The Carrizo Valley Ranch (CVR) has a sandpaper oak population that if managed could improve forage quality for deer, elk, and livestock. The CVR conducted a study to determine if mowing oak brush would improve its nutritional quality by reducing plant characteristics known to reduce palatability and digestibility.
This approach fits well with the ranch’s management paradigm to promote herbaceous plant diversity, protect and stabilize range resources, and provide high quality habitat for livestock and wildlife through grazing management and other vegetation manipulation practices.
Our objective was to determine the effects of manipulating sandpaper oak on nutritional quality and palatability indicators to better supplement livestock diets and to improve forage quality, quantity, and cover for wildlife.
Test plots were set up in July of 2008 by researchers and the rancher. Three different pastures were selected and three treatments were established. Plots were 60-ft wide by approximately 300-ft long. Plots were designated as control, growing season mow, and dormant season mow treatments. Newly sprouting and mature leaves were picked along the entire length of each plot and placed in separate paper bags to constitute one composite sample for each leaf age-class.
RESULTS & DISCUSSION
Mowing appeared to affect crude protein and lignin contents of young sprouting oak leaves. Crude protein was generally higher in young leaves of the mowed treatment when compared with the control. There was no difference in crude protein between dormant-mow and growing-mow treatments within leaf age-class. Mature leaves did not differ in crude protein content among treatments. Although there was considerable overlap in the estimates, crude protein was higher in young oak leaves of mowed plots. New growth of mowed oak brush could represent an important protein source for cattle, elk, and deer during certain times of the year.
A trade-off for cattle is that oak contains lignin and tannins, which reduce digestibility, digestive action, and protein intake and may be toxic at high intake. Deer would not be affected by tannin content and would benefit from the higher protein content of the young leaves, elk would likely benefit similarly. We observed substantially more available new growth on individual plants that had been mowed compared to control plots.
Although crude protein and lignin content of leaves were positively affected by mowing, condensed tannins and hydrolysable tannins (partially represented by total phenolics) have the potential to offset benefits. Condensed tannins (CT) and total phenolics (TP) are displayed by sample collection date as timing in the growing season has an impact on their concentration.
Young leaves had lower concentrations of CT than mature leaves because concentrations increase as the growing season progresses. Concentrations of tannins should be viewed within year as they vary from year to year. Although typically not toxic, CT binds with proteins and may lower protein intake as well as reduce digestive action. As CTs accumulate their astringency increases reducing palatability of leaves. Young leaves have lower concentrations of CT but TP concentrations tend to be higher in young oak leaves than mature.
The highest estimated concentration of TP was about 22% for young leaves in the dormant-mow sample taken in May of 2009. Hydrolysable tannins break down into their constituent phenolics and sugars making them more absorbable in a ruminants digestive tract. These phenolics cause poisoning in animals that do not produce tannin-binding proteins in their saliva.
Leaf and bud tannin concentrations responsible for oak poisoning in cattle are typically greatest early in their growth during sprouting. Deer are not known to get oak poisoning because of tannin-binding proteins in their saliva, which is likely similar for elk, although it has yet to be studied.
Oak poisoning is unlikely on light to moderately grazed rangelands. Heavily grazed rangelands or when oak comprises a large portion of diet greatly increases the possibility for oak poisoning.
Although we did not observe any symptoms, we did not specifically monitor cattle to determine if there was reduced digestive performance related to plant components. These phenomena deserve further study.
Sampling over more months beginning with leaf-out (May) to the dormant season (Nov) would provide a better understanding of changes in protein, lignin, and tannins during the growing season. Collecting grass samples every time oak samples are collected would allow for an in-depth comparison between grass and oak forages. Collecting base-line performance data on cattle including blood, urine, and feces for analysis before cattle are exposed to oak would allow a complete assessment of oak as a spring and fall forage for cattle. Collect the same performance data at regular intervals once cattle have been on oak forage for a few days. Determine the proportion of oak in cattle diets. Set trial cameras at appropriate locations to catalog use by wildlife and cattle of the various treatment plots.
Have the species of oak you wish to use identified and ask your county ag agent and neighbors if they are aware of oak poisoning problems in the area. Ask for literature on oak management poisoning. Start small and only mow a small area of oak. There was no clear benefit to dormant season mowing versus growing season so mow when it is most convenient. Familiarize yourself with the symptoms of oak poisoning. Ensure ample dormant grasses are present. Closely monitor your cattle for signs of oak poisoning. Consider using a dietary supplement when your cattle are on oak brush rangelands to ensure you will not experience problems.
This document will be published as an Extension publication at New Mexico State University and will be used at an oak brush workshop to be held at the CVR in the spring of 2010. A supporting PowerPoint presentation will be developed for use at the workshop. Results will be submitted for presentation at state and regional conferences on range management. This document will be expanded using statistical analysis and reformatted for submission to a peer reviewed publication.