Final answer:
When cows have a low Body Condition Score due to inadequate forage, the priority nutrient is energy. For soil fertility, nitrogen is a crucial soil-derived macronutrient necessary for plant growth.
Step-by-step explanation:
When forage is inadequate and cows have a low Body Condition Score (BCS), the nutrient that takes priority is energy. Ensuring adequate caloric intake is crucial for maintaining basic physiological functions and overall health. Without sufficient energy, cows cannot efficiently use other nutrients such as proteins, vitamins, or minerals. In the case of soil fertility issues, if a soil sample shows deficiencies in all soil-derived macronutrients, one major macronutrient derived from the soil that is essential for plant growth is nitrogen. Other important soil-derived macronutrients include phosphorus and potassium, but nitrogen is often the most limiting factor for plant growth and is a key component of amino acids, proteins, and chlorophyll.