growth characteristics, reproductive performance, … sires (i.e., international brangus breeders...

ABSTRACT: Balancing growth and reproductive performance in beef cattle managed in desert environ-ments is challenging. Our objectives were to 1) evaluate trends in growth and reproductive traits, and 2) assess associative relationships between growth characteristics and reproductive performance in a Brangus herd man-aged in a Chihuahuan Desert production system from 1972 to 2006. Data were from bull (n = 597) and heifer calves (n = 585; 1988 to 2006) and cows (n = 525; repeated records of cows, n = 2,611; 1972 to 2006). Variables describing the growth curve of each cow were estimated using a nonlinear logistic function (each cow needed 6 yr of data). Mixed-effect models and logistic regression were used to analyze trends across years in growth and reproductive traits (both continuous and categorical). For continuous traits of calves, a slight cu-bic response (P < 0.01) described the dynamics of birth weight, 205-d BW, and 365-d BW across years. For categorical traits of females, positive linear trends (P < 0.05) across years were observed in percent pregnant as yearlings, calved at 2 yr of age, and first-calf heifer re-breeding (slopes ranged from 0.007 to 0.014%/yr). Au-

tumn cow BW increased gradually until 1997 (509 kg ± 8.8) and then decreased gradually by 0.6 kg/yr, where-as pregnancy percentage decreased gradually until 1995 (78.4% ± 1.0) and then increased slightly by 0.2%/yr. A quadratic effect best described the dynamics of these 2 variables across years (P < 0.01) as well as estimates describing the growth curve of each cow. Specifically, asymptotic BW and age increased (P < 0.05) from 1972 to 1983 and 1990, respectively. Asymptotic age then decreased by 27% from 1983 to 1996 (P < 0.05). The maturing rate index was negatively correlated with age at first calving and calving interval (r = –0.42 and –0.18, P < 0.01), which suggested that early-maturing cows had enhanced fertility in this environment and production system. In summary, minimal changes were observed in measures of growth in bulls and heifers in a Brangus herd managed in the Chihuahuan Desert. Op-posing relationships were observed among measures of cow size and fertility; as growth curves shifted toward earlier maturity, measures of reproductive performance suggested that fertility improved.

Key words: Brangus, cattle, Chihuahuan Desert, fertility, growth curve

©2010 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2010. 88:1891–1904 doi:10.2527/jas.2009-2541

INTRODUCTION

Sustaining beef cattle production systems on range-lands such as the Chihuahuan Desert is challenging. Bos indicus-influenced cattle appear favorable to Brit-ish-Bos taurus cattle in these types of environments because heterosis helps them accommodate to high am-bient temperature and poor forage quality conditions (Winder et al., 1992; O’Rourke et al., 1995a,b,c).

Differences in body size and milk production in cattle result in different nutrient requirements for growth and reproduction (Arango and VanVleck, 2002; MacNeil, 2005; Calegare et al., 2009). Because of the relationship between body size and estimates of efficiency, variables

Growth characteristics, reproductive performance, and evaluation of their associative relationships in Brangus cattle managed

in a Chihuahuan Desert production system1

P. Luna-Nevarez,*2 D. W. Bailey,* C. C. Bailey,* D. M. VanLeeuwen,† R. M. Enns,‡ G. A. Silver,* K. L. DeAtley,* and M. G. Thomas*3

*Department of Animal and Range Sciences, and †Department of Economics and International Business, New Mexico State University, Las Cruces 88003; and ‡Department of Animal Science,

Colorado State University, Fort Collins 80523

1 Financial support provided by New Mexico Agricultural Experi-ment Station (Las Cruces) Project 216391. Collaboration developed from membership in the Western Educational/Extension and Re-search Activity committee for Beef Cattle Breeding (WERA-1). The authors acknowledge L. A. Holland, B. J. Rankin, and J. A. Winder of New Mexico State University for their herd management and data collection efforts. The paper is from the Western Section American Society of Animal Science writing workshop.

2 Current address: Departamento de Ciencias Agronómicas y Vet-erinarias, Instituto Tecnológico de Sonora, 5 de Febrero #818 Sur, CP 85000, Ciudad Obregón, Sonora, México.

3 Corresponding author: [email protected] September 30, 2009.Accepted January 19, 2010.

1891

Published December 4, 2014

associated with mature BW and age were developed (e.g., asymptotic age and BW; López de Torre et al., 1992; Kaps et al., 1999, 2000; Kratochvilova et al., 2002). Understanding factors that affect rate of ma-turity could thereby help identify the optimal genetic types needed for beef cattle production in specific envi-ronments and systems.

Since the late 1970s, the beef cattle inventory in the United States has decreased while the annual quan-tity of beef produced has increased, suggesting that cow size increased coincident with larger beef carcasses (Hughes, 2001). This trend may not be advantageous for beef cows grazing arid and semiarid rangeland be-cause optimal biological efficiency was observed in cows that were moderate in both BW and stature (Kattnig et al., 1993). Winder et al. (2000) reported similar find-ings in heat-tolerant composite breeds evaluated in the Chihuahuan Desert. Our goal was to learn more about factors influencing the performance of beef cattle man-aged in arid-land production systems. Specifically, the first objective was to evaluate trends in growth char-acteristics and reproductive performance in a Brangus herd managed in a Chihuahuan Desert production sys-tem from 1972 to 2006. A secondary objective was to evaluate relationships between growth characteristics and reproductive traits in this herd.

MATERIALS AND METHODS

Animals were handled and managed according to Institutional Animal Care and Use Committee guide-lines.

Experiment Station Objectives and Cattle

The New Mexico State University (NMSU) Brangus breeding program was established in 1966, and herd registration with the International Brangus Breeders Association began in 1979 at the Chihuahuan Desert Rangeland Research Center (CDRRC). The CDRRC is composed of extensive rangeland containing 19 pas-tures with an average size of 1,249.7 ± 222 ha. The 74-yr mean annual precipitation is 234 ± 11 mm. The experimental objective for this herd from 1966 to 1987 was to study crossbreeding and biological efficiency (Winder et al., 1992; Kattnig et al., 1993). Subsequent-ly, heat-tolerant composite breeds of Brangus, Barzona, and Beefmaster were evaluated for desert rangeland performance and diet selection characteristics (1988 to 1996; De Alba-Becerra et al., 1998; Winder et al., 2000). From 1997 through 2009, the research objective was to enhance knowledge of physiological genetics and performance by cattle in a desert production system, which included grazing distribution (Obeidat et al., 2002; Shirley et al., 2006; Thomas et al., 2007a,b; Gar-rett et al., 2008; Bailey et al., 2010).

Spring-born calves were tagged and weighed short-ly after birth and were branded and vaccinated for clostridial diseases at approximately 60 d of age. Calves were fall weaned at approximately 205 d, except when drought forced early weaning. Some limited culling (ap-proximately 10% of each calf crop) occurred at weaning on the basis of unfavorable birth (>45 kg) and weaning (<181 kg) weights, conformation, disposition, and bull calf sheath score.

From 1972 to 1987, heifer development consisted of grazing rangeland at the CDRRC from weaning to ex-posure to first breeding as yearlings. After these years and during the years of 1988 to 2006, weaned heifer calves were transported approximately 35 km to the NMSU campus farm each autumn for evaluation of postweaning BW gain, as described by Lopez et al. (2006) and Shirley et al. (2006). Similarly, weaned bulls were transported to the NMSU campus farm and evalu-ated in a postweaning BW gain test using the proce-dures described by Thomas et al. (2002, 2007b) and Garrett et al. (2008), which involved feeding the bulls to achieve a BW gain of 1.6 kg/d. Postweaning BW gain evaluations ended when cattle were approximately 365 d of age. Sire selection and heifer replacement deci-sions were based primarily on growth performance be-fore 1997; however, after 1997, these decisions included fertility records of the dams that grazed at the CDRRC, growth rate, and carcass trait genetic merit, which was derived from ultrasound measures. Growth trait EPD information and familial diversity (i.e., effort to mini-mize inbreeding) were also used as selection criteria. Breeding values of AI and natural-service sires from this breeding program are described in Table 1. Four of the AI sires were derived from this breeding program and had records for natural-service mating. On average, 18 ± 2.3% of the females were mated using AI each year; this value was small because of the limited ability to gather the cows from extensive rangeland and apply estrous synchronization procedures.

Heifers were estrous synchronized and artificially in-seminated (approximately April 15) based on estrus detection, and then transported from the campus farm to the CDRRC for natural-service breeding to calving-ease sires (i.e., International Brangus Breeders Asso-ciation birth weight EPD <0.5 kg). For calving as 2 yr olds, heifers were returned to NMSU, where they were pen-fed a mix of alfalfa and sudangrass hay dur-ing the months of February, March, and April. This management system allowed students to assist with dystocia when necessary. After calving, these females were estrous synchronized and bred by AI to the sires described in Table 1. After this procedure, they were again returned to the CDRRC for grazing and natural-service mating. Cows were used in various grazing stud-ies throughout their lives (Winder et al., 1992, 2000; Beck et al., 2007; Khumalo et al., 2007; Thomas et al., 2007a). Body weight and BCS data (range: 1 = emaci-

Luna-Nevarez et al.1892

ated, and 9 = obese) were collected each autumn at weaning and the weaning percentage was determined.

Natural-service breeding was from May 1 to August 1 each year in all ages of females grazing at the CDRRC. The breeding season of mature cows was preceded by estrous synchronization and AI in pastures and groups of cows that were accessible in this extensive grazing environment. Pregnancy percentage was determined by rectal palpation for heifers and cows at least 60 d af-ter the end of each breeding season. First-calving data were collected and used to calculate age at first calving. These data were also used to determine the percentage of heifers that calved at 2 yr of age. The difference in days between the first and second calving dates was reported as the calving interval. Before 1997, cows were culled from the herd if they failed to become pregnant after a second breeding season. After 1997, any female not pregnant at the time of autumn pregnancy palpa-tion was culled.

Nutritional management at the CDRRC included protein supplementation of approximately 30% CP and 70% TDN on a DM basis during peak lactation (March 1 to May 1; 0.9 kg/cow per day), followed by a low-protein, high-energy supplement of approximately 17% CP and 79% TDN fed on a DM basis after May 1 until the beginning of summer rains (i.e., approximately July 1), which initiated forage growth. These supplements and feeding strategies were initially described by Obei-dat et al. (2002), and the supplements were purchased each year from local companies willing to provide com-petitive price bids for the feeds. Annual precipitation from 1972 to 2006 was 265 ± 13 mm, and stocking rate was 0.03 ± 0.01 animal unit/ha, which was a target forage use amount of ≤30% unless grazing experimen-tation required heavier use (Winder et al., 1992, 2000; Beck et al., 2007; Khumalo et al., 2007; Thomas et al., 2007a). Forage composition and fecal nutrient composi-tion for these native rangelands were described by De Alba Becerra et al. (1998), Obeidat et al. (2002), and Bailey et al. (2010). Typically, perennial grasses ranged from 3 to 5% CP during dormancy and from 6 to 10% CP during the growing season (e.g., DM basis). The NDF and ADF values averaged approximately 59 ± 3.0 and 36 ± 2.0%, and approximately 62 ± 6.2 and 32 ± 3.2% during these seasons, respectively. Nutrient com-position of forbs was extremely variable on these range-

lands, depending on the pattern of rainfall. If the typi-cal precipitation did not begin by August 1 (drought ≤50% normal precipitation), calves were early weaned in August or September rather than in October. This procedure was implemented in 1973, 1975, 1989, 1994, 1995, 2001, 2002, and 2003 to prevent excessive grazing of grasses and other forage and to ensure sustainability of the rangeland condition.

Data and Traits

The data evaluated in this study were collected from 1972 through 2006, which included records of weaned bull and heifer calves, and mature cows. From 1972 to 1987, the Brangus herd was used for crossbreeding studies (Winder et al., 1992); therefore, in the current study, analyses of developing bull (n = 597) and heifer (n = 585) data included only the years from 1988 to 2006, when postweaning BW gain tests were conduct-ed at the NMSU campus farm. These data included birth weight, 205-d BW, and 365-d BW, which were adjusted for age and age of dam (Beef Improvement Federation, 2006). The ADG was calculated for bulls and heifers as the difference between initial and final BW divided by the number of days (approximately 112 d) in the postweaning growing program. Breeding soundness exams were completed on the bulls at the end of the postweaning BW gain tests, which included measurements for the trait scrotal circumference. Cow data included BCS, BW, and pregnancy percentage, which were collected each autumn via rectal palpation. Growth curve parameters (e.g., asymptotic BW, as-ymptotic age, and maturing rate index) were calculated for each cow. These estimates describe the locations on a growth curve where the animal reached maturity and the rate it took for the animal to reach maturity.

Reproductive information from heifers included the binary traits of pregnant as a yearling, calved as a 2-yr-old heifer, and primiparous cows that rebred (1 = yes, 0 = no). These traits were reported as percentages on a herd-wide basis, as were these types of traits in cows (i.e., pregnancy, calving, and weaning). Two numeric traits, age at first calving and calving interval (i.e., number of days between the first and second calving of a heifer), were also recorded. Dystocia scores, using the

Table 1. Arithmetic mean EPD and accuracy ± SE from Brangus sires used in a Chi-huahuan Desert beef production system from 1972 to 20061

Trait

AI (n = 47) Natural service (n = 40)

EPD Accuracy EPD Accuracy

Birth weight, kg 0.01 ± 0.01 0.69 ± 0.03 0.26 ± 0.1 0.46 ± 0.02205-d BW, kg 9.42 ± 0.9 0.71 ± 0.03 6.55 ± 1.0 0.49 ± 0.02365-d BW, kg 17.08 ± 1.5 0.58 ± 0.03 12.25 ± 1.5 0.35 ± 0.02Milk, kg 3.23 ± 0.4 0.59 ± 0.03 1.52 ± 0.5 0.32 ± 0.02

1Data obtained from International Brangus Breeders Association (2008).

Performance trends in desert Brangus cattle 1893

Guidelines for Uniform Beef Improvement Programs (Beef Improvement Federation, 2006; 1 = no assistance, to 5 abnormal presentation), were recorded from 1972 to 2006. Average scores were 1.01 ± 0.01 (n = 684); however, because there was so little dystocia, and sub-sequently limited variability in the scores, these data were not subjected to statistical analyses.

Nonlinear logistic functions (PROC NLIN; SAS Inst. Inc., Cary, NC) were used to estimate growth curve parameters of each cow, using 6 yr of data and the pro-cedures of Kaps et al. (1999, 2000) and Kratochvilova et al. (2002). In brief, asymptotic BW, an estimate of the BW at which the growth curve of a cow reached its asymptotic point, was calculated with the formula Wt = A/(1 + b0e

−kt), where Wt is BW at a given time (t) in years, A predicts asymptotic BW, b0 is the scaling parameter relating BW at time t = 0 (i.e., at birth) to mature size, e is the natural logarithm base, and k is the curve parameter of the ratio of maximum growth rate to mature size. The variable k is described herein as the maturing rate index. Asymptotic age, which was used to describe the age at which a cow reached 99% of mature BW, was calculated with the equation 1/k{ln b − ln[(0.99)−1 − 1]}.

Statistical Procedures

Statistical analyses were completed with SAS soft-ware. The MEANS procedure was used to compute arithmetic means ± SE of continuous traits. Normality of data distribution and equality of variances of these traits were evaluated using PROC UNIVARIATE, the Levene test, and PROC GPLOT, respectively. Binary trait percentages were calculated using PROC FREQ.

Bull and Heifer Trait Analyses. Mixed-effects models in PROC MIXED were used to determine whether continuous dependent variables regressed against year (i.e., 1988 to 2006) fitted linear, quadratic, or cubic trends. Dependent variables (continuous traits) were birth weight, 205- and 365-d BW, ADG, age at first calving, calving interval, and scrotal circumfer-ence. Logistic regression using PROC GLIMMIX was used for similar analyses for the binary traits pregnant as yearling, calved as 2-yr-old heifer, and primiparous cow that rebred, which were expressed as percentages. These traits were also evaluated within PROC MIXED to obtain polynomial equations synonymous with the outputs from analyses of continuous traits. The statisti-cal model included the fixed effect of year (from 1988 to 2006), the covariate effect of nominal day of birth (i.e., birth date), the fixed effect of age of dam categories (Beef Improvement Federation, 2006; i.e., 2, 3, 4 to 10, or 11 yr and older), and the random effect of sire. Year was coded with the formula year = calendar year − 1900 because of the limit-central theorem, which arises when the second derivative of the polynomial vanishes at maximum or greater levels of n.

To determine the most descriptive term for a trait regressed against year, the model was executed sequen-

tially with each of these codes: linear = year, quadratic = year × year, and cubic = year × year × year. The higher order term was chosen based on significance (P < 0.05) of the Type I error test. Means ± SE of each variable were calculated with least squares procedures. These procedures were also executed for the following cow trait and growth curve analyses.

Cow Trait Analyses. Mixed-effect models in PROC MIXED and PROC GLIMIX for predicting continuous and binary traits were used to analyze changes across time (i.e., 1972 to 2006) for autumn cow BW and pregnancy percentage. The model in-cluded the fixed effects of year (1972 to 2005), the covariate BCS, age at the time of measurement in days, and the random effect of sire. Grazing treat-ment was also tested as a source of variation in this model; however, it was not significant and was there-fore omitted (Beck et al., 2007; Thomas et al., 2007a).Year was coded with the following formula: year = calendar year − 1900. When regression analyses re-vealed a quadratic trend with year, the first derivative was used to estimate the maximum or minimum of the curve. Specifically, for f(x) = ax2 + bx + c, setting the first derivative, f′(x) = 2ax + b, to zero produced the year, x = −b/2a, at which the minimum or maxi-mum response was achieved. Finding f(yearmaximum) or f(yearminimum) then provided the estimate of the maxi-mum or minimum mean.

Analyses of Cow Growth Curve Traits. Be-cause of the limited number of purebred Brangus cows in the early years of this study, data were pooled into groups of cows born from 1972 to 1976, 1977 to 1980, 1981 to 1984, 1985 to 1988, 1989 to 1992, and 1993 to 1996. Cow numbers included in each of these groups averaged 87.5 ± 8.8. In addition, cows born after 1996 were omitted from these analyses because of the lim-ited numbers of cows available after destocking this ex-periment station during a drought from 2001 to 2003. A mixed model in PROC MIXED was used to evalu-ate trends across years in asymptotic BW, asymptotic age, and maturing rate index. The model included the fixed effect of year groupings from 1972 to 1996 and the random effect of sire. The linear, quadratic, and cubic effects of year were tested as described in previous sec-tions, and derivative calculations using the quadratic function were estimated when appropriate.

Correlation Analyses. To further evaluate as-sociative relationships among continuous traits within each class of animals, residual correlations were com-puted using the procedures of Bailey et al. (2001) and VanWagoner et al. (2006). The RESIDUAL option within PROC MIXED was used to compute the devi-ance from the mean of each value for each animal and trait, using the models described previously. Correla-tions were then estimated using these residual results and the Pearson correlation with PROC CORR. The associations among estimates of cow growth curve pa-rameters and primiparous cow reproductive traits were also estimated with this procedure.


RESULTS

Arithmetic means ± SE of performance and repro-ductive traits in bulls and heifers from 1988 to 2006 are reported in Table 2. This table also includes BW, BCS, reproductive and weaning traits, and descriptors of the growth curve from cows born from 1972 to 2006. Arithmetic means ± SE of autumn BW, BCS, and pregnancy percentage in cows by age classification are shown in Table 3.

Bull and Heifer Trait Analyses

Year, numeric day of birth within a year, age of dam, and sire were important (P < 0.05) sources of variation in predicting performance and reproductive traits in de-veloping bulls and heifers. A slight cubic response (P < 0.05) across years was observed in the traits evaluated, with values typically declining from the mid-1990s to 2000 and increasing by 2006 (Figures 1 and 2; note the years described as drought and the regression equation

Table 2. Number of observations and arithmetic means ± SE for performance and re-productive traits in Brangus bulls, heifers, and cows managed in a Chihuahuan Desert beef production system

Trait n Repeated records, n Mean ± SE

Bulls (1988 to 2006) Birth weight, kg 597 37.2 ± 0.2 205-d BW, kg 542 242.4 ± 1.9 365-d BW, kg 438 453.2 ± 2.1 ADG, kg/d 438 1.5 ± 0.1 Scrotal circumference, cm 422 34.7 ± 0.1Heifers (1988 to 2006) Birth weight, kg 585 35.4 ± 0.2 205-d BW, kg 560 221.4 ± 1.5 365-d BW, kg 448 327.4 ± 1.8 ADG, kg/d 448 0.9 ± 0.1 Age at first calving, d 262 722.4 ± 1.9 Calving interval, d 142 414.9 ± 5.4 Pregnancy, % 385 80.7 ± 2.9 Calved as 2 yr old, % 406 71.3 ± 3.4 First-calf heifer rebreeding, % 396 79.1 ± 3.1Cows (1972 to 2006) Autumn BW, kg 525 2,611 499.6 ± 0.1 Autumn BCS 525 2,611 4.9 ± 0.1 Pregnancy, % 525 2,611 84.8 ± 1.0 Calving, % 525 2,611 81.8 ± 1.2 Weaning, % 525 2,611 78.0 ± 1.3 Asymptotic BW, kg 525 1,606 530.9 ± 5.2 Asymptotic age, yr 525 1,606 3.7 ± 0.2 Maturing rate index1 525 1,606 1.7 ± 0.1

1Ratio between the maximum observed growth rate and the BW at maturity (e.g., large values describe early maturing cows).

Table 3. Number of observations and arithmetic means ± SE for autumn BW, BCS, and pregnancy percentage by Brangus cow age in a Chihuahuan Desert beef production system from 1972 to 2006

Cow age, yr n Autumn BW, kg Autumn BCS1 Pregnancy, %

1 561 442.8 ± 3.4 5.74 ± 0.03 80.7 ± 2.92 471 455.4 ± 3.8 5.26 ± 0.06 79.1 ± 3.13 363 494.8 ± 4.6 4.99 ± 0.08 85.4 ± 3.04 290 511.1 ± 5.0 5.00 ± 0.08 87.7 ± 3.15 233 525.0 ± 6.6 5.22 ± 0.09 88.9 ± 3.16 187 541.5 ± 7.8 5.35 ± 0.14 89.4 ± 3.17 143 551.2 ± 6.9 5.26 ± 0.13 91.8 ± 3.28 119 549.3 ± 8.3 5.55 ± 0.15 88.0 ± 3.19 88 546.6 ± 8.8 5.19 ± 0.14 88.6 ± 3.110 74 545.0 ± 9.1 5.15 ± 0.16 77.4 ± 2.711 51 546.6 ± 10.8 5.23 ± 0.18 76.5 ± 2.712 31 506.6 ± 20.4 5.18 ± 0.29 72.2 ± 2.5

1Range: 1 = emaciated; 9 = obese.


Figure 1. Least squares means ± SE adjusted for age of dam (aod) and age (birth date; bd) across years in traits of growing Brangus bulls (n = 597) managed in a Chihuahuan Desert beef production system from 1988 to 2006. X indicates years of drought, which caused early weaning.


Figure 2. Least squares means ± SE adjusted for age of dam (aod) and age (birth date; bd) across years in continuous traits of growing Bran-gus heifers (n = 585) managed in a Chihuahuan Desert beef production system from 1988 to 2006. X indicates years of drought, which caused early weaning.


in the panel of each trait). In contrast, calving interval decreased linearly by approximately 60 d across years (P < 0.01; Figure 2). For categorical heifer traits, a positive linear trend was observed across years for the traits pregnant as yearling (P < 0.01), calved as 2-yr-old heifer (P < 0.05), and primiparous cow that rebred (P < 0.05; Figure 3). Regression coefficients suggested these trait levels increased by 1.4, 0.7, and 1.1% each year from 1988 to 2006.

Cow Trait Analyses

Year, BCS, age at measurement, and sire influenced variation (P < 0.01) in autumn BW and pregnancy per-centage in cows. Autumn BW gradually increased from 1972 until 1997, reaching a maximum BW of 509.5 kg, and then gradually decreased by 0.6 kg/yr through 2006

(Figure 4). Pregnancy percentage decreased gradually from 1972 until 1995 to a minimum of 78.4%, and then increased slightly by 0.2%/yr through 2006. A quadrat-ic effect (P < 0.01) was the best-fitting descriptor of these traits across years (Figure 4). Similar results were observed when these analyses were computed in cows that were 3 yr of age (data not presented).

Analyses of Cow Growth Curve Traits

Year groupings and sire were significant (P < 0.05) sources of variation in the mixed model analyses of as-ymptotic BW, asymptotic age, and maturing rate in-dex. Asymptotic BW increased gradually from 1972 until 1990 (P < 0.01), reaching a maximum of 572.4 kg and then becoming static through 1996. Asymptot-ic age increased gradually from 1972 until 1983 (P <

Figure 3. Categorical reproductive trait measures adjusted for age of dam (aod) and age (birth date; bd) across years in Brangus heifers (n = 585) managed in a Chihuahuan Desert beef production system from 1988 to 2006. X indicates years of drought, which caused early weaning.


0.05), reaching a maximum of 4.0 yr and then decreas-ing to 2.9 yr (approximately 27.0%) through 1996. A quadratic effect described the phenotypic trends across years for asymptotic BW and asymptotic age (Figure 5). Maturing rate index increased quadratically across years (P < 0.05) from a ratio of 1.5 to 2.2 ± 0.2 in heif-ers born on or before 1985 relative to heifers born on or after 1986; larger values for the maturing rate index reflect earlier maturity.

Correlation Analyses

Residual correlation analyses suggested moderate positive associations (P < 0.01) among BW traits and scrotal circumference in developing bulls (Table 4). Similar associations (P < 0.01) were observed among growth traits in females, which included asymptotic BW (Table 5); however, moderate negative associa-tions (P < 0.01) were observed among traits describ-

Figure 4. Autumn BW ± SE and pregnancy percent adjusted for BCS and cow age across years in Brangus cows (n = 525; repeat records, n = 2,533) managed in a Chihuahuan Desert beef production system from 1972 to 2006. An asterisk (*) indicates extreme values. X indicates years of drought, which caused early weaning.

Table 4. Residual correlations among BW traits in Brangus bulls managed in a Chihuahuan Desert beef produc-tion system from 1988 to 2006

Trait Birth weight 205-d BW 365-d BW ADG Scrotal circumference

Birth weight 1.00 0.23** 0.21** 0.14** 0.07205-d BW — 1.00 0.70** 0.12* 0.42**365-d BW — — 1.00 0.47** 0.50*ADG — — — 1.00 0.13*Scrotal circumference — — — — 1.00

*Correlations are different from 0, P < 0.05. **Correlations are different from 0, P < 0.01.


ing postweaning BW gain and asymptotic age. More-over, negative associations (P < 0.01) were detected among the variables asymptotic BW, asymptotic age, and maturing rate index. In young females, age at first calving was negatively and moderately associated with asymptotic age, maturing rate index, and 365-d BW. This type of relationship was also detected for calving interval and maturing rate index (Table 6).

DISCUSSION

Our first objective was to analyze phenotypic trends across years in growth characteristics and reproductive performance in Brangus cattle managed in a Chihua-huan Desert production system from 1972 to 2006. Cat-tle were managed exclusively on rangeland from 1972 to 1988 (Winder et al., 1992). In 1988, bull and heifer

Figure 5. Asymptotic BW and age across years ± SE in Brangus cows (n = 525; repeat records, n = 1,606) managed in a Chihuahuan Desert beef production system from 1972 to 2006. An asterisk (*) indicates extreme values. X indicates years of drought, which caused early weaning.

Table 5. Residual correlations among BW traits in Brangus females managed in a Chihuahuan Desert beef pro-duction system from 1972 to 2006

ItemBirth weight 205-d BW 365-d BW ADG

Asymptotic BW

Asymptotic age

Maturing rate index

Birth weight 1.00 0.18** 0.63** 0.44** 0.60** –0.21 0.24*205-d BW — 1.00 0.45** –0.26** 0.47** –0.08 0.19**365-d BW — — 1.00 0.74** 0.47** –0.26** 0.29**ADG — — — 1.00 0.29** –0.29** 0.36**Asymptotic BW — — — — 1.00 –0.02 –0.30**Asymptotic age — — — — — 1.00 –0.78**Maturing rate index — — — — — — 1.00

*Correlations are different from 0, P < 0.05. **Correlations are different from 0, P < 0.01.


development programs were implemented, and slight changes in pre- and postweaning growth performance were observed from 1988 to 2006 (i.e., see traits of birth weight, 205- and 365-d BW, and postweaning ADG). These trait levels were greatly influenced by year, which was most likely a result of extremely dynamic seasonal rainfall amounts influencing forage production and range conditions. Excessive rainfall or drought in the Chihuahuan Desert can influence forage production for several consecutive years; thus, decreased animal growth rates in drought years were expected and ob-served in association with reduced forage quantity and quality. These years were identified, and the reports of Winder et al. (2000), Beck et al. (2007), (Khumalo et al. (2007), and Thomas et al. (2007a) describe the rela-tionships between rainfall dynamics, forage production, and cattle performance in the Chihuahuan Desert.

Trends across years in growth traits in these Brangus cattle were minimal relative to a multibreed producer-managed bull test facility in Tucumcari, NM (Garcia et al., 2004; Decker et al., 2008); however, data used in the current study were collected each year from all animals within this Chihuahuan Desert experiment sta-tion herd, whereas data collected from this public bull test facility were compiled each year from a few select-ed bull calves from numerous seed stock herds in New Mexico and surrounding states. The EPD of the sires used in this Brangus herd were included so that the level of genetic merit can be understood relative to the base EPD level of Brangus cattle established in 1973. It should be noted that sire was a significant source of variation in many of the analyses conducted. Given the relatively large growth EPD values of sires used in the breeding program, the harsh environmental conditions at the CDRRC may have limited improvement in phe-notypic measures of growth. This observation warrants further study of these data, particularly an evaluation of genetic improvement relative to phenotypic trait ex-pression.

Results from the Tucumcari Bull Test suggested that birth weight was relatively static from 1961 to 2000 (Garcia et al., 2004). Because the cattle in these stud-ies were from extensive rangeland production systems, genetic selection decisions probably placed considerable

emphasis on the need for cows to calve without assis-tance. There is evidence to suggest that selection for decreased birth weight and increased yearling weight can improve production efficiency through enhanced maternal performance (MacNeil et al., 1998, 2000). The incidence of dystocia was minimal in the Brangus cattle of the CDRRC, and other measures of heifer fer-tility exhibited improvement (i.e., pregnancy, calving, rebreeding percentage, and calving interval). Reports by Reynolds et al. (1991), Colburn et al. (1997), and Freetly and Cundiff (1998) described collateral observa-tions of reduced dystocia and improved rebreeding rate in heifers or young cows; however, a slight increase in age at first calving was observed in the present study. Because the breeding season of this herd was 90 d, a slight increase in age at first calving could occur with minimal detrimental effects on other measures of fertil-ity.

Improvement in the traits describing heifer fertil-ity were expected and observed in this herd after 1997 because selection and management required heifers to become pregnant as yearlings and each year thereafter. Nonetheless, improvement in primiparous cow rebreed-ing success was expected to be difficult because of the challenges of grazing desert rangeland (Hawkins et al., 2000; Winder et al., 2000; Beck et al., 2007), despite the fact that 2-yr-old heifers had their first calf in a pen-fed management system after 1988. Approximately 20% of the cows from 3 to 10 yr of age were culled each year. As a result of the nutritional challenge in the grazing environment, cow longevity was probably less than expected for Bos indicus-influenced cows in tropi-cal and subtropical production systems (Thrift and Thrift, 2003; Chase et al., 2005; Sanders et al., 2005); however, results were similar to reports involving these types of cows grazing other arid landscapes (O’Rourke et al., 1995a,b,c; Holloway et al., 2005). It should also be noted that it was challenging for this herd to achieve a pregnancy percentage greater than 90%, even though mean BCS appeared acceptable (approximately 5.0 on a scale of 1 to 9) in each class of cow. These results are likely a function of challenging grazing conditions, which can involve exposure to the various toxic plants that exist on these desert rangelands (Pieper, 1989; Cox and Ross, 2002). Our results parallel those from various commercial beef production systems (Dargatz et al., 2004; Minick Bormann et al., 2006).

Slight opposing trends in autumn cow BW and preg-nancy percentage were observed in this arid rangeland production system. These results agree with those of Kattnig et al. (1993), whose data were also collected at this Chihuahuan Desert experiment station, and support the concepts presented by López de Torre et al. (1992), Arango and Van Vleck (2002), and Cale-gare et al. (2009), which suggested that a larger body size decreased cow productivity. Similarly, Forni et al. (2009) suggested that selection for faster growth in-creased mature size and decreased reproductive rate, which increased maternal cost per animal slaughtered.

Table 6. Residual correlations among growth curve parameters in Brangus cows and reproductive traits in heifers managed in a Chihuahuan Desert beef produc-tion system from 1972 to 2006

TraitAge at first

calvingCalving interval

Asymptotic BW −0.14 −0.14Asymptotic age −0.32** 0.11Maturing rate index −0.42** −0.18*365-d BW −0.22* −0.09

*Correlations are different from 0, P < 0.05. **Correlations are dif-ferent from 0, P < 0.01.


A noteworthy observation was the somewhat cyclic na-ture of the pregnancy percentage data underlying the quadratic response curve. This was most likely due to the influence of rainfall on forage production and sub-sequent animal performance in the Chihuahuan Desert, where an extremely high or low amount of precipitation in a specific year can influence range conditions for sev-eral subsequent years (Winder et al., 2000; Beck et al., 2007; Khumalo et al., 2007; Thomas et al., 2007a).

Because of the need to study the effects of cow BW on various efficiency traits, fertility traits, or both, procedures to estimate asymptotic age and BW were developed by Kaps et al. (1999, 2000) and Kratoch-vilova et al. (2002). These estimates describe BW and age at maturity via analysis of a growth curve. After 1983, there was a steep decrease in asymptotic age, which suggests cows matured at younger ages after this year in the current study. This trend was most likely a consequence of the selection criterion, such as en-hanced performance in the postweaning BW gain test, increased yearling heifer pregnancy percentage, or se-lection for enhanced measures of ultrasound carcass fat at 365 d of age. The management decision in 1997 to begin culling all cows each year that failed to conceive that season also may have indirectly placed selection pressure on early maturity.

Evaluation of the associative relationships between BW traits and reproductive performance was our sec-ond objective. In developing bulls, residual correla-tions were observed among growth traits and scrotal circumference, which was expected based on previously reported data from this region of the United States (Thomas et al., 2002; Garcia et al., 2004); however, ADG was not strongly correlated with BW traits (e.g., 205- and 365-d BW) across years because these bulls were fed to gain 1.6 kg/d each year. Increased scrotal circumference historically has been associated with im-provements in male and female fertility traits (Martin et al., 1992; Patterson et al., 1992; Kastelic et al., 2001; Brito et al., 2007). Nonetheless, it appears that genetic improvement in scrotal circumference has decreased the variation associated with age of puberty and made these relationships difficult to detect (Martínez-Velázquez et al., 2003; Shirley et al., 2006).

Residual correlation analyses from heifers and cows revealed moderate positive associations among BW traits (e.g., birth weight, 205- and 365-d BW) and moderate to high associations among these traits and measures of cow size (e.g., autumn BW and asymptotic BW). These relationships were expected (Kaps et al., 1999, 2000; Garcia et al., 2004). Inverse relationships were observed among measures of maturity and young female rebreeding traits, such as age at first calving and calving interval, suggesting improved fertility for a primiparous cow that achieved maturity quickly. This observation appears inverse to studies of Bos indicus females selected and managed for increased yearling weight in tropical environments (Cooke et al., 2008; Shiotsuki et al., 2009).

Several studies have reported significant correla-tions between estimates of growth curve parameters and reproductive traits in cattle (Beltrán et al., 1992; Menchaca et al., 1996; Bayram et al., 2004; Nesetrilova, 2005). Results indicated a negative association between the maturing rate index and the young cow reproduc-tive traits age at first calving and calving interval, sug-gesting that early-maturing cows were younger at first calving and had shorter calving intervals, both indi-cators of improved fertility. In addition, females with greater 365-d BW had improved age at first calving. These results may seem contradictory to the positive relationship of 365-d BW and asymptotic BW; how-ever, because this trait was negatively associated with asymptotic age, growth curves of these Brangus cattle seem to have shifted toward earlier maturity.

Results of our study with data from 1972 to 2006 suggest that Brangus cattle managed in a Chihuahuan Desert production system had slight changes in growth performance in bulls and heifers, with improvement in traits related to heifer puberty. As cow size increased, pregnancy percentage tended to decline. Growth curves of Brangus cows grazing this arid rangeland shifted to-ward an earlier age at maturity, which was associated with enhanced fertility in young cows.

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