Trends in Age at First Calving and Calving Intervals
Posted: January 4, 2006
The first major trend was that age at first calving decreased markedly between 1980 and 2004. Average age at first calving in 2004 ranged from 24 mo. for Jerseys to 28 mo. for Ayrshires (see figure 1). Furthermore the distribution of age at first calving for Holsteins (not shown) is much more uniform than it was in 1980.This decrease is due to better management of calves and heifers and realization of the economic importance of earlier first calving.
Figure 1. Trend in age at first calving by year of first calving for Ayrshires (■), Brown Swiss (▲), Guernseys (Х), Holsteins (•), and Jerseys (♦). (J. Dairy Science 89:365, 2006)
That was the good news. The second trend presented in this report was the significant increase in calving intervals during this 24 year period. The data summarized by USDA showed that across parity, annual increase in calving interval was similar for all breeds (0.9 to 1.07 days/year) except Jersey (0.49 day/year). Figure 2 illustrates trends in calving interval between first and second parities.
Figure 2. Trend in calving interval between first and second parities by year of first calving for Ayrshires (■), Brown Swiss (▲), Guernseys (×), Holsteins (•), and Jerseys (◊). (J. Dairy Science 89:365, 2006).
Across all parities over this period of time, the average calving intervals were 390, 398, 404, 406 and 407 for Jersey, Ayrshire, Holstein, Guernsey and Brown Swiss, respectively. This trend is similar to those in other major dairy countries that select for high milk and component yield. For more detailed information on this study and other research articles from USDA AIPL access the following website:
Although the heritability of reproductive traits is low compared to production traits, over time there is an impact. Dr. Chad Dechow, dairy geneticist at Penn State University, states that the genetic correlation between milk yield and Daughter Pregnancy rate in US Holsteins is -0.35, indicating that selection for higher yield alone will result in lower pregnancy rates. He further states that genetic and phenotypic estimates from USDA indicate average pregnancy rate of a cow born in 2000 is about 6 percent less than the average cow born in 1975. About half of this change is due to genetics.
Those are some of the genetic trends. What physiological changes have occurred over time? This is difficult document but we know from several intensive studies describing estrous behavior that on average the duration of estrus in Holsteins is only about 7-8 hours and higher producing cows have a shorter duration of estrus and fewer standing to be mounted events than lower producing herdmates. Cows with greater negative energy balance and significant loss in body condition have an increased interval to first ovulation and lower first service conception rate. Research from Wisconsin suggests that higher milk production with associated higher dry matter intake may increase liver metabolism of steroid hormones important to reproductive function. The impact of higher milk yield, postpartum problems and mastitis on uterine and embryo physiology is currently being investigated. There is the whole complex issue of environmental effects on reproductive performance including susceptibility to heat stress, cow comfort, footing surface, pre and postpartum cow environment and management.
Reproductive management is a challenge. However, summarization of reproductive performance comparing 21-day pregnancy rates from a large number of DHIA herds consistently show on average equal and in some studies better reproductive performance in higher producing herds compared to lower producing herds. Designing and complying with effective protocols for reproductive management, feeding management, herd health and cow comfort are crucial to minimizing the effects of high milk yield on reproductive performance.
Michael O’Connor, Dairy and Animal Science Extension