Impacts of grazing cattle in a 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. To this day, travel to almost any Mediterranean coastal area and you’ll see rock-walled sheep paddocks with rows of ancient olive trees covering the hillsides. These farms were first created by ancient Greeks and Romans some 2000 to 3000 years ago. On land unsuited for grain crops, ancient farmers produced meat, wool, oil and fruit all from the same acreage. Today’s native pecan groves are just a reflection of this ancient agricultural system.
Cattle help diversify farm income but grazing the pecan grove impacts the ground cover and the trees in ways not easily recognized. In Figure 2, 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 under a native pecan grove. The tall weed growing on the pastured side of the fence is ironweed (Vernonia sp.). 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 (Elymus canadenis), 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 pecan grove ground-cover composition. On the plus side, cattle will help control poison ivy, Virginia creeper and wild grape, keeping these 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 an explosion of chickweed in thinly-covered, over-grazed pecan groves. Chickweed is a common, winter-annual weed that germinates in the fall at a time when cattle have been removed from the grove in preparation for harvest. Chickweed makes a thick mat of intertwined vegetation, creating a ground cover that prevents harvesters from efficiently sweeping up nuts.
The biggest impact cattle have on the pecan grove is on soil nitrogen. In the past, I’ve talked to cattle “experts” and asked what I thought was a simple question: “How much nitrogen does cattle production use per acre?” The knee-jerk response I got was: “Cattle don’t use nitrogen, they add nitrogen via urine and manure.” Well, I knew that couldn’t be right. A 100-pound calf in the spring turns into a 700-pound calf by fall and part of that weight gain is nitrogen-containing proteins and amino acids.
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 pounds of nitrogen during the grazing season. Of the nitrogen ingested, 10 pounds are retained by the animals and 270 pounds are excreted as urine and manure. But here’s the problem. The nitrogen returned to the soil surface via urine and mature is concentrated in a relatively small soil area. A fresh cow pie or urine puddle overwhelms the soil system and 30-50 percent of the nitrogen contained in these cattle excretions end up volatilizing into the air. Cattlemen call the pungent odor of cattle dung “the smell of money”. But that strong ammonia odor is the smell of dollars you spent on fertilizer floating off into the air.
On a per-acre basis, grazing cattle remove only about 2.5 pounds 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 pounds N/acre. Taken together, grazing cattle remove between 20 and 36.25 pounds of nitrogen per acre from the pecan grove.
Cattle have had a traditional 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 limit 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 pounds N/acre to replace the N lost to cattle production.
Recently, questions are being raised by federal agencies as to the “food safety” of grazing animals in pecan groves. I am unaware of any reported case of human disease caused by consuming native pecans harvested from grazed groves. Nutmeats are tightly held in a shell that is sanitized before cracking. I don’t see the problem that needs government regulation. But, I’ll let others fight this political battle. I’ve got to watch my blood pressure.