0.1 kg/yr for females, for the NeS, NeT, and NeC lines, respectively. Corresponding values for H550 were 0.25 +/- 0.03, 0.24 +/- 0.04 and -0.04 +/- 0.03 cm/yr for females. Heifers and cows from NeS and NeT were 19% and 15% heavier and 4% taller at the start of the breeding season than those from NeC. No significant differences between selected (NeS and NeT) and control females were detected for body condition scores and for reproductive performance. The results indicate that selection for body weight promoted high and consistent weight and height responses both at the yearling and later ages, without compromising the reproductive performance of the cows with respect to days to calving and calving success.

The objectives of this study were to estimate genetic parameters for indicator traits of feed efficiency and to recommend traits that would result in better responses to selection for increased weaning weight (weaning weight adjusted to 210 d of age [W210]), ADG, and metabolic BW (BW(0.75)) and lower DMI. Records of W210 from 8,004 Nellore animals born between 1978 and 2011 and postweaning performance test records from 678 males and females born between 2004 and 2011 were used. The following feed efficiency traits were evaluated: G:F, partial efficiency of growth (PEG), relative growth rate (RGR), Kleiber's ratio (KR), residual feed intake (RFI), residual weight gain (RWG), and residual intake and gain (RIG). Covariance and variance components were estimated by the restricted maximum likelihood method using multitrait analysis under an animal model. Estimates of genetic gain and correlated responses were obtained considering single-stage and 2-stage selection. Heritability estimates were 0.22 ± 0.03 (W210), 0.60 ± 0.08 (DMI), 0.42 ± 0.08 (ADG), 0.56 ± 0.06 (BW(0.75)), 0.19 ± 0.07 (G:F), 0.25 ± 0.09 (PEG), 0.19 ± 0.07 (RGR), 0.22 ± 0.07 (KR), 0.33 ± 0.10 (RFI), 0.13 ± 0.07 (RWG), and 0.19 ± 0.08 (RIG). The genetic correlations of DMI with W210 (0.64 ± 0.10), ADG (0.87 ± 0.06), and BW(0.75) (0.84 ± 0.05) were high. The only efficiency traits showing favorable responses to selection for lower DMI were G:F, PEG, RFI, and RIG. However, the use of G:F, PEG, or RFI as a selection criterion results in unfavorable correlated responses in some growth traits. The linear combination of RFI and RWG through RIG is the best selection criterion to obtain favorable responses in postweaning growth and feed intake of Nellore cattle in single-stage selection. Genetic gains in feed efficiency are expected even after preselection for W210 and subsequent feed efficiency testing of the preselected animals.

This study was conducted to evaluate associations of performance traits, feed efficiency, and blood variables with residual feed intake (RFI) in growing Nellore cattle. A total of 118 growing Nellore animals, 62 males and 56 females, were used. A diet containing 2 Mcal/kg was offered ad libitum and individual DMI was measured over a period of 84 d. Animals were classified as low (≤0.128 kg/d; = 40), medium (-0.128 to 0.135 kg/d; = 42), or high RFI (>0.135 kg/d; = 36). Blood samples were collected at the beginning and at the end of the test for determination of plasma concentrations of different blood variables. Data were analyzed using a mixed model that included the random effect of facility the fixed effects of sex, RFI class, and linear effect of the covariate age within sex; and the interaction between RFI class and sex. Least squares means were compared using a -test. Animals of different RFI classes presented similar performance traits and different DMI. Low-RFI animals consumed, on average, 0.670 kg/d less DM than high-RFI animals. Among the blood variables analyzed, significant differences among RFI classes were observed for urea, IGF-1, and insulin. Plasma urea concentrations were lower (5.58 vs. 5.91 mmol/L) and insulin (4.45 vs. 3.70 μIU/mL) and IGF-1 (433 vs. 399 ng/mL) concentrations were higher in low-RFI animals when compared to high-RFI animals. Plasma concentrations of urea, IGF-1, and insulin can be used as indicators of feed utilization efficiency in Nellore cattle.

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ABSTRACT. Real-time ultrasound is currently used for in vivo carcass evaluation of beef cattle. We estimated heritability and repeatability coefficients for ultrasound-measured carcass traits in Nelore cattle. We measured longissimus muscle area, backfat thickness, and rump fat thickness in male and female animals ranging in age from 10 to 26 months. The variance components were estimated by single-trait analysis using the derivative-free restricted maximum likelihood method, under an animal model. The complete data file included 3077 records of 1172 animals born between 2003 and 2008. Two analyses were performed using a repeatability model: a) all records of longissimus muscle area, backfat thickness, and rump fat thickness of animals aged 10 to 26 months, and b) considering two age periods (11 to 17 and 18 2945©FUNPEC-RP www.funpecrp.com.br Genetics and Molecular Research 10 (4): 2944-2952 (2011) Repeated measures of LMA and fat thickness in Nelore to 24 months). The heritability estimates ranged from 0.33 to 0.46 for longissimus muscle area, from 0.20 to 0.26 for backfat, and from 0.26 to 0.29 for rump fat, demonstrating that direct selection for these traits can provide genetic gain. The repeatability estimates were moderate, ranging from 0.42 to 0.73. The highest repeatability estimates were obtained for longissimus muscle area in both the first (0.70) and second (0.73) analyses. The repeatability estimates suggest that ultrasound measures are precise; higher precision was obtained for longissimus muscle area than for subcutaneous fat thickness. The latter could be obtained during periods of greater feed availability to increase precision.