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EXPANSION AND EVALUATION OF A DYNAMIC, MECHANISTIC MODEL OF NUTRITIONAL AND REPRODUCTIVE PROCESSES IN DAIRY CATTLE
The effects of nutrition and genetics on fertility are multiple and although we do have a large knowledge base and good management practices; reproductive efficiency does not match the biological potential. In part, this is because we lack a full systems approach to managing the genetics and nutrition of cows to improve reproduction. Our objective was to expand the integration of nutritional and reproductive processes in a mechanistic, dynamic model of the dairy cow; suitable for evaluation of data, concepts, and hypotheses regarding underlying genetic, nutritional, and physiological control of reproduction. A model of metabolism (Molly, UC Davis); which describes nutrient metabolism, as well as tracking energy transactions; was integrated with a model of reproductive processes, which describes growth and decay of the follicles and corpus luteum, gonadotropin releasing hormone, follicle stimulating hormone, luteinizing hormone, progesterone, estrogen, oxytocin, and prostaglandin F2α over time. The models are integrated at specific points based on available literature data, for example: glucose and IGF-I affect rates of synthesis and release of follicle stimulating hormone, luteinizing hormone, and follicular growth according: follicular_growth = (follicular_rate_constant = hp_fsh_mod + (follicular_ rate_factor_IGF_1(0.001833) * (IGF_1 - average_IGF_1))), where follicular_rate_constant is the rate of follicular growth, hp_fs_mod is a Hill function describing the effect of FSH on growth, and follicular_ rate_factor_IGF_1 which affects follicular growth. Degradation of estrogen and progesterone is a function of metabolic rate in visceral tissues of Molly (AtAdV), for example: progesterone degradation = 0.0005669 (deg_const_P4 = progesterone degradation factor (0.0005669) + (metab_rate_degradation_factor_P4 * (AtAdV -avg_ATADV)). During pregnancy, cycling ceases and the model maintains progesterone concentrations and describes fetal growth. A modeling analysis that varied milk production from 25 to 55 kg/d DMI from 18.8 to 27.3 kg DMI, gave a range of metabolic rate from 1090 to 1426 M/d and a range of IGFI from 86.4 to 106.4 ng/L). Increasing IFGI increased follicular growth] , while increasing metabolic rate increased the degradation of estrogen and progesterone . Because most reproductive systems have negative and positive effects on each other, it is the interaction of these systems which provided an interesting pattern of change in follicular growth and steroid degradation. This model should be of use in testing hypotheses about effects of genetic selection and nutritional management in dairy cattle.
Keywords: systems biology, reproduction, nutrition