Sandy Johnson, extension beef specialist, Colby
In an ideal world, each time a cow was inseminated (intentionally inseminated that is, this is an “ideal” world) it would result in a live and healthy calf. Unfortunately, even though fertilization rate is about 95 – 98%, by day 28 after mating, only 70 % are still pregnant and there are further losses before calving. While the significance of this loss is easy to see, research to reduce this loss is still hampered by the lack of a reliable early pregnancy test. Nevertheless, we can improve our understanding of factors that are known to contribute to embryonic loss and seek to minimize the impacts.
Known stressors that affect embryonic loss and pregnancy rates include disease and inflammation, nutrition, temperament, temperature, and transportation. For this article, the focus will be on nutrition. We are most familiar with situations where inadequate nutrition delays puberty in heifers, the return to normal estrous cycles in postpartum cows, or a severe nutritional shortage can cause cycles to stop. These changes reflect nutrient intake over weeks and months. We are less familiar with the impacts of shorter-term nutritional changes that might occur during the production cycle perhaps due to unintended situations like drought or limited forage availability.
The impact of nutrient availability on reproduction is mediated by several metabolic hormones that can influence oocyte quality and embryo development. Insulin is one of these metabolic hormones. When nutrition is restricted, circulating concentrations of insulin decrease. If nutrition is restricted and insulin levels are replaced, the result is a larger dominant follicle and improved ovulation rate. When embryos are cultured, insulin provided in excess of normal physiological levels results in a decrease in embryo growth. It could be elevated insulin levels play a role in reduced fertility in over-condition animals.
Dunne and co-workers altered the pasture allowance of 18–24-month-old beef heifers to achieve either 2 times the maintenance energy requirements (H) or .8 of maintenance energy requirements (L) to evaluate pre-breeding and post-breeding nutrition. Heifers had either low or high energy 10 days before breeding and high or low energy after breeding to achieve four treatment groups, L-L (low pre-breeding and low post-breeding), L-H (low pre-breeding and high post-breeding), H-H (high pre-breeding and high post-breeding), and H-L (high pre-breeding and low post-breeding). Embryo survival rate was lower in the H-L group compared to the other three treatments. The observation that embryonic loss was only impacted in H-L group, implies that the developing embryo was sensitive to the decrease in forage allowance whereas increased or no change in forage allowance did not impact embryo survival.
In a series of studies by Perry and co-workers at SDSU, replacement heifers developed in a dry lot since weaning and then moved to summer pasture immediately after AI lost weight and had reduced pregnancy rates even if forage quantity and quality were adequate. These decreases were not evident if replacement heifers had post-weaning grazing experience. Pedometers on the dry lot only reared heifers indicated they took significantly more steps the first few days on pasture compared to those with post-weaning grazing experience. Even though forage was available, either the novelty of the new surroundings, inexperience grazing and or time spent looking for a feed bunk likely impacted short-term gain and reproduction.
The effect of a post fixed-timed AI diet changed was further evaluated in yearling heifers fed to gain 1.5 lb/day pre-breeding (Kruse et al, 2017). After AI, half of the heifers remained on the pre-breeding diet and the other half were fed to lose weight (50-80% of NRC). Embryos were collected on day 6 and evaluated for quality. Embryos from heifers fed to lose weight were delayed in stage of development and lower in quality (scored from 1=excellent to 5=degenerate) than those from heifers continuing to gain weight.
These are examples of relatively short periods of reduction in nutrient availability and subsequent negative impacts on embryo health or survival. The limited data presented here are an oversimplification but represent repeatable effects. In the long run, we may want to identify those animals that were able to produce high quality embryos in the face of reduced nutrient intake. Until our understanding reaches that point, early embryo survival will be improved if diets are at the same level or improving as females enter the breeding season.
