Written by: Matthew Jenkins, MoFCB 4R Specialist & Inspector
Believe it or not, grazing management can have a large impact on soil fertility over time. If you’ve been to a Missouri Grazing School, you’ve probably seen the image below from the Dairy Grazing Manual. That image demonstrates how going from a few large paddocks to many small paddocks creates a more uniform distribution of manure piles. Oftentimes, I hear producers interpret this as “Once you start an intense rotational system you won’t have to fertilize anymore.” While there may be some truth in that, I don’t think that’s always the case and here’s why.
According to the Beef NRC, 80 percent of the phosphorus retained in beef cattle is tied up in bones and teeth. In growing cattle, Watson and colleagues found that you can expect anywhere from 3 to 6 grams of phosphorus to be retained in the animal for every pound of gain they grow. In terms of phosphorus fertilizers, this is equivalent to 7-14 grams of P2O5 plant food.
For example, if you have steers gaining 2 lb/hd/d on pasture, you may expect roughly 25 grams of P2O5 to leave the pasture. If you are stocked at 3 calves per acre and were able to get 60 days of grazing that year, you’ll be taking around 10 lb of P2O5 per acre in that year. You can see how after several years, that might add up to detectable amounts in a soil sample.
The reality is that on average, your pasture fertility may not drop that much as long as you are feeding minerals, grain, or unrolling hay from another farm. Keep in mind that taking at least one soil sample within each paddock every 4 years to get a better idea of the nutrients removed from your field.
Key Points
- Growing calves remove 7-14 grams of phosphate per pound of weight they grow.
- Calf P2O5 removal equation (in pounds/acre):
- =(lb gain/day) x (7 to 14 g P2O5) x (# of calves/ac) 454
- Feeding hay, mineral, and other feeds will add fertility back to the soil.
- These values are only relevant to growing and finishing cattle; mature cattle will not retain much phosphorus because their skeletons are done growing.
- For more information, visit the UNL Beef Report at https://www.sciencedirect.com/science/article/pii/S1080744618300457