Genetic analysis of gain from birth to weaning, milk production, and udder conformation in Line 1 Hereford cattle

The objective of this research was to partition phenotypic variation in calf gain from birth to weaning, and milk production measured, by the weigh-suckle-weigh method, and udder score of cows into genetic and nongenetic components. Data were from the Line 1 Hereford population maintained by USDA-ARS at Miles City, MT, and included observations of pre-weaning gain (n = 6,835) from 2,172 dams, milk production (n = 692) from 403 cows, and udder score (n = 1,686) from 622 cows. Data were analyzed using a Gibbs sampler for multiple-trait animal models. Results are reported as means +/- SD derived from the posterior distributions of parameter estimates. Mean estimates of the phenotypic variance of preweaning gain, milk production, and udder score were 476.3 kg2, 8.88 kg2, and 1.89 (1 to 9 scale), respectively. Estimates of phenotypic correlations between preweaning gain and milk production, preweaning gain and udder score, and milk production and udder score were 0.37 +/- 0.04, - 0.07 +/- 0.04, and - 0.09 +/- 0.05, respectively. Estimates of heritability for direct and maternal preweaning gain, milk production, and udder score were 0.13 +/- 0.03, 0.25 +/- 0.04, 0.25 +/- 0.06, and 0.23 +/- 0.05, respectively. Genetic correlations of milk production with maternal preweaning gain and udder score were estimated as 0.80 +/- 0.08 and - 0.36 +/- 0.16, respectively. Posterior distributions of the other genetic correlations all contained 0.00 within the respective 90% probability density posterior intervals. Estimates of repeatability of maternal preweaning gain, milk production, and udder score were 0.43 +/- 0.03, 0.39 +/- 0.05, and 0.34 +/- 0.03, respectively. Breeding value for maternal gain from birth to weaning was highly predictive of breeding value for milk production. Direct measurement of milk production to use in genetic improvement may not be justified because it is difficult to measure, and selection based on the breeding value for maternal preweaning gain may be nearly as effective in changing milk production as direct selection. A potentially undesirable consequence of selection to increase milk production is the degradation of udder quality. However, this correlation is not so strong as to preclude simultaneous improvement of milk production and udder quality using appropriate predicted breeding values for each trait.     

Within-herd variation in energy utilization for maintenance and gain in beef cows

Fourteen mature, nonpregnant, nonlactating Angus cows (498 kg) were individually fed through two consecutive phases (maintenance [M], 80 d and ad libitum [A], 70 to 79 d) to estimate within-herd variation in individual cow ME requirements for maintenance (MEm) and to identify factors contributing to this variation. Body composition was determined at initiation of phase M, at termination of phase M (also initiation of phase A) and at the end of phase A by a two-pool D2O dilution technique. Daily MEm averaged 156.7 kcal/kg BW.75 (SD = 18.4 kcal/kg BW.75) and efficiency of ME use for tissue gain or loss averaged 76% (SD = 30%). Estimates of ME intake to maintain 1 kg of protein or 1 kg of fat were 192.9 (SE = 24.8) or 20.7 (SE = 21.5) kcal. These data indicate that among cows of similar fat masses, those with larger protein masses had higher energy requirements for maintenance. Daily MEm was positively correlated (P less than .16) with liver weight (r = .40) and relative proportions of liver (r = .44; P less than .16) and heart (r = .48; P less than .10) in the empty body. Also, daily MEm was correlated negatively (P less than .05) with weight (r = -.71) and relative proportion of omental and mesenteric fat (r = -.78). Estimates of ME required for deposition of 1 kcal of protein or fat were 5.56 (SE = 1.01) or 1.26 (SE = .09) kcal. Weight of liver and the sum of liver, spleen, kidney and heart weights increased 1.58 (R2 = .47) and 1.95 kg (R2 = .52) per kilogram of daily weight gain during phase A. These results indicate that increased performance caused increased organ mass (liver).     

Effects of sire misidentification on estimates of genetic parameters for birth and weaning weights in Hereford cattle

Birth weights (4,155) and weaning weights (3,884) of Line 1 Herefords collected at the Fort Keogh Livestock and Range Research Laboratory in Miles City, MT, between the years of 1935 to 1989 were available. To study the effect of misidentification on estimates of genetic parameters, the sire identification of calf was randomly replaced by the identification of another sire based on the fraction of progeny each sire contributed to a yearly calf crop. Misidentification rates ranged from 5 to 50% with increments of 5%. For each rate of misidentification, 100 replicates were obtained and analyzed with single-trait and two-trait analyses with a restricted maximum likelihood (REML) algorithm. Two different models were used. Both models contained year x sex combinations and ages of dam as fixed effects, calendar birth date as a fixed covariate, and random animal and maternal genetic effects and maternal permanent environment effects. Model 2 also included sire x year combinations as random effects. As the rate of misidentification increased, estimates of the direct-maternal genetic correlation increased for both traits, with both models, for all analyses. With singletrait analyses, estimates of the fraction of variance that were due to sire x year interaction effects increased slightly for birth weight (near zero) and decreased slightly (0.015 to 0.004) for weaning weight as misidentification increased. With two-trait analyses, estimates of fraction of variance that were due to sire x year effects gradually decreased for weaning weight as misidentification increased. With the two-trait analyses, and with both models, as the level of sire misidentification increased, estimates of the genetic correlation between direct effects gradually increased, and estimates of the correlation between maternal effects gradually decreased. Estimates of the direct-maternal genetic correlation were more positive with Model 2 than with Model 1 for all levels of misidentification. Results of this study indicate that misidentification of sires would severely bias estimates of genetic parameters and would reduce genetic gain from selection.     

Synthesis of direct and maternal genetic components of economically important traits from beef breed-cross evaluations

Published information on relative performance of beef breed crosses was used to derive combined estimates of purebred breed values for predominant temperate beef breeds. The sources of information were largely from the United States, Canada, and New Zealand, although some European estimates were also included. Emphasis was on maternal traits of potential economic importance to the suckler beef production system, but some postweaning traits were also considered. The estimates were taken from comparison studies undertaken in the 1970s, 1980s and 1990s, each with representative samples of beef breeds used in temperate agriculture. Weighting factors for breed-cross estimates were derived using the number of sires and offspring that contributed to that estimate. These weights were then used in a weighted multiple regression analysis to obtain single purebred breed effects. Both direct additive and maternal additive genetic effects were estimated for preweaning traits. Important genetic differences between the breeds were shown for many of the traits. Significant regression coefficients were estimated for the effect of mature weight on calving ease, both maternal and direct additive genetic, survival to weaning direct, and birth weight direct. The breeds with greater mature weight were found to have greater maternal genetic effects for calving ease but negative direct genetic effects on calving ease. A negative effect of mature weight on the direct genetic effect of survival to weaning was observed. A cluster analysis was done using 17 breeds for which information existed on nine maternal traits. Regression was used to predict breed-cross-specific heterosis using genetic distance. Only five traits, birth weight, survival to weaning, cow fertility, and preweaning and postweaning growth rate had enough breed-cross-specific heterosis estimates to develop a prediction model. The breed biological values estimated provide a basis to predict the biological value of crossbred suckler cows and their offspring.     

The impact of direct-maternal genetic correlations on international beef cattle evaluations for Limousin weaning weight

In beef cattle maternally influenced traits, estimates of direct-maternal genetic correlations (r_dm) are usually reported to be negative. In international evaluations, r_dm can differ both within countries (r_{dm_WC}) and between countries (r_{dm_BC}). The r_{dm_BC} are difficult to estimate and are assumed to be zero in the current model for international beef cattle evaluations (Interbeef). Our objective was to investigate re-ranking of international estimated breeding values (IEBVs) in international beef cattle evaluations between models that either used estimated values for r_dm or assumed them to be 0. Age-adjusted weaning weights and pedigree data were available for Limousin beef cattle from ten European countries. International EBVs were obtained using a multi-trait animal model with countries modeled as different traits. We compared IEBVs from a model that uses estimated r_{dm_BC} (ranging between −0.14 and +0.14) and r_{dm_WC} (between −0.33 and +0.40) with IEBVs obtained either from the current model that assumes r_{dm_BC} to be 0, or from an alternative model that assumes both r_{dm_BC} and r_{dm_WC} to be 0. Direct and maternal IEBVs were compared across those three scenarios for different groups of animals. The ratio of population accuracies from the linear regression method was used to further investigate the impact of r_dm on international evaluations, for both the whole set of animals in the evaluation and the domestic ones. Ignoring r_{dm_BC}, i.e., replacing estimated values with 0, resulted in no (rank correlations > 0.99) or limited (between 0.98 and 0.99) re-ranking for direct and maternal IEBVs, respectively. Both r_{dm_BC} and r_{dm_WC} had less impact on direct IEBVs than on maternal IEBVs. Re-ranking of maternal IEBVs decreased with increasing reliability. Ignoring r_{dm_BC} resulted in no re-ranking for sires with IEBVs that might be exchanged across countries and limited re-ranking for the top 100 sires. Using estimated r_{dm_BC} values instead of considering them to be 0 resulted in null to limited increases in population accuracy. Ignoring both r_{dm_BC} and r_{dm_WC} resulted in considerable re-ranking of animals’ IEBVs in all groups of animals evaluated. This study showed the limited impact of the current practice of ignoring r_{dm_BC} in international evaluations for Limousin weaning weight, most likely because the estimated r_{dm_BC} was close to 0. We expect that these conclusions can be extended to other traits that have reported r_dm values in the range of r_{dm_WC} values for weaning weight in Limousin.     

Effects of body composition, pre- and postpartum energy level and early weaning on reproductive performance of beef cows and preweaning calf gain

Mature Charolais x Angus rotational cross cows were adjusted to moderate body condition by d 190 of gestation then randomly blocked to a maintenance (ME) or low-energy (LE) diet. At parturition, the 128 cows were randomly allotted within prepartum (PRP) diet to a high-energy (HE) or LE diet. At d 30 postpartum (PP), cows were randomly blocked to two treatments in which calves were weaned early (EW) or normally (NW) at 7 mo of age. Cows receiving a LE PRP diet had lighter calves at birth (34.7 vs 39.0 kg) and 105 d (127.9 vs 144.6 kg). Prepartum and PP energy interacted to affect postpartum anestrous interval (PPI, d) and cycling activity (%), respectively (LE-LE = 72.6, 33.3; LE-HE = 54.3, 56.3; ME-LE = 65.7, 52.9; ME-HE = 68.4, 54.3). High PP energy averaged over PRP diet increased (P less than .10) pregnancy rate by 22.7% and 105-d calf weight by 15.1 kg. Early weaning reduced PPI by 24.3 d (P less than .01) and first service conception rate by 21.7% (P less than .10). Cycling activity within 60 d PP was affected (P less than .01) by PRP diet and suckling status (LE-EW = 62.5, LE-NW = 26.7, ME-EW = 88.9, ME-NW = 13.3%). Thin cows had a longer PPI but had a higher first service conception rate than moderate and fleshy cows. Higher pregnancy rates were observed in cows approaching or maintaining average body condition from parturition to conception than for cows moving away from moderate body condition. Results suggest that fleshy and thin cows at parturition should be managed to approach moderate body condition before the breeding season to optimize reproductive performance and preweaning calf gain.     

Selection for weaning weight and postweaning gain in Hereford cattle. II. Response to selection

Single trait selection was practiced in three lines of Hereford cattle at two locations. Bulls were selected within sire families for increased weaning weight (WW) in the WW line (WWL), for postweaning gain (PG) in the PG line (PGL) and at random in the control line (CTL). Data include the performance of 2,467 calves produced from 1967 to 1981. Environmental effects were estimated from CTL (method I) and from multiple regression procedures (method II). Phenotypic and environmental time trends were negative for WW and generally were positive for PG. Estimated genetic gains for WW in WWL were 1.07 +/- .51 kg/yr in bulls and .62 +/- .36 kg/yr in heifers using method I and .50 +/- .31 kg/yr in bulls and .10 +/- .17 kg/yr in heifers using method II. Corresponding values for PG in PGL were .85 +/- .40 and 1.03 +/- .24 kg/yr in bulls and .30 +/- .28 and .37 +/- .12 kg in heifers. Correlated genetic gains for WW in PGL were larger than direct WW gains, whereas genetic gains for PG in WWL were smaller than direct PG gains. From method I, estimates of realized heritability (h2R) for WW were .31 +/- .18 in bulls and .22 +/- .13 in heifers. For PG, h2R was .31 +/- .13 in bulls and .06 +/- .12 in heifers. Using method II, h2R for WW was .09 +/- .08 in bulls and .02 +/- .07 in heifers. Corresponding values for PG were .29 +/- .10 and .11 +/- .08. Joint estimates of the realized genetic correlation between WW and PG were .69 +/- .18 and .46 +/- .31 for methods I and II, respectively. Variation in selection response was evaluated using quasi-replicates. Results of this study indicate that selection for PG improved both WW and PG faster than selection for WW.     

Genetic relationships among direct and maternal components of milk yield and maternal weaning gain in a multibreed beef herd.

Data spanning 1980 to 1993 from a multibreed beef herd including primarily eight breeds (Angus, Charolais, Gelbvieh, Hereford, Maine-Anjou, Pinzgauer, Simmental, and Tarentaise) were used to obtain 2,207 records on 200-d weaning gain (WG) and 1,826 records on 200-d milk yield (MY), obtained by machine milking after oxytocin injection. Estimates of (co)variances for the two traits (WG and MY) were obtained with REML with breed of calf, breed of cow, and heterotic effects modeled for the two traits. Animal effects of calf (CalfWG, CalfMY) and cow (CowWG, CowMY) contributions to each trait were modeled including 2,926 animals. The permanent environmental effect of the cow was modeled for MY, with 693 levels. Estimates of breed differences were generally similar to literature estimates. Simmental, Charolais, and Maine-Anjou were highest for CalfWG, and Tarentaise, Simmental, Gelbvieh, and Maine-Anjou were highest for CowMY. Heterosis was estimated at 8.00, 2.58, 4.05, and 5.50% of the mean for CalfWG, CowWG, CalfMY, and CowMy, respectively. Variance attributable to repeated records on CowMy represented 9% of phenotypic variance. Heritabilities estimated were .22 and .24 for CalfWG and CowWG and .04 and .35 for CalfMY and CowMY. Genetic correlations estimated between CalfWG and CowWG and between CalfMY and CowMY were -.35 and -.64, respectively. A genetic correlation between CowWG and CowMY of .76 indicates that maternal weaning gain evaluations are a good predictor of a cow's potential for milk yield.     

Genetic trends and breed overlap derived from multiple-breed genetic evaluations of beef cattle for growth traits.

Genetic evaluations for a multiple-breed population of beef cattle were used to estimate genetic trends for five breeds, and genetic differences and overlap among 14 breeds. Genetic evaluations studied were for direct contributions to birth weight, gain from birth to 200 and 365 d, and maternal contribution to gain from birth to 200 d. Almost all genetic trends were positive, but the magnitude of the trends varied among breeds. Trends were nonlinear between 1985 and 1995 for most breed and trait combinations. The rates of increase in genetic trends were generally higher for the lighter weight breeds, and lighter weight breeds had faster growth rate genetic trends at 1995 than the heavier breeds. Genetic trend estimates for yearling gain at 1995 were 2.46, 2.23, 1.73, 1.70, and 1.46 kg/yr for Angus, Hereford, Limousin, Charolais, and Simmental, respectively. Corresponding birth weight genetic trends were .130, .226, .049, .130, and .048 kg/yr. Mean genetic differences between breeds have been decreasing in magnitude due to these differences in genetic trends between heavier and lighter breeds. Genetic variation for the traits studied seemed to be greater within than between breeds for calves born and cows calving between 1993 and 1995. Genetic trends at 1995 suggest that ratios of within:between breed variation will increase and that across-breed genetic improvement initiatives for growth traits will become more important in the future.     

Genetic parameters for stayability, stayability at calving, and stayability at weaning to specified ages for Hereford cows

Genetic parameters for stayability to six ages (ST1, . . ., ST6), for five measures of stayability to calving (SC2, . . ., SC6), and for five measures of stayability to weaning (SW2, . . ., SW6), were estimated using records of 2,019 Hereford cows collected from 1964 to 1979 from a selection experiment with a control line and three lines selected for weaning weight, yearling weight, and an index of yearling weight and muscle score. The model included birth year of the cow as a fixed effect and the cow's sire as a random effect. Analyses were performed with 1) a generalized linear mixed model for binary data using a probit link with a penalized quasi-likelihood function, and 2) with a linear mixed model using REML. Genetic trends were estimated by regressing weighted means of estimated transmitting abilities (ETA) of sires by birth year of their daughters on birth year. Environmental trends were estimated by regressing solutions for year of birth on birth year. Estimates of heritability (SE) for ST were between 0.09 (0.08) and 0.30 (0.14) for threshold model and between 0.05 (0.04) and 0.19 (0.09) for linear model. Estimates of heritability from linear model analyses transformed to an underlying normal scale were between 0.09 and 0.35. Estimates of heritability (SE) for SC were between 0.29 (0.10) and 0.39 (0.11) and between 0.18 (0.09) and 0.25 (0.08) with threshold and linear models. Estimates of heritability transformed to an underlying normal scale were between 0.30 and 0.40. Estimates of heritability (SE) for SW were between 0.21 (0.14) and 0.47 (0.19) and between 0.12 (0.08) and 0.26 (0.12) with threshold and linear models, respectively. Estimates of heritability transformed to an underlying normal scale were between 0.21 and 0.50. Estimates of genetic and environmental trends for all lines were nearly zero for all traits. Correlations between ETA of sires for stayability to specific ages, for stayability to calving, and for stayability to weaning with threshold and linear models ranged from 0.09 to 0.82, from 0.68 to 0.90, and from 0.67 to 0.87, respectively. Selection for stayability would be possible in a breeding program and could be relatively effective as a result of the moderate estimates of heritability, which would allow selection of sires whose daughters are more likely to remain longer in the herd. Selection for weaning and yearling weights resulted in little correlated response for any of the measures of stayability.     

Milk yield of primiparous beef cows from three calving systems and varied weaning ages

Primiparous beef cows produced in 3 calving systems were used in a 2-yr study with a completely random design to measure milk yield throughout a 190-d lactation (2002, n = 20; 2003, n = 24 per calving system). Calving occurred in late winter (average calving date = February 4 +/- 2 d), early spring (average calving date = March 30 +/- 2 d), and late spring (average calving date = May 26 +/- 1 d). Additionally, cows used in this study had been weaned at varied ages as calves, creating 6 dam treatments. Dam age at weaning was 140 (late spring), 190 (late winter, early spring, late spring), or 240 (late winter, early spring) d of age. Milk production was measured by using the weigh-suckle-weigh technique at an average of 20, 38, 55, 88, 125, 163, and 190 d in milk. Milk yield for the 190-d lactation period was calculated as area under the curve by trapezoidal summation. Data were analyzed with a model containing treatment, year, and their interaction. Orthogonal contrasts were used to separate effects when treatment was significant (P < 0.10). Total milk yield did not differ (P = 0.42) between cows in the late winter and early spring systems, but cows in the late spring system tended to differ (P = 0.09) from the average of the other 2 systems. Cows in the late spring calving system had increased milk yield in 2002 and lesser milk yield in 2003 compared with the other calving systems (treatment x year interaction, P < 0.001). Cows born in late spring that had been weaned at 140 d of age produced more (P = 0.05) total milk than those weaned at 190 d of age. Peak milk yield was affected (P < 0.001) by treatment and showed a treatment x year interaction (P = 0.006). Day of peak lactation differed among treatments (P = 0.002), with cows in the late winter system peaking later (P = 0.007) than early spring cows, and late spring cows peaking earlier (P = 0.004) than the average of late winter and early spring cows. The average date of peak lactation was May 4 for the late winter system, May 31 for the early spring system, and July 19 for the late spring system. Calf ADG differed (P < 0.001) for the late spring system compared with the average of the late winter and early spring systems, but the relationship interacted with year (P < 0.001). Cow BW and BW change differed among treatments (P < 0.004), with much of the difference associated with the amount of milk produced or the timing of peak lactation. Season of calving affects milk yield of primiparous cows grazing Northern Great Plains rangelands and ADG of their calves.     

Synchronized estrus and fertility of beef cows after weaning calves for short intervals

In Exp. 1, the objective was to determine if interval of separating calves from cows (24 or 48 h) immediately before insemination affects detection and precision of estrus and pregnancy rates of lactating beef cows implanted with norgestomet. Separation of calves from cows for 24 h (n = 418) lengthened intervals to estrus, did not affect precision of estrus, reduced success of detecting estrus and lowered pregnancy rates relative to positive controls (48 h separation, n = 508). Cows with poor body condition, and not suckled for 24 h, conceived at lower rates than cows with similar condition that were not suckled for 48 h. Adverse effects of separation for only 24 h on fertility are apparently due to inadequate intervals between estrus and insemination at 48 h after removing implants. In Exp. 2, the objective was to determine effects of separating calves from cows for 48 h immediately before insemination on detection and precision of estrus and on pregnancy rate of ovulatory lactating beef cows injected twice with prostaglandin F2 alpha (PGF2 alpha). Weaning increased detection of estrus but overall pregnancy rates did not differ between suckled (n = 256) and nonsuckled (n = 221) cows. But, weaning calves improved pregnancy rates of young (2 to 3 yr) cows and reduced fertility among middle (4 to 6 yr)-aged cows. Increased pregnancy rates after weaning calves for 48 h are due largely to greater detection of estrus and inseminating more cows.(ABSTRACT TRUNCATED AT 250 WORDS)     

Estimation of net energy requirements (NEm and NE delta) of lactating beef cows.

Spring-calving Angus cows (n = 24) in moderate body condition were assigned to either a high (H), maintenance-high (MH) to support superior milk, maintenance-low (ML) to support average milk, or low (L) energy diet at 12 d (SD = 4) postpartum. Energy balance for individual cows was determined by body condition change, weight change, and weigh-suckle-weigh milk production estimates. High energy intake increased (P < .05) BW, body condition score (BCS), and megacalories of body energy (BE) by 94 d postpartum. Neither dietary nor BCS accounted for significant (P > .30) variation in days to first ovulation or conception. Fasting heat production was estimated to be 72.5 kcal/BW.75 from the regression of log daily heat production/BW.75 on daily ME intake/BW.75. Rate of daily BW change did not affect concentration of energy in the weight change. Body condition score change was highly correlated (r = .98) to BW change, with each unit of BCS (1 to 5 scale) change associated with 68 kg of BW change. Two methods were used to determine NE for weight change (NE delta) based on empty body weight (EBW) change. Method 1 used the equation: BCS change = -.404 + .0147 (BW change) and Method 2 used only the regression coefficient of this equation to predict daily BCS change. Methods 1 and 2 resulted in similar regression equations: NE delta (Mcal/kg EBW change) = 1.590 + 1.251 (BCS) and NE delta (Mcal/kg EBW change) = 1.317 + 1.251 (BCS). Ranges of estimated protein and lipid in the EBW change, respectively, were 10.0 to 13.7% and 17.1 to 77.2%.     

Development of a model to predict dietary metabolizable energy from digestible energy in beef cattle

Understanding the utilization of feed energy is essential for precision feeding in beef cattle production. We aimed to assess whether predicting the metabolizable energy (ME) to digestible energy (DE) ratio (MDR), rather than a prediction of ME with DE, is feasible and to develop a model equation to predict MDR in beef cattle. We constructed a literature database based on published data. A meta-analysis was conducted with 306 means from 69 studies containing both dietary DE and ME concentrations measured by calorimetry to test whether exclusion of the y-intercept is adequate in the linear relationship between DE and ME. A random coefficient model with study as the random variable was used to develop equations to predict MDR in growing and finishing beef cattle. Routinely measured or calculated variables in the field (body weight, age, daily gain, intake, and dietary nutrient components) were chosen as explanatory variables. The developed equations were evaluated with other published equations. The no-intercept linear equation was found to represent the relationship between DE and ME more appropriately than the equation with a y-intercept. The y-intercept (−0.025 ± 0.0525) was not different from 0 (P = 0.638), and Akaike and Bayesian information criteria of the no-intercept model were smaller than those with the y-intercept. Within our growing and finishing cattle data, the animal’s physiological stage was not a significant variable affecting MDR after accounting for the study effect (P = 0.213). The mean (±SE) of MDR was 0.849 (±0.0063). The best equation for predicting MDR (n = 106 from 28 studies) was 0.9410 ( ± 0.02160) +0.0042 ( ± 0.00186) × DMI (kg) – 0.0017 ( ± 0.00024) × NDF(% DM) – 0.0022 ( ± 0.00084) × CP(% DM). We also presented a model with a positive coefficient for the ether extract (n = 80 from 22 studies). When using these equations, the observed ME was predicted with high precision (R² = 0.92). The model accuracy was also high, as shown by the high concordance correlation coefficient (>0.95) and small root mean square error of prediction (RMSEP), <5% of the observed mean. Moreover, a significant portion of the RMSEP was due to random bias (> 93%), without mean or slope bias (P > 0.05). We concluded that dietary ME in beef cattle could be accurately estimated from dietary DE and its conversion factor, MDR, predicted by the dry matter intake and concentration of several dietary nutrients, using the 2 equations developed in this study.     

Estimates of genetic and phenotypic covariance functions for postweaning growth and mature weight of beef cows

Annual weights of cows from 19 to 119 months of age in two herds were analysed fitting a random regression model, regressing on orthogonal polynomials of age in months. Estimates of covariances between random regression coefficients were obtained by restricted maximum likelihood, and the resulting estimates of covariance functions were used to construct covariance matrices for all ages in the data. Analyses were carried out fitting regression coefficients corresponding to overall animal effects only and fitting regressions for animals' additive genetic and permanent, environmental effects. Different definitions of fixed effects subclasses were examined. Models were compared using likelihood ratio tests and estimated standard deviations for the ages in the data. Cubic regressions were sufficient to model both population trajectories and individual growth curves. Random regression coefficients were highly correlated, so that estimation forcing their covariance matrices to have reduced rank (2 or 3) did not reduce likelihoods significantly, allowing parsimonious modelling. Results showed that records were clearly not repeated measurements of a single trait with constant variances. As cows grew up to about 5 years of age, variances. As cows grew up to about 5 years of age, variances increased. Estimates of genetic correlations between 3-year-old and older cows were close to unity in one herd but more erratic in the other. For both herds, genetic correlations between weights on 2-year-old cows and older animals were clearly less than unity.     

Estimation of direct, maternal, and grandmaternal genetic effects for weaning weight in several breeds of beef cattle.

Weaning weights from nine parental breeds and three composites were analyzed to estimate variance due to grandmaternal genetic effects and to compare estimates for variance due to maternal genetic effects from two different models. Number of observations ranged from 794 to 3,465 per population. Number of animals in the pedigree file ranged from 1,244 to 4,326 per population. Two single-trait animal models were used to obtain estimates of covariance components by REML using an average information method. Model 1 included random direct and maternal genetic, permanent maternal environmental, and residual environmental effects as well as fixed sex x year and age of dam effects. Model 2 in addition included random grandmaternal genetic and permanent grandmaternal environmental effects to account for maternal effects of a cow on her daughter's maternal ability. Non-zero estimates of proportion of variance due to grandmaternal effects were obtained for 7 of the 12 populations and ranged from .03 to .06. Direct heritability estimates in these populations were similar with both models. Existence of variance due to grandmaternal effects did not affect the estimates of maternal heritability (m2) or the correlation between direct and maternal genetic effects (r(am)) for Angus and Gelbvieh. For the other five populations, magnitude of estimates increased for both m2 and r(am) when estimates of variance due to grandmaternal effects were not zero. Estimates of the correlation between maternal and grandmaternal genetic effects were large and negative. These results suggest that grand-maternal effects exist in some populations, that when such effects are ignored in analyses maternal heritability may be underestimated, and that the correlation between direct and maternal genetic effects may be biased downward if grandmaternal effects are not included in the model for weaning weight of beef cattle.     

The effect of individual liveweight and condition of beef cows on their reproductive performance and birth and weaning weights of calves

AIM: To examine of the influence of liveweight (LW), condition (CS) and age of dam on inter-calving interval (ICI), date of calving, days from joining to calving, and birth and weaning weights of calves. METHODS: LW and CS were measured in a herd of mixed-aged (3-8 years) Hereford x Friesian beef cows on four occasions, annually, over a 3-year period from joining (the date bulls were introduced into the herd at the start of a restricted breeding season) in November 2000 to weaning in March 2004. The four dates of weighing and condition-scoring were joining (November), weaning (March), winter (June), and pre-calving (August). Calves were tagged, identified to their dam, and weighed within 24 h of birth, and at weaning. Date of calving, ICI and days from joining to calving were determined. RESULTS: The CS and LW of cows varied between years. LW increased in cows up to 6 years of age. Three-year-old cows had a longer ICI than older cows. Change in CS and LW from winter to pre-calving and pre-calving to joining were negatively correlated with ICI for younger cows but not for cows > or = 5 years (CS) and 6-8 years (LW) old. No difference in CS or LW precalving was evident between cows that subsequently became pregnant compared with non-pregnant cows, although cows that became pregnant gained more condition from pre-calving to joining than non-pregnant cows. Pregnant cows were heavier and had higher CS at joining than non-pregnant cows. CONCLUSIONS: Young cows, up to second-mating as 3-year-olds, would benefit from separate nutritional management from older cows, to ensure ICI and days to conception are kept at targeted levels. In addition, increasing CS from pre-calving to joining and higher LW and CS at joining resulted in higher pregnancy rates.     

Effect of split nursing on variation in pig growth from birth to weaning

OBJECTIVE: To determine the effect of split nursing (i.e., removal of the larger pigs in a litter from the dam for a short period within 24 hours after birth to allow the smaller pigs in the litter uninhibited access to the dam) on variation in growth from birth to weaning among pigs. DESIGN: Clinical trial. ANIMALS: 1,193 pigs in 118 litters. PROCEDURE: Litters were randomly assigned to 1 of 3 treatment groups: control group (pigs were not split nursed), group 1 (heaviest 50% of pigs in the litter were removed from the sow for 2 hours), or group 2 (heaviest 50% of pigs were removed from the sow for 2 hours and, after the heaviest pigs were returned to the sow, the lightest 50% of pigs were removed for 2 hours). Birth weights and weaning weights were measured. RESULTS: Significant differences in average daily gain (ADG), weaning weight, or serum IgG concentration among groups were not detected. However, significant linear improvements in the SD and coefficient of variation of the ADG were detected, but only for litters with > or = 9 pigs born alive. Percentages of pigs from split-nursed litters that weighed < 3.6 kg (8 lb) at weaning (1.3 and 1.6% for groups 1 and 2, respectively) were lower than percentage of control pigs that did (3%). CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that split nursing will decrease variation in ADG of pigs from birth to weaning but only for pigs from litters with > or = 9 pigs born alive.