Diet Selection by Cattle under High- Intensity Low-Frequency, Short Duration, and Merrill Grazing Systems

A study was conducted to evaluate standing crop of forage and cattle diets for a ‘I-pasture high-intensity low-frequency (HILF) grazing system, and a ‘I-pasture short-duration grazing (SDG) system on the same area. A epasture, 3-herd (Merrill) deferred rotation grazing system was sampled as a standard for comparison. Standing crop of forage was highest for the HILF grazing system compared to the SDG and Merrill grazing systems. The Merrill system with brush control (Pasture 10) had a greater standing crop than the Merrill system without brush control (Pasture 16) or the SDG system. Standing crop in Pasture 16 was comparable to the SDG system. Cattle diets from the HILF system varied significantly between collections at the beginning and end of each grazing period. A significantly higher percentage of forbs were consumed at the beginning of each grazing period (Period A) compared to the end (Period B). Cattle selected the greatest amounts of pricklypear at the end of each collection period during fall and spring, but not during the winter. Greater amounts of pricklypear were selected when mature grasses and oak and juniper browse were the primary alternatives. Crude protein (CP) levels of diets from the HILF system decreased with shifts in forage selection from Texas wintergrass and forbs (Period A) to pricklypear and dry grass (Period B). Diets from the SDG system were characterized by higher percentages of grass and less forbs and pricklypear compared to the HILF grazing system. Also, there were no major shifts in forage selection between collection period A and B for the SDG system. This resulted in a non-significant difference in CP values for diets collected in period A compared to period B. However, a significant decline was recorded for digestibility of diets between the two collection periods. CP and digestibility were higher for diets from the SDG system compared to the HILF grazing system. Botanical composition and diet quality were comparable for the SDG and Merrill grazing systems. Competition between different kinds of animals may be reduced by changing management from a HILF to a SDG system. This would be important where combinations of animals such as cattle, sheep, goats, and deer utilize a common range. Based on diet quality, livestock production from a SDG system should be equivalent to a Merrill system. Most rangelands on the Edwards Plateau have been grazed yearlong and too intensively for almost 100 years, resulting in a significant decline of their productivity. By using proper kinds of animals, stocking rate, and system of grazing, Merrill (1959) found that declining productivity of these rangelands could be reversed. Acocks (1966) described non-selective grazing as an effective scheme for range improvement but indicated that it might reduce nutrient intake. At the time of the research authors were research scientist, professor, professor in charge, and research assistant, Texas Agricultural Experiment Station. This report is Technical Article No. 15258 from the Texas Agricultural Experiment Station. C.T. Taylor’s and LB. Merrill’s present address is P.O. Box 918, Sonora, Texas; M.M. Kothmann’s is Dept. of Range Science, Texas A&M Univ., College Station, Texas; and D. Elledge’s is 110 N. Elm, Perryton, Texas. Manuscript received July 11, 1979. 428 During the past two decades, new grazing schemes have evolved utilizing several pastures and fewer herds of livestock (Booysen and Tainton 1978). They described two types of intensive rotational grazing systems. High utilization grazing is comparable to high-intensity low-frequency (HILF) grazing used in Texas and highperformance grazing is comparable to short duration grazing (SDG). Howell’s (1978) experience with the systems led to a shortening of the grazing and rest periods with subsequent improvement in animal performance. Savory (1979) reported new concepts in ranch fencing designs to facilitate frequent movement of animals under short duration grazing systems. Corbett (1978) found that steers on a HILF grazing system needed supplements during the latter part of the grazing periods throughout the year. HILF grazing systems using grazing periods longer than 2 weeks have resulted in reduced animal performance (Robbins 1975; Kothmann et al. 1975). The cause of reduced animal performance must be identified to improve the design of intensive rotational grazing systems. The objectives of this study were to determine the standing crop of forage in seven pastures grazed first under a HILF system and then under SDG and in two pastures grazed under a 4-pasture, 3-herd (Merrill) deferred rotation system. The botanical and nutrient compositions of diets selected by cattle under these grazing systems were also determined. Methods and Materials Botanical and nutritive compositions of cattle diets were determined on the Texas A&M University Agricultural Research Station at Sonora, Texas. Two studies were conducted, the first during 197 l-72 and the second during 1978-79. Experiment I The study site was a 7-pasture, high intensity low-frequency grazing system using cattle, sheep, and goats. All livestock were in one herd and were moved every 3 weeks, giving each pasture 21 days of grazing followed by an 18-week deferment. The stocking rate was equivalent to 4.8 ha/auy (54 animal units per section) yearlong. The stocking ratio was 60% cattle, 20% sheep, and 20% goats. Standing crop of available forage was determined immediately prior to grazing in each of the seven pastures, using 100 plots measuring 0.3 X 0.6 M (1 X 2 ft) by the method described by Edlefsen et al. (1960). Data were summarized by species and forage classes within pastures. Data were averaged across all seven pastures to give mean composition of available forage during the study. Species composition was based on standing crop prior to grazing. These data were not analyzed statistically. Diet samples were collected from five esophageally cannulated heifers in each pasture during the morning and afternoon of each of the first 3 days (Period A) and last 3 days (Period B) of each grazing JOURNAL OF RANGE MANAGEMENT 33(6), November 1980 period. Diets collected during period A were representative of light utilization and period B a higher degree of use. Animals were rotated between morning and afternoon collections, but were not used for both on the same day. A subsample was removed from each diet collection and dried at room temperature for determination of botanical and chemical composition. Diet samples were analyzed microscopically to determine the relative proportion of plant fragments of each species as described by Kothmann (1968). Percent composition of diets based on plant fragment counts was corrected to percent composition by weight. Known weights of single species diet samples were mixed together and analyzed botanically to determine the percent fragments of each forage class. The following equations were developed to determine the relative weight of fragments of forbs (X21, browse Xi) and pricklypear (X4’) as compared to grass (Xl’). Xl = % grass by fragments YI = % grass by weight X2 = % forbs by fragments X3 = % browse by fragments Y2 = % forbs by weight Ys = % browse by weight X4 = % pricklypear by fragmentsY4 = 9% pricklypear by weight xi= 1 xz’ = YPXlXl x; =YBXIXi xi Y4XlXi x2 (I-Y2) X3( 1 -Y3) X4( 1 -Y4) The relative weight per plant fragment of each forage class based on grass as equal to one Xi = 1, Xi = 3.15, Xi = 2.62, and Xl = 19.24. Once relative weight had been determined for all forage classes, the following formula was used to correct the species composition from percent fragments to percent by weight(Y). Computations of actual percent by weight are made by the appropriate adjustment of the numerator (XX’) over the common denominator.