1a. Objectives (from AD-416):
In Objective 1, we will determine genetic, nutritional, and physiological factors influencing development of the female reproductive tract from sexual differentiation through sexual maturity. Sub-objective 1.A focuses on developing methods to combine reproductive tract scoring and Lutalyse (prostaglandin F2'' to minimize labor while maximizing life time productivity and calf weaning weights in beef heifers. Sub-objective 1.B will validate the influence of polymorphisms in the glutamate receptor, ionitropic, AMPA 1 (GRIA1) gene on antral follicle counts in beef heifers. In Objective 2, we will examine the relationship between genes controlling carcass and production traits and reproductive function to identify biological pathways that mediate the genetic and phenotypic antagonism that exists between production traits and reproductive performance. We will do this by determining if polymorphisms in genes that influence carcass traits have a detrimental effect on reproductive traits (Sub-objective 2.A). In Objective 3, we will identify ovarian, embryonic, and uterine contributions to conception failure and apply them to increase lifetime productivity in beef cattle. We will do this by determining differences in uterine function between heifers with low and high antral follicle counts (Sub-objective 3.A). In sub-objective 3.B, we will combine analysis of ovarian vasculature and the vascular endothelial growth factor system with in vitro fertilization to determine if differences in ovarian vascularity contribute to the variation in oocyte quality in heifers differing in antral follicle counts.
1b. Approach (from AD-416):
The overall objective of this project is to understand the factors controlling establishment of the ovarian reserve (i.e. the number of ovarian follicles that a heifer has at birth) and development of the female reproductive tract. The Reproduction Research Unit at the U.S. Meat Animal Research Center conducts research to identify highly fertile replacement females. Identifying these females early in life can improve overall production efficiency. For example, a replacement heifer must wean 3 to 5 calves to recoup her development costs or incur a net loss for the enterprise. The most practical method for evaluating reproductive capacity in beef heifers is palpation of the reproductive tract. This allows the palpator to identify heifers with small under-developed reproductive tracts. Adding ultrasonography to this process in the previous project demonstrated that antral follicle counts were also an indicator of heifer fertility. Limitations of palpation based evaluation of heifers are the number of animals that can be examined in a day and the age at which these exams can be performed. Understanding the physiological, genetic, and nutritional factors that influence development of the female reproductive tract will allow the implementation of novel technologies to aid in identifying replacement heifers at a very young age. Once they are identified, management strategies can be administered more efficiently to prepare them to conceive as early as possible in their first breeding season. Understanding the interaction of the genes controlling reproductive tract development with genes controlling production traits will determine the impacts of using genetic markers for production traits on reproductive performance in beef cows.
3. Progress Report:
Parturition and weaning data were collected for heifers prepared for breeding with reproductive tract score and prostaglandin treatments (Objective 1A), heifers from the Germplasm Evaluation Population (Objective 1B), heifers from the UNL herd (Objective 1B), and heifers with the F94L myostatin polymorphism (Objective 2). Sequencing and genotyping of the GRIA1 gene were initiated to begin determining the association of the gene with antral follicle count in cows (Objective 1B). Heifers were submitted for ultrasonographic evaluation to identify 10 heifers with high antral follicle counts and 10 heifers with low antral follicle counts that were subsequently slaughtered to collect reproductive tracts (Objective 3A). Oocytes were aspirated from the ovaries and used for in vitro fertilization studies. Uteri were flushed and protein concentration was determined. Protein samples were run on gels to identify differences in specific proteins. Total cellular RNA was extracted from uterine endometrial samples for construction of cDNA libraries.
1. Influence of calving date on reproductive traits. Studies conducted by ARS researchers at Clay Center, Nebraska, in collaboration with scientists at South Dakota State University and the University of Nebraska at Lincoln confirmed that heifers that calve early in their first season remain in the production herd longer. These heifers weaned heavier calves through their first six parturitions, resulting in increased revenues of $500-$2000 per heifer. Results indicated that post-partum interval to conception was not a major contributor to increased herd life. Additional research demonstrated that heifers that calved later and left the production herd earlier had smaller ovaries, and reduced uterine horn diameters along with fewer follicles in their ovaries. Together, these results indicate that pre-breeding ultrasonographic evaluation of the reproductive tract can be a useful tool for removing less productive heifers from the herd before first breeding to increase reproductive efficiency in the cow herd.
2. Mechanisms responsible for placental separation and release in ruminants. Calving difficulty and retained placenta are economically detrimental reproductive ailments that may decrease post-partum reproductive performance in cows. ARS researchers at Clay Center, Nebraska, demonstrated that enzymes involved in placental separation are under multiple levels of regulation resulting in delayed protein synthesis. Subsequently, these researchers developed a tissue sampling device and methodology to facilitate multiple tissue collections from a single animal. As part of a series of experiments designed to elucidate the mechanisms responsible for placental separation, these results are important for resolving how normal placental separation and expulsion occurs after parturition, and identifying uterine factors that may contribute to lifetime reproductive function. Applying these findings towards the development of a treatment for retained placenta is estimated to save $180 million dollars annually in lost revenues.
3. Feed efficiency as growing heifers and subsequently as lactating beef cows. Residual feed intake is an indicator of feed efficiency that measures variation in feed intake independent of body weight, growth rate, and milk production. Studies using growing animals have shown moderate levels of heritability for residual feed intake; however, few studies have compared an individual animal’s residual feed intake between two physiological states, e.g., growing heifer and lactating cow. In industry, it is common to measure feed efficiency as a growing heifer and then to assume that this efficiency is representative of that individual as a cow. Research was conducted in Florida with University of Florida scientists (Marianna, Florida) and an ARS researcher located in Clay Center, Nebraska, to evaluate residual feed intake (and other performance traits) in growing heifers and subsequently as 3-year-old lactating beef cows. Rank correlation between efficiency as heifers and the same animals as cows was very low. Although residual feed intake for postweaning and lactation phases did not appear to be related, it appears that selection of the most efficient heifers based on postweaning residual feed intake may have economic implications by reducing feed costs and maintaining similar cow performance.
Aad, P.Y., Echternkamp, S.E., Sypherd, D.D., Schreiber, N.B., Spicer, L.J. 2012. The hedgehog system in ovarian follicles of cattle selected for twin ovulations and births: evidence of a link between the IGF and hedgehog systems. Biology of Reproduction. 87(4):79.