2. CÁLCULOS JUSTIFICATIVOS
2.2 Centros de transformación
Carol Delaney
chapter 14
A
ny trait you want to change or select for needs to be measured and recorded somehow. You can make up your own evaluation system or borrow recommended measurements but it needs to con-tain strong elements of objectivity and consistency.If milk yield and days in milk are important to you, their performances in the first year of lactation are very good indicators for future performance. thus, you can base your selection of replacements on the yearlings’ lactation performances.
Ease of kidding (Photo 14.1) is a worthy trait to track for its effect on economics (costs of labor, vet intervention, medical supplies, etc.) and produc-tion (time lost to recovery and uncaptured peak production potential).
the obvious genetic traits that researchers and farmers have focused on are milk production and yield of protein and fat since these directly deter-mine income for fluid milk and cheese yield for farmstead cheese makers. However, if you select goats for higher milk production only and ignore personality and body traits, your herd may become malformed in relation to the milk yield. In the 1990s in France they emphasized high milk production in goats for a decade and they had to weed out the many goats with bad udders. It is best to consider soundness traits as well as production traits so as
not to compromise the goats’ ability to do well in the environment provided to them.
One milking trait of goats that has high heritabil-ity (65%) is the speed of milking. (For background, the degree of heritability roughly means how much of the trait, based on 100%, directly comes from the genes of the goat with the remaining difference in percentage coming from the influence of the envi-ronment.) thus, if you have a very high producer but she is a slow milker, that adds milking time and you risk over-milking the other goats waiting for her to finish. It would be prudent to avoid retaining replacement does from her offspring because the heritability of the trait is so high.
Milk and protein yield have heritabilities of about 30–40% so their improvement can be made more gradually by measuring an individual goat’s milk content and doing a comparison among the does.
It is more difficult to increase the solid content (fat, protein, etc.) of milk while choosing for increased milk production because milk amount is inversely related to the percent of milk fat and protein in the milk.
Sometimes there is a question about selecting off-spring from animals with high somatic cell counts (SCC). SCC is not a trait that can be selected for or against based on the actual milk measurement.
88 A GuiDe To STARTinG A CoMMeRCiAl GoAT DAiRy
However, an animal with consistently high SCC due to recurring or chronic mammary infections should be culled to prevent infecting other animals.
Mastitis can most easily begin and spread in the milking parlor (machines or protocol), a high risk area for infection because the teat ends are opened.
the second area where infection can occur is in the bedded area especially if the does lie down on dirty bedding soon after milking. At Langston Univer-sity, a mastitis rate of 5% is considered acceptable for a commercial herd (at any one time, 5% of milk-ers exhibit mastitis).
For an example of a selection program for a goat herd, we can look north of the border to research done recently in Canada1. In Québec and Ontario, starting in 2001, they enlisted 26 herds of goats where farmers followed selection protocol-based traits that were balanced to 60% production and 40%
conformation. the production traits were as follows:
• milk, protein, fat yield;
• fat and protein percent;
the conformation traits of general appearance are:
photo 14.1 Good kidding and mothering traits demonstrated.
• leg strength;
• dairy character;
• body capacity;
• median suspensory ligament;
• front and rear attachment of the udder;
• teat quality. females. Scientifically, the desired system of genetic improvement is to select different male offspring from the best goats in various herds and then breed them to many herds (using artificial insemination).
these bucks are then ranked based upon the perfor-mance of their respective daughters’ many herds.
this is done in the United States for bulls of dairy breeds and you can get very detailed information on proven bulls, including how well they improve a certain trait like milk yield. Some of this is done for dairy goats, too, and genetic improvement in herds can be rapid using semen from proven ani-mals. However, because this work requires lots of records of daughters and takes years to get data from a couple of lactations because there are not many proven bucks in the U.S. In Canada, they knew this would take a lot of time for dairy goats so they focused instead on selecting the offspring of known superior animals in each herd that had been evaluated.
After 4 years of continued selection on these 26 dairy goat farms, they published the results for the Saanans and Alpines in the study. they found that the average annual production improvement over the 4 years was between 15.5–18 kilograms of milk (34.1 –39.6 pounds) per goat with an annual aver-age increase of .32–.52 kilograms (0.7–1.14 pounds) butterfat and .33–.47 kilograms (0.7–1.0 pounds) milk protein per goat. this progress, they esti-mated, gave an increased return of $1400–$1600 per year per farm, minimum. And this rate of annual improvement superseded that found in the U.S. and France in the previous 5 and 10 years.
THe iMPACT oF GeneTiC SeleCTion 89 In the United States, the American Dairy Goat
Association is a good resource for finding goats critically subjected to type or physical evaluation (linear appraisal) and milk production and content measurement. Records of many animals and their pedigree (shared relatives) can be analyzed and the predicted transmitting ability (PtA) is ranked based on or their type and milk production (Production type Index). Basically, this means you are able to find animals that have been found to improve phys-ical traits and milk production and content based on actual records. this can be found on the internet at
www.adga.org.
Another point to make is that annual progress is affected by the replacement rate you choose to follow. It is like investing money at different rates of interest. the higher the interest rate, the higher the compounded return over time. For example, if you decide that you will selectively replace 10% of your goats per year with your best replacements you will see only 25% of the improvement than if you chose to replace 40% of your goats with better replacements over a period of 10 years. If you prac-tice a higher replacement rate, the genetics in your herd will become more concentrated or similar so you will eventually want to look for new genetics to inject into your herd to prevent higher degrees of inbreeding. As animals become more inbred, their milk production tends to drop in comparison to their genetic potential.
Inbreeding is caused by mating individuals with the same ancestors. Some purebred breeders do employ line breeding to specifically concentrate the genes of an extraordinary individual in new generations. they keep detailed milk production records and have their animals subjected to linear appraisal by a third party. For goats registered in the American Dairy Goat Association, for example, the inbreeding coefficient is supplied by the pedigree service. Watching for defects, these breeders can see up to 20% concentration of the same ancestor’s genes in offspring. However, unlike line breeding, inbreeding is more common when breeders do not intentionally mate animals with similar ancestry. It is recommended to limit the inbreeding to no more than 10% in any one individual.2 In general, for
every 1% increase in inbreeding above 10%, there is a 1% decrease in production traits.
Genetic selection of your animals based on mea-sured traits that fit your management system will result in progress toward your goals. the rate of progress will depend upon:
1. the intensity of selection (e.g., replacement rate per year 5, 10, 20, 40%?) and the number of traits you select for and your emphasis on each trait. the more traits included will slow the progress but has the potential to create more balanced animals in physical and production traits;
2. the ability to accurately measure the traits. (What traits have you observed in a goat that lives the longest, is the healthiest and produces what you want? How is this measured?) the great advan-tage of DHIA testing is that it can usually do a better job than you at identifying how much of the difference of milk yield in your does is due to genetics versus their environment;
3. the variability of the trait in your herd. (Do you have a wide range of low producers and high producers to choose from? If not, you can’t improve much unless you import other genetics by buying breeding stock or using AI;)
4. the interval of a generation. (i.e. the average age of the parents at the first birth of their descendants.
Are you breeding before one year of age or wait-ing another year?)
Once you decide what measurements to make on which traits, your annual replacement rate will determine how quickly you make progress.
Endnotes
1. Colloque sur la chevre 2005, L’innovation, un outil de croissance! L’impact de la génétique sur les enter-prises caprines. Laurence Maignel, pp. 28–34. www.
agrireseau.qc.ca/caprins/documents/gingras_sophie.
2. Proceedings of the National Goat Conference, Breed-ing for Improved Dairy Goat Production: A Genetic toolbox, pp. 14–31 and presentation, Dr. terry Gib-son, Langston University, September 13, 2010, talla-hassee, Florida.
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