Reproduction of the cow herd R. Mark Enns Department of Animal Sciences Colorado State University Historical perspective on improving fertility Reproductive ability is very difficult to improve. Lowly heritable traits Focused on properly managing cows rather than on selection for improved fertility Appropriate supplementation and body condition score before calving Example traits Traits Heritability Calving Date .06 Calving Interval .11 Conception Rate .05 Calving Rate .09 Days to Calving .08 Traditional Perspective on reproduction Lowly heritable Focus on managing your cows properly rather than on selection and genetic improvement to improve fertility Difficult to quanitify Binary Trait (yes/no) If all animals in a contemporary group have the same observation, the data is not useful for selection For example, every heifer comes in pregnant after breeding season New perspective Despite low heritabilities we can make genetic improvement in reproduction Driven by the establishment of large databases of performance and pedigree information New statistical techniques – threshold model Heritability of the traits tends to be higher (on an underlying scale) What evaluations for reproductive ability are in use? U.S. Dairy Industry Daughter Pregnancy Rate Percentage of nonpregnant cows that become pregnant during each 21-day period. Bull 1 ―EPD‖= 1 Bull 2 ―EPD‖= 0 Bull 1’s daughters are 1% more likely to become pregnant during that estrus cycle Each increase of 1% equals a decrease of 4 days open Heritability = .04 What evaluations for reproductive ability are currently in use? Australia Days to calving How do you account for open cows? Scrotal circumference Why scrotal circumference? Highly heritable Related to age of puberty in a bull’s daughters Justification: earlier puberty results in more heifers pregnant More on scrotal circumference Early and current research indicates SC is related to yearling and lifetime pregnancy rate Genetic relationship between SC and heifer pregnancy rate = .57 Genetic relationship between SC and lifetime pregnancy rate = .34 We have done a lot to improve SC since this study: Average SC on 13 month bulls was 27.7 cm 27.7% of heifers did not reach puberty before the breeding season Scrotal Circumference may not tell the whole story… Changing Relationships Evans et al., 1999 Why are we concerned with age of puberty in females? Heifer pregnancy Should explain more of the genetic differences in 1st breeding season fertility than scrotal circumference (age of puberty) alone Heifer Pregnancy Heifer pregnancy EPD – What is the probability that a sire’s daughters will conceive in a restricted breeding season? Heritability Angus = .13 Red Angus = .24 Limousin = .20 Challenge – knowing what heifers had the opportunity to conceive What about cow longevity? The ability of a cow to produce a calf each year on a consistent basis Stayability Stayability – Given the female has a calf at age one, what is the probability that she will stay in the herd and raise a calf at age 6? Why 6 years of age? Guttierez et al. showed this was typically the average age a cow paid for herself (and the cows that didn’t make it to that age) Stayability (continued) Challenges – lack of total herd reporting systems that have cow disposal information… In the perfect world would require the cow to have a calf every year Heritability estimates (higher than many other measures): Red Angus = 0.08 Simmental = 0.11 Gelbvieh = 0.12 What evaluations for reproductive ability are out there? U.S. Beef Industry Breed Traits Evaluated Angus Scrotal Circumference Red Angus Heifer Pregnancy, Stayability Limousin Scrotal Circumference, Stayability** Gelbvieh Stayability Simmental Stayability ** Brangus Collecting data for stayability Hereford Scrotal Circumference How important is reproductive ability? The largest contributor to profitability of the cow/calf operation. Ponzoni and Newman, 1989; Melton, 1995 Examples of the value associated with genetic improvement of fertility Change in net income as a function of sires’ heifer pregnancy EPD and herd phenotypic heifer pregnancy level - 1000 cow herd, 18% replacement rate Change in Net Income Change in net income ($) per 1000 base cow herd -- stayability Net Income Herd Stayability Stayability EPD Change in number of mature cows– stayability Challenges of stayability EPD Getting ―reasonably‖ high sire accuracies at early ages For a cow to have an observation she must be 6 years of age Strategies for overcoming this challenge Find genetically correlated traits Correlated traits Body Condition Score Questions: Devori Beckman, PhD student Is it heritable? (requirement) Is it genetically related to stayability? Phenotypically??—yes! Observations on BCS Balancing management and genetic improvement… Contemporary groups must have variation. Variability in BCS 3000 2500 2000 1500 2 Year-old 3 Year-old 1000 4 Year-old 500 0 BCS 2 and BCS 4 BCS 5 BCS 6 BCS 7 and 3 8 BCS – is it heritable? Age BCS Is it heritable? recorded Heritability YES!!! 2 Years .20 Is it genetically 3 Years .17 related to stayability? 4 Years .23 Our theory—genetic optimum for fertility Heifer pregnancy rate BCS < 3 BCS 4 to BCS 6+ 6 Another option Use earlier measures of fertility to increase accuracy of stayability EPD Heifer pregnancy Stayability at earlier ages Stayability to 3 years of age Summary Historically reproductive ability of the cow herd has been difficult to improve Large breed association databases have given us the opportunity to develop tools for genetic improvement of fertility Stayability (length of productive cow life) Heifer pregnancy Questions?
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