Additive Breed Effects Research Articles

Estimation of additive and non-additive genetic parameters for carcass traits on bulls in dairy, dual purpose and beef cattle breeds

Carcass records on 4749 bulls (from 153 sires) slaughtered at an average age of 20 months were collected from slaughter houses in the Netherlands. Bulls had a variable proportion of genes from Dutch Black and White (DBW), Dutch Red and White (DRW) and Pimontese (PIM). Additive genetic and crossbreeding parameters were estimated for fleshiness score (FLESH), fat covering score (FAT) and carcass weight (CWT) using three fixed models (additive breed, Dickerson and Kinghorn model) and the corresponding three mixed models with random sire effects. These models involved additive breed, heterosis (or dominance), and recombination loss (or epistatic loss) effects (i.e., crossbreeding parameters) as regression covariates. In addition, an additive sire group model was used as a mixed model. FLESH and FAT were defined numerical values ranging from 1 (poorly muscled; very much fat) to 15 (very muscled; very little fat). When comparing models, estimates of crossbreeding parameters over breeds and breed combinations showed similar patterns, although absolute values between the same parameters differed up to a standard error between fixed and mixed models and between Dickerson and Kinghorn models. Mixed models showed larger standard errors relative to fixed models. Relative to DBW, PIM showed significantly positive favorable breed effects for FLESH (ranging from 5.8 to 6.3 for different models) and CWT (62.6 to 111.8 kg), and unfavorable for FAT (−1.7 to −2.6). DRW showed favorable, but smaller than PIM, additive genetic effects for all three traits (2.2 to 2.3 for FLESH, 0.1 to 0.3 for FAT and 18.1 to 24.2 kg for CWT). Significant heterosis was only found for FAT (0.4 to 0.7) and for CWT (12 to 14 kg) in the cross DBW×DRW. Significant recombination and epistatic effects were obtained for CWT in the Dickerson and the Kinghorn models. Models not including non-additive effects over-estimated genetic variances and heritabilities for FLESH and FAT.

Heterogeneity of variances for carcass traits by percentage Brahman inheritance.

Heterogeneity of carcass trait variances due to level of Brahman inheritance was investigated using records from straightbred and crossbred steers produced from 1970 to 1988 (n = 1,530). Angus, Brahman, Charolais, and Hereford sires were mated to straightbred and crossbred cows to produce straightbred, F1, back-cross, three-breed cross, and two-, three-, and four-breed rotational crossbred steers in four non-overlapping generations. At weaning (mean age = 220 d), steers were randomly assigned within breed group directly to the feedlot for 200 d, or to a backgrounding and stocker phase before feeding. Stocker steers were fed from 70 to 100 d in generations 1 and 2 and from 60 to 120 d in generations 3 and 4. Carcass traits included hot carcass weight, subcutaneous fat thickness and longissimus muscle area at the 12-13th rib interface, carcass weight-adjusted longissimus muscle area, USDA yield grade, estimated total lean yield, marbling score, and Warner-Bratzler shear force. Steers were classified as either high Brahman (50 to 100% Brahman), moderate Brahman (25 to 49% Brahman), or low Brahman (0 to 24% Brahman) inheritance. Two types of animal models were fit with regard to level of Brahman inheritance. One model assumed similar variances between pairs of Brahman inheritance groups, and the second model assumed different variances between pairs of Brahman inheritance groups. Fixed sources of variation in both models included direct and maternal additive and nonadditive breed effects, year of birth, and slaughter age. Variances were estimated using derivative free REML procedures. Likelihood ratio tests were used to compare models. The model accounting for heterogeneous variances had a greater likelihood (P < .001) than the model assuming homogeneous variances for hot carcass weight, longissimus muscle area, weight-adjusted longissimus muscle area, total lean yield, and Warner-Bratzler shear force, indicating improved fit with percentage Brahman inheritance considered as a source of heterogeneity of variance. Genetic covariances estimated from the model accounting for heterogeneous variances resulted in genetic correlations of or near unity. These results suggest that different genetic values be considered for genetic evaluation of carcass yield and shear force traits from steers with different degrees of Brahman inheritance.

Heterosis and breed additive effects for Hereford, Tarentaise, and the reciprocal crosses for calf traits.

Records from 595 straightbred Hereford (HH), straightbred Tarentaise (TT), and reciprocal-cross (HT) females, randomly mated to HH, TT, or HT bulls, were analyzed for estimates of heterosis and breed additive effects for calf traits that included birth weight (BWT), calving difficulty (DIFF), prebreeding (PRE) and postbreeding (PST) weight, weaning weight (WWT), weaning condition score (WCS), weaning hip height (WHH), and preweaning average daily gain (ADG). The statistical model included year, age of dam, sex, regression on age at time of measurement, and regressions for the genetic effects of breed individual, breed maternal, breed grandmaternal, individual heterosis, and maternal heterosis. Breed effects were coded to reflect TT- HH differences. Age at the time of measurement, year, age of dam, and sex were significant for most traits. Individual breed effects were important (P < .05) for BWT, PRE, PST, WCS, and WHH but not for WWT, resulting in lower weights, less condition, and taller animals for TT. Maternal breed effects did not influence BWT, but they were important ( P < .05) for PRE, PST, WWT, WCS, WHH, and ADG. Grandmaternal effects were only important for BWT and ADG. Individual and maternal heterosis were important (P < .05) for most traits measured, resulting in increased calf size, weight, and body condition.

Estimation of individual and maternal additive genetic and heterotic effects for preweaning traits of crosses of Ayrshire, Brown Swiss and Sahiwal cattle in the lowland tropics of Kenya

A total of 1443 records were used to estimate individual and maternal additive genetic breed and heterotic effects for preweaning growth and survival traits of crosses of Ayrshire (A), Brown Swiss (B), and Sahiwal (S) in a dairy herd in the lowland coastal tropics of Kenya. Individual and maternal additive genetic effects for A and S were not significant (P > 0.05) for any of the traits. Similarly, the individual heterotic effect of Bos taurus-B. indicus heterozygosity was not significant (P > 0.05) for calf performance. A small effect of AS individual heterosis on preweaning average daily gain (DG) was found. It was estimated that F1 AS calves gained 26 g/day faster (P < 0.1) than the mean of the purebred A and S. The maternal heterotic effect of B. taurus-B. indicus heterozygosity for birth weight (BW) was −3.19 ± 1.25 kg (P < 0.05) and of opposite sign to estimates reported in the literature. The absence of any major variation among calf genetic groups showed that the individual and maternal breed and heterotic effects of the sire breeds and dam genotypes summed to give very similar aggregate preweaning performance for all traits except BW. Therefore no emphasis should be given to preweaning performance when selecting among these breeds and mating systems. Instead, breeding decisions aiming to increase herd productivity will be determined by relative lactation and reproductive performances of cows.

Genetic parameters for reproduction traits in crosses between Finnish Landrace and Ile de France sheep

Genetic parameters of reproduction traits were estimated from crosses between Ile de France (IdF) and Finnish Landrace (FL). In 20 years, 6611 joinings and 4724 lambings out of 1383 ewes were recorded in an 8 months lambing cycle. Crossbreeding effects were estimated from 13 different crossing types. Data was analyzed with a linear mixed model with a random additive genetic effect for ewe and fixed effects of lambing year, mate genotype, an interaction between lambing season × lambing age × interval before lambing, and crossbreeding effects. Crossbreeding effects were estimated by regression on either crossbreeding parameters (effects of individual and maternal breed, of individual and maternal heterosis and of individual recombination) or on crossbreeding group. Variance components were estimated by REML. Litter size traits showed significant additive genetic differences between breeds. For litter size at birth, at 24 h of age and at weaning, these contrasts were, respectively, 1.35 (±0.13), 0.89 (±0.12) and 0.79 (±0.12) in favour of the FL. Litter size at birth showed a significant maternal genetic contrast (0.17 ± 0.07) in favour of IdF. Other traits like percentage of ewes mated and lambed showed no significant additive breed effects. Most of the traits analyzed showed a significant positive individual heterosis, e.g., for litter size traits 9.3 to 15.1%, and for percentage of ewes mated and lambed 5.0 and 15.1%, respectively. Heritabilities for litter size at birth and litter size at weaning were 0.16 and 0.08 respectively. This study showed that the large additive genetic effect of FL and a substantial heterosis on litter size traits and on ewes mated and lambed resulted in an efficient and prolific synthetic, which is well adjusted to the 8 months lambing cycle.

Breed and crossbreeding effects on weight, yield and quality of heavy italian dry-cured hams

The aim of this study was to estimate direct heterosis and breed additive effects of Belgian Landrace (BL), Duroc (DU) and Spotted Poland (SP) in comparison with Italian Large White (LW) on weight, losses and percentage yields during processing, and qualitative traits of heavy dry-cured hams. Hams from 290 pigs of 16 genetic types, produced by LW, BL × LW, DU × LW and SP × LW gilts mated with LW, BL, DU and SP boars, were controlled during industrial processing; a sub-sample of 116 dry-cured hams were deboned and chemical composition, colour and organoleptic properties were assessed. Technological processing traits were slightly influenced by additive effects and the imported breeds gave yields of dry-cured hams comparable to those of LW. BL crossbred pigs reduced the incidence of bone on the weight of seasoned ham (P < 0.01), but tended to have a higher percentage of ham discarded during processing than LW and a lower ham flavour score (P < 0.05). These findings suggest a lesser dry-curing suitability of BL-derived hams, even though no pigs gave PSE meat. DU decreased the incidence of bone (P < 0.05), protein content (P < 0.05) and flavour score (P < 0.10) and increased dry matter and sodium chloride (P < 0.05) of dry-cured hams with respect to LW. SP gave lighter boned hams than LW (P < 0.01), but with higher trimming discards (P < 0.05), and did not exert any influence on quality traits of seasoned hams. Direct heterosis never affected weight, losses and quality traits of seasoned hams, suggesting a negligible effect of crossbreeding on processing traits of pork meat.

Genetic parameters of purebred and crossbred Milking Criollos in tropical Mexico

AbstractRecords on total of 1746 calvings of principally Milking Criollos and their crosses were collected between 1972 and 1990 at the experiment station of the Mexican Association of Animal Production on the Gulf Coast of Mexico. Traits of interest were 305-day milk yield, days in lactation, number of services per lactation (raised to power 1/2), age at first calving and lifetime milk yield. Data were analysed by restricted maximum likelihood (REML) under an individual animal model based on 584 animals of which 146 were female ancestors and 35 were male ancestors. Heritabilities of 305-day milk and age at first calving were 0·17 and 0·07, respectively, but heritabilities of other traits were close to zero. Genetic trend in the Milking Criollos for 305-day milk was small and not significant (0·76 (s.e. 2·38) kg/year). Of crosses with Criollo sires, those involving Holstein and Canadienne breeding had highest 305-day and lifetime milk yields and those involving Brown Swiss and native Mexican (mostly Oaxaca) had lowest yields. Jersey crosses were intermediate for 305-day yield. The F1 Criollo × Jersey cross had highest lifetime yield, but backcrosses involving Jersey breeding were poor for lifetime milk. Additive breed effects for Jersey and Canadienne, relative to Milking Criollo, were 88 (s.e. 91) and 227 (s.e. 74) kg 305-day milk, respectively. Heterosis was 144 (s.e. 55) kg (11·6%) for 305-day milk, 16–4 (s.e. 9·6) (5·0%) for days in milk, −0·107 (s.e. 0–042) (7·7%) for number of services per lactation raised to power 1/2, −25·6 (s.e. 41·4) days (2•3%) for age at first calving and 1789 (s.e. 664) kg (60·0%) for lifetime milk. A plan was designed to develop a nucleus breeding scheme utilizing multiple ovulation and embryo transfer technology (MOET) for the genetic improvement of the Milking Criollo breed in Mexico.

Reproductive and maternal performance of rotational three-breed, and inter se crossbred cows in Florida

Reproductive, calf growth, and cow weight data were collected during a 16-yr period in southern Florida. Data included 1,767 calves at weaning from purebred, F1, backcross, F2, and three-breed crossbred cows of the Angus (A), Brahman (B), and Charolais (C) breeds. The purebred cows primarily produced purebred calves, the F1 and F2 Cows produced inter se crossbred calves, and the backcross cows produced 3/8:5/8 calves. The 1/2 C:1/4 A:1/4 B three-breed crossbred cows were mated to A series, whereas three-breed crossbred dams with breed compositions of 1/2 B:1/4 A:1/4 C, and 1/2 A:1/4 B:1/4 C were mated to C and B sires, respectively. For analysis of additive breed and heterosis effects, pregnancy rate and cow weight were considered to be traits of the dam alone. The A additive breed effect increased (P < .05) pregnancy rate but reduced (P < .001) cow weight. The effects of AB, AC, and BC heterozygosity all increased pregnancy rate, but the advantage was greater for the crosses that involved B. Both AB and BC heterozygosity increased (P < .001) cow weight, whereas AC did not. The additive direct effect of B and C increased birth weight (BWT) and weaning weight (WWT). The additive maternal effect of B reduced BWT. The direct effect of AB heterozygosity increased BWT and WWT, and that of BC increased only WWT (P < .001). A comparable pattern was observed for maternal heterosis on weight traits. A much smaller effect of AC maternal heterosis on WWT was found. Pregnancy rates of F1, backcross, and three-breed dams were similar.(ABSTRACT TRUNCATED AT 250 WORDS)

Fixed effects in the formation of a composite line of beef cattle: I. Experimental design and reproductive performance

Red Angus (RA) dams were mated to Charolais (C) or Tarentaise (T) sires to produce crossbred (F1) progeny. Members of the F1 generation, differing in breed direct effects, were mated to produce an F2 generation with an expected breed composition of 1/2 RA, 1/4 C, and 1/4 T. Two breed groups within the F2 generation differing in breed maternal effect ([C x RA] x [T x RA] and [T x RA] x [C x RA]) were identified separately. These breed groups were crossed to produce an F3 generation and, likewise, the two resulting F3 generation breed groups were crossed to produce an F4 generation. No distinction was made among breed groups subsequent to the F3 generation. Pregnancy rates averaged 90.9% over 11 yr, with 82.0% of cows exposed weaning a calf. Among formative generations of this composite population, F2 had greatest pregnancy, calving, and weaning rates. Age of dam significantly affected pregnancy rate, calving difficulty, and gestation length. Older cows tended to express higher pregnancy rates and longer gestation lengths than did younger cows (P < .01). Males calves had a 1.7% greater weaning rate than female calves (P < .05), but matings producing male calves had longer gestation lengths (P < .05) and were 8.5% more likely to experience calving difficulty (P < .01). Individual breed additive effects (calves from C or T sires mated to RA dams) were important for calving difficulty only (P < .05), where C-sired matings experienced greater calving difficulty. Differences between C x RA and T x RA dams, indicative of maternal breed additive effects, were not detected.(ABSTRACT TRUNCATED AT 250 WORDS)

Fixed effects in the formation of a composite line of beef cattle: II. Pre- and postweaning growth and carcass composition

Generation, age of dam, sex of calf, and certain covariates were studied to elucidate their effect on traits related to growth and carcass characters measured on individuals from a stabilized three-breed composite (1/2 Red Angus [RA], 1/4 Charolais [C], 1/4 Tarentaise [T]). There was evidence that sires had been selected for yearling weight. Thus, an animal model was fitted to the data to estimate the effects free of bias due to selection. Differences between generations were not different from zero (P > .05) for birth weight, weaning weight, and preweaning ADG. There were few important differences between generations for carcass traits as well. This may have been caused by confounding of individual and maternal heterotic effects with direct and maternal components of the model, or the partial confounding of years, age of dam, and generation. Birth weight was curvilinear with respect to calving day (P < .01). Age of dam was important for all growth traits except postweaning ADG. In general, growth of calves increased with increasing age of dam, as did carcass weight and predicted retail product. Individual breed additive effects (differences between calves sired by C and T sires mated to RA dams) were positive for birth weight (P < .01), weaning weight (P < .05), carcass weight (P < .05), and predicted retail product (P < .05). Maternal breed additive effects (differences between calves out of C x RA or T x RA dams) were also positive for weaning weight (P < .05), carcass weight (P < .05), and fat depth (P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Crossbreeding Ayrshire, Friesian, and Sahiwal Cattle for Milk Yield and Preweaning Traits of Progeny in the Semiarid Tropics of Kenya

Individual and maternal additive genetic and heterotic effects were estimated for preweaning, reproductive, and lactation traits of the Sahiwal breed and its crosses with Bos taurus (Ayrshire and Friesian). The individual additive genetic breed effect influenced all cow traits except annual milk yield, and the maternal additive genetic effect influenced all yield traits. The individual and maternal genetic effects between B. taurus and Sahiwal breeds for lactation milk yield were 746 and –601kg, respectively. Individual heterosis for lactation milk yield of 73kg (5%), estimated using the mean of the reciprocal F1 crosses, was not significant but increased to 374kg (28%) when estimated using only the F1 cross sired by B. taurus, showing the important influence of maternal effects. Individual additive genetic effects influenced preweaning daily gain and weaning weight. Maternal additive genetic and heterotic effects affected birth weight, and individual heterosis significantly influenced all calf traits. Maternal additive genetic effects may have biased the estimation of individual heterosis for milk yield in other tropical studies. When stratification is not possible or when reproductive rates are low, two-breed rotational crossbreeding is an efficient system to exploit additive and nonadditive genetic variation for tropical dairy production.

Estimated breed additive effects and direct heterosis for growth and carcass traits of heavy pigs

A trial was carried out using 290 pigs of 16 genetic types, 128 gilts and 162 barrows, produced from Italian Large White (LW), Belgian Landrace (BL) × LW, Duroc (DU) × LW and Spotted Poland (SP) × LW sows inseminated by LW, BL, DU and SP boars. The aim of the research was to estimate direct heterosis and breed additive effects on growth performance, carcass traits and wholesale cuts when slaughter weight averaged 145 kg; the breed additive effects are expressed as differences from purebred LW. The BL breed had a negative effect on postweaning growth rate (−42 g/d, P<0.10), but favourably affected commercial carcass composition (+2.7% lean joints, P<0.05) and wholesale cuts (+1.4% ham, P<0.01) compared to LW. The DU breed additive effect improved average daily gain (+34 g/d, P<0.10) and positively influenced carcass composition, with higher yield and incidence of total lean joints (+1.9%, P<0.10) and ham (+1.6%, P<0.01). The SP breed additive effect did not affect growth, but gave less percentage weight of total lean joints (−3.7%, P<0.01) and more total percentage weight of fatty joints (+4.1%, P<0.01) than LW. Direct heterosis positively influenced postweaning growth rate (+34 g/d, P<0.05) but was small and not significant for most carcass traits. The DU breed offers some advantages in crossbreeding plans involving LW to produce heavy industrial pigs; BL can also be utilized for this purpose, with less favourable results, while the SP breed does not seem suitable for heavy pig production.

Additive and Nonadditive Genetic Effects on Milk Production in Dairy Cattle: Evidence for Major Individual Heterosis

Coefficients for individual and maternal breed composition and the expected contributions of individual and maternal heterosis and breed source of cytoplasm were assigned to 42,554 primiparous Holstein-Friesian, Jersey, and crossbred cows. The individual additive genetic breed effect influenced all milk production traits. Highly significant maternal additive genetic breed effects equivalent to 3% of the mean were identified for milk yield and milk fat percentage. Individual heterosis was highly significant for milk yield and milk fat yield. A primiparous first cross cow produced 6.1% more milk and 7.2% more milk fat than the average of straightbred cows of both breeds. For milk fat yield, the individual heterosis effect was higher than the individual additive genetic breed difference between Jersey and Holstein-Friesian. A small negative maternal heterosis and a small effect of breed source of cytoplasm were estimated for milk fat percentage. Results suggest that individual heterosis is a major genetic effect for milk yield and milk fat yield. This heterosis could be utilized through a stratified breeding scheme in which high genetic merit nucleus herds maintain genetic progress in the two straightbred populations, and commercial dairy herds employ a rotational crossbreeding scheme to take advantage of both the additive genetic progress and nonadditive genetic effects.

Direct and maternal genetic effects due to the introduction of Bos taurus alleles into Brahman cattle in Florida: I. Reproduction and calf survival.

Pregnancy rate, calf survival rate to weaning and calf age at weaning of several types of crossbred cows (2/3 or more Brahman) were compared to those of straightbred Brahman and Angus cows over a 12-yr period at Subtropical Agricultural Research Station near Brooksville, FL. The purpose of this study was to determine the relative importance of additive vs nonadditive genetic effects on reproductive and calf survival traits in a population of cattle whose foundation was selected on the basis of superior reproductive performance under harsh environmental conditions. Best linear unbiased estimates (BLUE) of direct additive effect (measured as the deviation of Brahman additive breed effect from Angus) for pregnancy rate and calf age, measured as traits of the dam, were 6 +/- 3% and -7.2 +/- 2.1 d, respectively. Thus, Bos taurus germ plasm did not increase pregnancy rate but resulted in an earlier calving date. The BLUE of nonadditive (intralocus) direct genetic effects measured as deviations from intralocus group genetic effects in the parental breeds on pregnancy rate and calf age at weaning were 25 +/- 4% and -6.4 +/- 2.5 d. Nonadditive effects on pregnancy rate were the primary cause of the superior reproductive rates observed in Brahman crossbred cows. Calf survival was considered to be a trait of the calf, and BLUE of direct additive, direct nonadditive, maternal additive and maternal nonadditive genetic effects was obtained. Only maternal nonadditive genetic effects were found to have a significant effect on survival rate (9 +/- 4%).

Influence of Nonadditive Effects on Estimation of Genetic Parameters in Dairy Cattle

A population was simulated having progeny that descended from sires and dams with various fractions of genes from two breeds. Additive breed effects and nonadditive effects from breed crosses were simulated.Data on performance were analyzed using mixed models that accounted for fixed additive genetic group effects and random sire effects. Three additive models, with genetic groups defined according to 1) breed composition of the progeny, 2) breed composition of the sire and the dam, or 3) linear regression on breed fraction of the progeny, were compared with a nonadditive model with a linear regression on breed fraction, heterozygosity and recombination in the genome of the progeny. Variance components were estimated using restricted maximum likelihood.Additive genetic variance and heritability were overestimated for an additive model with progeny groups. Additive models gave estimates for breed difference, group effects, and breeding values that were not equal to true additive genetic values. Breed differences were overestimated when sire groups were used. Estimators for each parameter were unbiased with the nonadditive model.

Heterosis for milk production traits by crossing Red Danish, Finnish Ayrshire and Holstein-Friesian cattle

A large experiment was initiated in Denmark in 1972 to examine benefits from systematic crossing of the dairy cattle breeds Red Danish (RD), Finnish Ayrshire (FA), Danish Friesian (DF) and Holstein-Friesian (HF). The data were analysed by three versions of a mixed model. The first included additive breed effects and F 1 heterosis (dominance model). Additionally, the second included effects of recombination (recombination model). In the third version the effects of each genetic group were estimated (group model). Means of the genetic groups predicted by the dominance and the recombination model were compared with the means estimated in the group model. The estimates of F 1 heterosis from the dominance model were 20, 6, 20 kg of fat, 18, 8, 18 kg of protein and 428, 206, 537 kg of milk for 305-days-yield records of RD × FA, FA × HF and RD × HF, respectively. The corresponding estimates from the recombination model were 22, 24, and 10 kg fat, 20, 30 and 7 kg of protein and 486, 820 and 210 kg of milk. The estimated effects of recombination were negative and eliminated the positive effect of F 1 heterosis in the later generations.

Litter live weight at birth with varying levels of D’Man germplasm

A total of 208 litter birth weights, with live lambs produced in the northwest of Morocco was analyzed both by factorial analysis and by multiple regression analysis using least squares procedure. In the first model, breed group of dam and age of dam were included as fixed effects. In the second model, maternal additive and heterotic effects were included instead of breed group of dam. Results of the first analysis indicated that breed group of dam influenced litter live weight at birth (LLWB). The crossbred S x D dams (with the sire breed listed first), and the purebred D’Man dams produced the heaviest live litters, in average 4.25 and 4.08 kg, respectively. The purebred Beni Guil dams produced the lightest live litter, 3.32 kg in average. The second analysis showed that maternal additive effects were important for LLWB, and that maternal heterosis was of little importance. The ranking of maternal additive breed effects for LLWB was D’Man, Sardi, and Beni Guil in descending order.

Age and Zebu-Holstein Additive and Heterotic Effects on Lactation Performance and Reproduction in Brazil

Data of 6482 lactations from 14 crossbred (Holstein×Zebu) herds in Brazil were used to study breed additive and heterosis effects for first, second, third, and first to fifth lactation milk yields, age at first calving, calving interval, and milk yield ÷ calving interval, as well as the effect of age at calving on milk yield. Holstein additive expressed as deviation from Zebu and heterosis effects were highly significant for all traits. For each percentage of Holstein gene contribution an increase of 10.02, 12.02, 12.51, and 12.15kg of milk were expected for first, second, third, and first to fifth lactation yields, respectively. Corresponding heterosis effects on those traits were 3.80, 3.39, 4.02, and 3.90kg of milk for each percentage of heterozygosity. Replacement of pure Zebu genes by Holstein genes reduced age at first calving by 6 mo and shortened calving interval by 37 d. Holstein×Zebu heterotic effect decreased age at first calving by 2 mo and calving interval by 39 d. Holstein additive and heterosis effects for milk yield ÷ calving interval were 3.4 and 1.3kg of milk/d, respectively. Fitting breed additive and heterozygosity effects accounted for 99% of the genetic effects except for first to fifth lactation milk yield.

Periodic rotational crosses. I. Breed and heterosis utilization.

Periodic rotational crosses differ from conventional rotations by using sire breeds an unequal number of generations but in a regular sequence. Formulas for breed composition and heterosis in offspring from periodic rotations at equilibrium averaged over all generations of a cycle and for each generation of the cycle are presented. Coefficients of squares and products of breed differences and heterosis used to calculate inter-generational variance are also derived. Several specific periodic rotations utilizing a range of breed proportions were found to use from 70 to 95% as much heterosis as conventional rotations using the same number of breeds equally. Estimates of swine and cattle additive breed and heterosis effects taken from the literature were applied to the formulas. In both the swine and cattle examples, periodic rotations were found that equaled or exceeded the conventional rotations using the same number of breeds and also had lower inter-generational variance.

Periodic rotational crosses. II. Optimizing breed and heterosis use.

Choice of the optimal number and sequence of breeds in a periodic cross was determined by comparing the trade-off between increased utilization of breed differences and decreased utilization of heterosis. It was shown that the change in mean efficiency resulting from adding the next best breed to the best conventional n-breed rotation is always less than the change in efficiency predicted from the increase in heterosis. Periodic rotations were generally optimized by decreasing the proportion of poorer performing breeds in the rotation. However, efficiency of periodic rotations can exceed that of the better breed even when the difference in additive breed effects for efficiency is almost twice the effect of heterosis on efficiency. The periodic rotation that was optimal also tended to have the lowest inter-generational variance. It was suggested that inter-generational variances of component traits, which are not necessarily minimized when crosses are selected on a combined efficiency trait, can be considered by including inter-generational variance in an index or by introducing maximum thresholds.