|Beef cattle grazing in a native pecan grove|
The grazing of animals under tree crops is an age old agricultural practice that started soon after man first learned to tend livestock. Travel to almost any Mediterranean coastal area and you'll see the small, rock-walled sheep paddocks with ancient olive trees still dotting a landscape first created by ancient Romans.
In the photo at left, the pecan grove on the left side of the fence is pastured while the grove on the right side is mowed. Cattle are picky eaters. Over time, intensive grazing will change the composition of the plants that make up the ground cover. The tall weed growing on the pastured side of the fence is ironweed. This plant has stems as tough as iron, can grow 5 feet tall and is totally avoided by grazing cattle. On the right side of the fence, you see mostly Canadian wild rye, a common native in our river bottom and great cool season forage grass. You don't see wild rye in the grazed area because the cattle have already chewed it into the ground.
Grazing has both positive and negative impacts on the pecan grove ground-cover composition. On the plus side, cattle will help control poison ivy and wild grape, keeping those troublesome vines from climbing trees. Over-grazing, on the other hand, fosters the growth of undesirable plants in the ground cover. Mowed-off ironweed stems can damage the rubber fingers of your pecan harvester. Nutsedge, another plant cattle will not eat, will quickly carpet the orchard floor during the summer months. Nutsedge secretes allelopathic substances into the soil that can inhibit the growth of other plants and possibly reduce pecan tree vigor.
Last year, I saw and explosion of chickweed in thin, over grazed pecan groves. Chickweed is a winter annual weed that germinated in the fall after the cattle were removed from pecan groves in preparation for harvest. Chickweed made such a thick mat of intertwined vegetation that harvesters were unable to effectively pick up the nut crop.
I finally discovered some beef research that estimated the nitrogen uptake by a grazing cow/calf pair. The only problem with reading beef research is everything is reported in animal units not on a per acre basis. So, in calculating nitrogen use by cattle, I will assume that each cow/calf pair utilizes 4 acres of grazing land.
It is estimated that a cow/calf pair ingests 280 lb. of nitrogen during the grazing season. Of the nitrogen ingested, 10 lbs are retained by the animals and 270 lbs are excreted as urine and manure. But here's the problem. The nitrogen returned to the soil surface via urine and manure is so concentrated in a relatively small area that it overwhelms the soil system and 30-50% of the nitrogen contained in cattle excretions end up volatilizing into the air.
On a per acre basis, grazing cattle remove only about 2.5 lbs. of N per acre as increased body weight. However, nitrogen volatilization from cattle urine and manure causes a net loss from the pecan grove of between 17.5 to 33.75 lbs. N/acre. Taken together, cattle grazing in the pecan grove removes between 20 and 36.25 lbs of nitrogen per acre.
Cattle do have a place in native pecan groves but knowing the impacts of grazing on the system is important for growers to understand. Growers should practice good pasture management techniques to limits the spread of troublesome weed species and be careful not to over graze the orchard floor. Growers should also consider increasing their spring fertilizer nitrogen application by 30 lbs N/acre to replace the N lost to cattle production.