Blue Oaks: Forage Production and Quality

The value of blue oaks in terms of forage production has long been debated. In northern California blue oaks have been shown to decrease total forage production as compared to that in open grassland. However, at the San Joaquin Experimental Range (SJER) total forage production has been found to be greater beneath the canopy of blue oaks. A variety of reasons have been identified for this difference: scattered blue oaks occur at SJER as opposed to dense stands further north and climate differences. A cooperative study between CATI, C.S.U. Fresno and U.C. Cooperative Extension (funded by the University of California Integrated Hardwood Range Management Program) has been studying the effect of blue oak canopy on seasonal forage production and quality to better understand this relationship. The study has been conducted in an area of SJER which has approximately 7% blue oak canopy cover, less than 15% slope and a general southwest aspect on an Ahwahnee soil. The years of this study (1986-90) have coincided with the drought cycle we have been experiencing. Annual precipitation at SJER has averaged about 11 inches, compared to a long term average of 19+ inches.

In general, there has been no significant difference in forage produced beneath blue oaks or in open grassland until March. In March forage produced beneath the canopies of blue oaks was greater than that in the open grassland, by an average of about 60 percent. This same relationship was also apparent at peak standing crop, when over 80% more forage was found beneath blue oaks than in open grassland.

1986-90 Average Forage Production (lb/acre)

In addition, clear differences in forage quality were measured between the open grassland and blue oak forage. Under blue oaks the forage was consistently higher in crude protein concentration and lower in fiber and lignin. On average the crude protein concentrations in forage under blue oaks were 54% higher than in the open grassland forage. When combined with the differences in forage abundance between the two situations there was twice the amount of protein available (lb/acre) under oaks than in the open grassland.

1987-89 Crude Protein Content of Forage (percent)

To estimate the integrated effect of forage abundance and quality on grazing animals, crude protein intake of a nursing cow was calculated in both situations (open grassland and beneath blue oak canopy) and related to her daily requirement. The open grassland had only one brief period (March) where daily dietary crude protein intake was in excess of the requirement, while beneath blue oak canopies an excess of crude protein is provided from the period from mid-December through May.

These results are a comparison of two ends of the spectrum: complete blue oak canopy cover and those with no oak canopy. While the typical situation is a mixture of the two, these results clearly demonstrate the value of maintaining an abundance of scattered blue oaks for grazing animals.

prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc