The average Holstein cow today produces milk for 29 months. She calves the first time at 23 months of age, milks for 13 months, is dry 50 days, calves the second time at 38 months of age, milks for 15 months, is dry 60 days, calves the third time at 55 months, has metabolic disease, a failed udder or is chronically lame, and is sold (or dies) 1 month after calving… milked a total of 29 months spread over a 33 month time period from a 56 month lifetime. Thus the average cow is only in production 51.75% of her total lifetime… but she was eating your feed for 100% of that lifetime.
Statistically she produces 2.75 live calves, 54% bulls and 46% heifers, thus 1.25 heifers in her lifetime.
12% of all heifers never make it to a first calving, thus your net harvest is 1.1 bred heifer per cow life. This heifer will average 17 months of age (six months shy of first calving) at the time of her death, so the average Holstein cow has not fully replaced herself at the time she leaves your herd. This is the single most important reason why “gender selected” (sexed) semen, in spite of higher prices and lower conception rates, is so popular in the high production, technology-adapted dairy world— cow turnover rates force us to raise and freshen increased numbers of heifers.
Genetic “value” is imputed on the sire stack, and on that basis when using “ranked” mating sires, this heifer is “genetically superior” to her momma (with her now obsolete pedigree, no great loss?) BUT I would beg to ask a simple question: How can I do genetic selection among my heifers if I must calve every one to keep the barn full? This is why genetic value is just an “imputed” (potential) value, only realized if we have surplus animals available that can be sorted for the improvement of our production profitability.
Profitability is not in genetic “value” but in gene-derived heritable value transmission
All cows and bulls possess a genetic blueprint in every body cell called the DNA which is a bundle of the gene pairs arranged on chromosome chains. As this DNA blueprint interacts with its environment (first the womb, then the calf pen, then the heifer groups, then the breeding group, then the milking barn and at every step, the nutritional input, the weather input, the facility space, contemporary competition and the human input) and the result of that DNA/environmental interaction we call the phenotype… in other words, the realized cow as a physical living organism (rather than enzyme microcosm).
In Genomic theory as applied today, out of the three billion base pairs of genes, with 22,000 different gene types arranged on 60 chromosomes within each cell of our cow, 64,000 gene “haplotypes” are referenced to determine “genetic value” for one or some of the three conception traits, eight fitness traits, thirteen linear type traits, and five production traits now published by CDCI. Thus the genetic valuation of your cow economically is based on a small fraction of her total gene pair possession.
Given 80% of her genes are “in common” with all other mammals, this is not as reductionist as it sounds initially. These “marker” genes are the ones with the highest association with measured traits. However there is no proof that these marker genes are causative. Thus production traits are only 70% Reliable, type traits 55% Reliable, fitness traits 40% Reliable after Genomic calculations (which still include the “parent average” pedigree input we used to use, at 40% of the total value imputed, for “stability”).
In traditional breeding programs, the physical mating effect and the genetic trait selection were known to be different heritable gene-derived effects. In other words, the genes that dictate realized skeletal and organic genesis and development are separate from the genes that dictate potential relative performance within a production management system.