High And Low Fertility In Dairy Cows

Background to declining fertility

Historically, the Irish dairy herd was primarily comprised of the British Friesian. Ireland’s first national selection index, known as the Relative Breeding Index (RBI), was established in 1990.

The RBI was a single trait selection index, focusing solely on milk production traits. Selection Indexes that encouraged use of high milk production bloodlines were common at the time. Selecting solely for milk production traits had been carried out in the North American Holstein Friesian (NAHF) breed for decades. From 1990 to 2001 the proportion of NAHF genetics within the Irish dairy herd increased from 9 per cent to 65 per cent.

In the same period, the average pedigree index for milk yield and actual phenotypic milk yield increased by 275kg and 742kg, respectively (Evans et al., 2006). This so called ‘Holsteinisation’ of national herds occurred in most major milk production countries around the world.

The NAHF cow was selected under high energy density total mixed ration feeding systems and total confinement housing, as is commonly practised in the US. However, in countries like Ireland that employ strict seasonal-calving pasture-based systems, diets consisting primarily of grazed grass reduced daily energy intake potential; ergo, energy intake was inadequate relative to potential for milk production.

The resulting cows were susceptible to negative energy balance and excessive mobilisation of body reserves in early lactation, failed to regain body reserves during mid to late lactation, and had markedly depressed reproductive efficiency. The overall effect was to reduce the profitability of pasture-based systems.

In 2001, the Irish Cattle Breeding Federation (ICBF) established a multi trait selection index to address the problem of declining fertility in the national herd, known as the Economic Breeding Index (EBI). As the name ‘Economic Breeding Index’ infers, the EBI is designed to identify genetically superior animals for profitability in typical Irish milk production systems.

The genesis of a profitable cow is expressed in the EBI through six subindexes (relative emphasis): milk production (38.1 per cent); fertility/survival (34.8 per cent); calving performance (10.3 per cent); beef carcase (7.2 per cent); maintenance (6.1 per cent); and, health (3.6 per cent). The fertility subindex is comprised of calving interval (CI), which is the single biggest contributor to the EBI at 23.2 per cent, and survival (11.6 per cent). The trends in genetic merit for CI and survival of progeny-tested AI bulls during the last 40 years are illustrated in Figure 1. Deterioration in both fertility traits is evident from the early 1980s until around 2000, followed by a period of rapid improvement in the last 10 years.

FIGURE 1: Genetic merit of progeny-tested bulls for fertility and survival traits.

Source: www.icbf.com

Establishment of Moorepark High/Low fertility herd

To improve our knowledge in key areas of dairy cow reproductive physiology, a long-term project using cows with similar genetic merit for milk production traits, but with extremes of high (Fert+) or low (Fert–) genetic merit for fertility traits, was initiated in autumn 2007.

In collaboration with the Irish Cattle Breeding Federation (ICBF), the national herd was screened for heifers of NAHF ancestry, calving for the first time in spring 2008. Within this population of heifers, large variation in genetic merit for fertility was apparent (Figure 2).

Two populations of heifers with extremes for either high or low genetic merit for fertility were identified. Within these two populations, eligibility was restricted to only heifers with high genetic merit for milk production. Heifers identified as available for purchase were examined (size, body condition, lameness, etc.), screened for infectious diseases and subsequently moved to Moorepark Research Farm in their last trimester of gestation.

On arrival, the high and low fertility animals were managed as a single herd in a typical grass-based spring-calving production system. By standardising genetic merit for milk production, proportion of NAHF ancestry and environment (i.e., management practices, diet), it was envisaged that the underlying physiological basis of declining reproductive performance in lactating dairy cows could be elucidated using this genetic model of high and low fertility.

FIGURE 2: Heifers with high and low genetic merit for calving interval were identified. Within both groups, selection criteria were restricted to heifers with high genetic merit for milk production.

FIGURE 3: Survival analysis curve showing the interval from mating start date (MSD) to conception in high and low fertility cows.

Results to date

Following parturition, cows were turned out to grass in early February until mid November and grazed under a rotational grazing system. During the entire first lactation, mean milk production (17.6 vs. 17.7kg/day), milk solids yield (1.25 vs.1.26kg/day), milk fat concentration (39.0 vs. 38.5g/kg) and milk protein concentration (33.1 vs. 33.6g/kg) did not differ between the Fert+ and Fert– groups, respectively. This indicated that the energy demands associated with lactation were similar between groups.

The breeding season commenced in mid April 2008 and lasted for 20 weeks. Cows were inseminated to observed oestrous to sires of their own genetic group to generate replacement heifers. There was no difference in the calving to service interval for both groups (averaging 77 days), but there was a large difference in the calving to conception interval, which is a vital statistic in a seasonal calving system.

On average, Fert–cows took an extra 28 days to conceive, increasing the inter-calving interval to 395 days compared to 367 days for the Fert+ cows. The reproductive performance of both groups is best illustrated in a survival analysis curve (Figure 3), which plots the rate at which both groups successfully conceived. The y-axis is the proportion of animals not pregnant and the x-axis is the interval from mating start date (MSD) to conception. The Fert+ cows readily went in calf and would survive in a seasonal-calving system of production. Reproductive performance was suboptimal in the Fert–cows; these animals required more services to get pregnant (2.9 vs. 1.8) and had a higher non- pregnant rate (28 per cent vs. 11 per cent) at the end of the breeding season.

Large differences in reproductive performance were observed, indicating for the first time that phenotypic reproductive efficiency is critically dependant on genetic merit for fertility traits. However, the underlying physiological controls responsible for the observed differences in reproductive performance remained unknown. With this in mind a detailed study of ovarian dynamics was carried out on the high/low fertility herd in 2009, then in their second lactation.

The function of corpus luteum (CL) is to produce progesterone (P4), a steroid hormone essential for pregnancy establishment. Following a synchronised oestrous, the Fert+ and Fert–cows underwent daily transrectal ultrasonography and blood sampling three times daily, with a view to monitoring CL volume and P4 concentrations throughout the duration of the oestrous cycle. The results are illustrated in Figure 4. The CL volume was larger and circulating P4 concentrations were greater in the Fert+ cows (depicted in blue) during the first 16 days of the cycle, compared to the Fert– cows. This is the first time genetic merit for fertility traits has been shown to alter both the size of the CL and circulating concentrations of P4. The results concur with a large body of evidence demonstrating the beneficial effects of elevated P4 concentrations on early embryo survival.

FIGURE 4: Temporal profile of corpus luteum volume and circulating progesterone (P4) concentrations during the oestrous cycle of high and low fertility lactating dairy cows.

Benefits to industry

A unique genetic model has been established whereby animals exposed to similar energetic demands associated with milk production display large differences in reproductive efficiency, at least partially explained by differences in CL volume and circulating P4 concentrations. This study clearly demonstrates that it is possible to simultaneously select for improved milk production traits and fertility traits as part of a balanced breeding objective. This animal model represents a unique and powerful tool to improve our understanding of poor fertility and elucidate the critical events associated with pregnancy establishment.