√ Fat supplements
Recent focus has been on feeding specific fat supplements during the transition period for improving reproductive efficiency. Although fat is a very dense source of energy, the focus has been on specific fatty acids and their effects on the function of the reproductive organs, not feeding the fat as an energy source. The focus has been on feeding sources of linoleic acid (C18:2) to close-up and fresh cows with the idea that the linoleic acid is important for prostaglandin synthesis that will aid in uterine involution. Several commercial sources of fat are available that are high in linoleic acid, but typical feed sources such as soybeans and cottonseed also can be good sources. During the breeding period, the focus is on providing adequate eicosapentaenoic acid (EPA) and/or docosahexaenoic acid (DHA), which are thought to help maintain a viable corpus luteum to aid in maintaining a pregnancy. Again, a few commercial sources are available, and fish meal and other marine feed sources contain appreciable concentrations of these fatty acids.
√ Protein nutrition
The focus on protein relative to reproduction has been on not feeding it in excess, especially not feeding excessive amounts of rumen degradable protein (RDP). Milk and blood urea nitrogen (MUN and BUN, respectively) are monitored in reference to this aspect. Caution should be exercised if MUN for the herd or high cow group exceeds 18 mg/dL and/or BUN exceeds 20 mg/dL. The actual targets for MUN are 10 to 14 mg/dL with the aim at reducing feed costs and nitrogen excretion by animals. To reduce nitrogen excretion, some scientists are even lowering the recommended MUN concentration to 8 mg/dL with strategic formulation of diets for amino acids. The high circulating concentrations of ammonia and urea may be toxic to sperm, ova, or embryos or may reduce the binding of luteinizing hormone to ovarian receptors, which leads to a decrease in serum progesterone. In general, the focus has been the potential for embryo mortality with high BUN, which is reflected in higher MUN. Although this is a potential effect to keep in mind, other influences (e.g., heat stress, disease, etc.) may more likely be causing the embryo mortality. This association of protein on reproduction has been speculated by some to occur because the highly RDP sources fed to dairy cattle are often legumes (e.g., soybean meal), and these legumes are sources of estrogen that could be affecting the reproductive cycles. However, this relationship has not been substantiated by research.