When considering Ca and P as a nutrient, most ruminant nutritionist will consider them together. Specifically, we usually consider the Ca:P ratio. These nutrients are tied very closely together in how they interact within the body. They are both major constituents of bone and have unique and powerful roles in energy metabolism, growth and overall development. They are vital macro elements that must be considered carefully during every stage of production.
Why do we consider the requirements of these nutrients as a ratio? Absorption of each of these is related to the other. In addition, bone retention or release of either of these elements is also related to the other. There are homeostatic mechanisms that closely regulate blood Ca and P, due to these mechanisms and their close relationship, we consider them together.
How important is the Ca:P ratio
The importance of a specific ratio is not as critical as some may lead you to believe. There is research going back 50 years that would indicate that growth and development are not impacted with a Ca:P ratio of 1:1 to 7:1. There is also data to suggest that a minimum of 1.5:1 and a maximum of 4:1 is needed to maintain maximal growth potential. At the end of the day, many unknowns still exist regarding Ca:P so our goal is to make sure we ‘stay between the ditches’.
The most critical aspect of the Ca:P ratio is to ensure that the ratio does not become inverted, where we are feeding more P that Ca. In this event, you can expect for absorption of Ca to be impaired. Due to the homeostatic mechanisms, bone Ca will be released to maintain blood Ca. In this event, bone density may be reduced impairing bone integrity.
Make sure we supply adequate phosphorous
Phosphorous is an important element to energy metabolism and growth. Achieving an adequate Ca:P ratio starts with ensuring we supply adequate phosphorous and building Ca around this basis. Cows will need approximately 0.12 to 0.20% dietary P (DMB) depending on level of lactation. Growing animals, including bulls and heifers, will need between 0.2 to 0.3% dietary P (DMB) depending on the stage of production and expected growth rates. Starting calves in the feedlot, it is recommended to provide 0.25-0.3% dietary P while bred replacement heifers may need up to 0.35% dietary P. As a general rule, I do not run diets over 0.35% P to help prevent the incidence of urinary calculi.
Minimum Calcium for Growth
Using a Ca:P ratio of 1.5:1 is adequate to maintain animal performance and production of the average animal. As easy as it is to achieve, I’d rather feed to the stage of production and allow my ratio’s to vary based on the changing needs of the animal.
- Cow diets: Using a 2:1 Ca:P ratio should always be adequate assuming P requirement is met. A ratio of 1.5 is likely workable but I would only recommend using this ratio when higher than required P levels (0.2% or more) are fed.
- Calves/replacement heifers: Target 0.55 – 0.6% Ca in diets of cattle with a high structural growth rate and bred replacement heifers. The final ratio may fall slightly less than or greater than 2:1 but this level of Ca is adequate to maintain and promote structural integrity.
- Growing to Finishing – 0.55% Ca in growing animals down to approximately 0.45% Ca in finishing cattle. I personally like to target 0.5% Ca in finishing diets as I believe there is benefit in the buffering capacity of limestone in the rumen.
Imperfect Science!
Requirements of Ca, P and the proper Ca:P ratio is based on the knowledge we have today. Mineral nutrition is an imperfect science and there is plenty to learn in the future. When analyzing cattle Ca:P requirements, first make sure that P requirement are met and then balance Ca based on the stage of production. Whether the Ca:P ratio is 1.3:1 or 2.5:1, these general guidelines will keep you ‘between the ditches’. We can definitely over think mineral nutrition. My simple advice is to prevent excessive phosphorous and follow some general Ca guidelines to get the most out of your cattle.