Relationships between weaning weight, maternal weaning weight, Relationships between weaning weight, maternal weaning weight, and milk production in polled Hereford cattle and milk production in polled Hereford cattle

Summary Performanc e data from a Polled Hereford herd selected for improved feed conversion were used to calculate a variety of genetic parameters. Heritabilities were .14 for weaning wt, .18 for maternal weaning wt., and .19 for milk productio n. The genetic correlations were –.10 between weaning weight and maternal weaning weight and 0 between weaning weight and milk production. However, the genetic correlation between maternal weaning weight and milk was . 99, indicating they are essentially the same trait. Milk EPDs published by most breed associations are calculate d as the maternal weaning weight. Our study strongly supports this method of estimating an animal's genotype for milk production.


Introduction
Today, beef pr oducers have a wide array of informatio n to assist them in making sound selection decisions.Heritabilities provide informatio n regarding the variation in traits due to inheritance, as well as information about expecte d response from selection.Genetic correlation s indicate how traits are genetically related o r how the same genes affect more than one trait.Our purpose was to estimate the heritabilities and genetic cor r elations of weaning weight, maternal weaning weight, and milk production.

Experimental Procedures
Performanc e data were collected on 1459 animals from a Polled Hereford herd at Kansas State University, from 1967 through 1979.Foundatio n animals were donated by breeders from several states and were a representative sample of the Polled Hereford breed.Animals from the original herd were use d to establish a selectio n herd.Following establishment of the selectio n herd, the original herd then was used as an unselected control.Replacements were selected from within each he r d.Two bulls were selected based on individual feed conversion and used fo r 2 consecutive years in the selected herd.In each year, the first bul l born, sired by the oldest herd sire in the control herd, was select ed to replace his sire.These bulls were used in the control herd for approximately 6 years.Cows in both herds were maintained on native pasture throughout the year and were supplemented in the win t er.Cows were bred to calve in Marc h and April.Calves were weaned in the fall at an average age of 196 days.Bull calves were individually fed for 140 days postweaning , which allowed selection for feed conversion (feed/g ain).Heifers were group-fed and, thus, were not selecte d on individual feed conversion.C ows were culled according to the follow ing: (1) open at the end of the breeding season , (2) severe structural problems, and (3) horned.Milk produce d by a sample of 59 cows at the end of the study (weigh-suckle-weigh technique ) was measured each month prewe aning, during a 3 year period.The numbers of observations, means, and standard deviations for weaning weight and milk production are presented in Table 1.A multiple-trait derivative-free, restricted maximum likelihood (MTDFREML) procedure , incorporating a full numerator relationshi p matrix, was used to analyze the data.The mixed linear animal model included age of dam (2, 3, 4, 5-10, and >10 yr) and contemporary group (sex an d year of birth) as fixed effects for weaning weight.Yea r of milking and age at milking (2 , 3, 4, 5-10, and >10 yr) were included as fixed effects in the milk production model.We aning weights were regressed to the averag e weaning age.Individual animal effect was included as a random effect for weaning weight and milk production, with maternal effect and permanent environment a l effect being included for weaning weight.

Results and Discussion
Heritabilitie s and genetic correlations for weaning weight, maternal weaning weight, and milk productio n are presented in Table 2.The heritabilit y for weaning weight (.14) is lower than estimat es previously reported, whereas the heritabilities for maternal weaning weight (.18) and milk production (.19) are similar to others.The strong positive genetic correlation of .99 betwee n maternal weaning weight and milk indicate s that th e same genes affect both traits.Maternal weaning weight commonly is used as an indicat ion of milk production by breed associations publishing milk EPDs.This study agrees with others, by indicating that maternal weanin g weight is a good predictor of milk production.

Table 1 . Number of Observations (n), Means, and Standard Deviations (SD) for Each Trait Analyzed
a

Table 2 . Heritabilities and Genetic Correlations for Each Trait Analyzed a
Heritabilities are on the diagonal; genetic correlations are below the diagonal.