DEFINE BREEDING GOALS
Breeding goals should be considered carefully taking into account the particular situation of each dairy producer. Ultimately, the goal of selection is to obtain more desirable cows—cows that give the producer the highest profitability. The characteristics that make cows profitable most often include the following:
- Production of large amounts of milk each lactation;
- Longevity (many lactations);
- Milk produced has the highest possible market value.
Production Traits and Milk Pricing
Cows that produce large amounts of milk are more profitable because, overall, they require less feed per unit of milk produced than cows of lower milk production. The market value of milk is an important factor to include in defining the goals of selection. The first two factors mentioned above are biological and common to all cows in the world, but milk market price makes the goal of selection very different from one country to the next and even from one region to another within a country. Depending on the way milk price is established, the most profitable strategy for a dairy producer may be to select cows that produce:
- The highest volume of milk without regard to composition;
- The highest volume of milk and amount of fat;
- The highest amount of fat and protein with no regard to the volume of milk per se;
- The highest amount of fat and protein in the minimum volume of milk.
In choosing a strategy for selection, remember that selection should be set for the long term. Goals that can remain unchanged over many years bear more fruit because selection has small but cumulative effects over successive generations of cows. In addition, the longer a goal is maintained, the greater the genetic gain over time. For example, it took about 20 years of selection (from 1965 to 1985) to improve the genetic merit for milk production by 1,000 kg in the United States. However, even the most conservative estimates indicate that in the next 20 years the genetic merit for milk production may increase by more than 6,000 kg.
Longevity (Lifetime Production) and Conformation
Longevity is a desired trait for many producers. Longevity does not mean old age; cows that live longer are only valuable for their tendency to have higher life time milk production. Selection for longevity itself is inefficient because it is influenced by many factors, most of which are nongenetic in nature. Heritability of herd life is approximately 8% (low heritability). In addition, it might take as many as seven or eight years for daughters of a bull to complete their herd lives in order to get reliable PTA estimates for sires. By then, if the sire is not dead, his genetic merit for production traits will likely be surpassed by younger sires. In reality, the longevity of cows in many herds depends primarily on three criteria—a cow usually remains in the herd as long as she:
- Remains free of serious mastitis;
- Remains free of serious reproductive problems (ability to reproduce);
- Produces at a level acceptable to the farmer.
Functional Type: Udder, Feet and Legs
Functional type is a term that has been coined recently to refer to the body conformation associated with lifetime performance. Functional type differs from the conventional “ideal” body conformation that may be of value for producers who have animals that get high classification scores, win show-ring contests, and may be sold as breeding stock.
In general, conformation traits are inaccurate predictors of longevity. Research shows that production traits are far better predictors of longevity than any conformation traits. Thus selection of a bull should be done on production first and conformation traits should come next. Actually, longevity would be automatically selected for by building an index that eliminates the use of bulls with low PTA for production and with obvious conformation or reproductive defects. Of all conformation traits, udder characteristics—in particular teat placement, udder depth and fore udder attachment—are most closely associated with longevity. Research has shown that cows with an intermediate udder depth stay in the herd longer than cows at both extremes (shallow udder and deep udder).
It is likely that cows with shallow udders tend to be poorer producers and cows with udders that are too deep are more prone to mastitis or physical injuries. Despite the importance that many producers place on foot and leg traits, lifetime performance studies suggest that foot and leg traits have a much smaller impact on herd life than do production traits or udder traits.
Production Traits versus Functional Type
Despite the common belief that functional traits improve the longevity of dairy animals, cows are rarely culled for poor conformation. There is no doubt that udder injuries are more frequent on cows with pendulous udders, and cows with serious leg and feet problems must sometimes be culled. However, as long as the animals remain productive, most farmers choose to keep them in the herd. Commercial producers are often required to cull cows with health, metabolic or reproductive disorders regardless of their conformation. Thus it is important to keep in mind that, economically, production traits are generally much more important than conformation traits. Most economic studies in the United States suggest that production traits should receive three to five times theeconomic weight that non-yield traits receive in determining the priorities of selection.
Big Cows versus Small Cows
In the past 25 years, the size of dairy cows in the United States has increased. As with all the other traits we have discussed so far, this change has both a genetic and an environmental component. Bigger cows eat more and, so the theory goes, produce more. However, larger size is not closely associated with milk production. In other words, selection for milk production does not necessarily increase body size. In fact, a research project that started in 1968 shows that genetic gain for milk production is as rapid for small cows as it is for large cows. However, large cows have substantially greater dry matter intake for maintenance requirements. Thus, when milk production is equal, small cows are clearly more efficient producers than large cows. A detailed study in the United States has shown that dairy producers oriented toward raising stock for sale at the show ring prefer taller, stronger animals. These animals tend to receive better final classification scores and rank better in the show ring.
How many traits should be included in a selection program?
When selection is performed on more than one trait, genetic gain is less rapid than if selection is performed on one trait alone. In general, more than one trait is desirable; however, selecting for more than four to five traits at the same time will reduce the rate of genetic gain considerably. Table 1 shows the loss of selection pressure as traits are added in a selection program. For example, if the selection decision is based on two traits, genetic progress of either of the two traits will be only 71% of the progress made when one of the two traits is selected alone. traits.
Consider the Correlation between Traits
In addition, when deciding which traits should be selected and what the relative weight of each trait in the selection program should be, remember the correlation that exists between Table 2 shows the average PTA of 10 sires for six traits (these sires were the top 10 sires on the “net merit” index in January 1995 in the United States). For example, PTA for milk yield averaged 2,274 pounds, but these top sires for milk yield also had the following PTA averages for other traits: type 1.03, fat percentage -0.06%, protein 62.5 pounds, fat 70.4 pounds, protein percentage -0.04%. Table 2 shows that the highest average PTA for a trait is obtained by selecting the top 10 sires for a particular trait (gray boxes). However, correlation also brings about change in other traits that is sometimes significant. In our example, selection for milk yield alone (Line 1 in Table 2) is also associated with a PTA for protein yield of 62.5 pounds—a genetic gain only a few pounds short of the possible gain obtained by concentrating all the selection pressure on protein yield (Line 4, 66.3 lbs.). Another interesting observation is that putting all selection pressure on fat percentage (Line 3) would make us choose bulls among the highest for PTA fat yield (74.9 lbs.) but less so for milk yield (average PTA = 1,312 lbs).
On the other hand, focusing selection efforts on pounds of fat (Line 5) makes us choose bulls that have a substantial positive PTA for fat percentage (0.078) and milk yield (1,831 lbs).
How to Achieve the Goals
Choosing the right sire for artificial insemination on a farm is the most effective and cheapest way to make progress toward genetic goals. Sires should be chosen on the basis of their predicted transmitting ability for the desired traits. Reliability should be used only to determine how intensely a particular sire should be used. The actual price structure of milk (the price of fat, protein and other adjustment factors used to calculate the price of milk) may be used in calculating an index of selection to help in identifying the best bull to meet a selection goal. The construction of an index of selection has been described in this series of publications (see “Choosing a Bull”). Many of the current indices of selection give unnecessary importance to type traits (e.g., Total performance index—TPI—calculated by the Holstein Association). However, newer indices try to give more importance to lifetime production and resistance to mastitis (Table 3).
