R.C. Cochran

VEGETATION TRENDS IN TALLGRASS PRAIRIE FROM BISON AND CATTLE GRAZING

Comparisons between how bison and cattle grazing affect the plant community are understood poorly because of confounding differences in how the herbivores are typically managed. This 10-year study compared vegetation changes in Kansas (USA) tallgrass prairie that was burned and grazed season-long at a moderate stocking rate by either bison or cattle. We held management practices constant between the herbivores and equalized grazing pressure by matching animals so that the total body mass in all pastures was similar each year. Trends in species cover and diversity indices in the bison and cattle pastures were compared with ungrazed prairie that also was burned annually. We found that little bluestem (Schizachyrium scoparium) cover decreased over time in bison pastures, and big bluestem (Andropogon gerardii) cover increased over time in cattle pastures. Grazing by either herbivore increased the canopy cover of annual forbs, perennial forbs, and cool-season graminoids, but both annual and perennial forb cove...

Grazing management effects on plant species diversity in tallgrass prairie

Abstract A 6-year study was conducted in tallgrass prairie to assess the effects of grazing management (cattle stocking densities and grazing systems) on plant community composition and diversity. Treatments included sites grazed season-long (May to October) at 3 stocking densities (3.8, 2.8, and 1.8 hectares per animal unit), ungrazed control sites, and sites under a late-season rest rotation grazing system at this same range of stocking densities. Plant communities were sampled twice each season using a nearest-point procedure. Native plant species diversity, species richness, and growth form diversity were significantly higher in grazed compared to ungrazed prairie, and diversity was greatest at the highest stocking density. This enhancement of plant species diversity under grazing was not a result of increased frequency of weedy/exotic species. There were no significant effects of grazing system on plant diversity, nor any significant stocking density × grazing system interactions, indicating that animal density is a key management variable influencing plant species diversity and composition in tallgrass prairie and that effects of animal density override effects of grazing systems. Increasing cattle stocking densities decreased the abundance of the dominant perennial tall grasses, and increased abundance of the C4 perennial mid-grasses. The frequency of perennial forbs was relatively stable across grazing treatments. Abundance of annual forbs varied among years and grazing treatments. In half of the years sampled, annual forbs showed the highest frequency under intermediate stocking density. Patterns of responses among plant groups suggest that some species may respond principally to direct effects of grazers and others may respond to indirect effects of grazers on competitive relationships or on the spatial patterns of fuel loads and fires. Thus, this study suggests that large grazer densities, fire, and annual climatic variability interact to influence patterns of plant community composition and diversity in tallgrass prairie. Effects of varying management such as stocking densities and grazing systems on plant species diversity and the relative abundances of different plant growth forms or functional groups may have important consequences for grassland community stability and ecosystem function.

Effect of rumen-degradable intake protein supplementation on urea kinetics and microbial use of recycled urea in steers consuming low-quality forage

We evaluated the effect of increasing amounts of rumen-degradable intake protein (DIP) on urea kinetics in steers consuming prairie hay. Ruminally and duodenally fistulated steers (278 kg of BW) were used in a 4 x 4 Latin square and provided ad libitum access to low-quality prairie hay (4.9% CP). The DIP was provided as casein dosed ruminally once daily in amounts of 0, 59, 118, and 177 mg of N/kg of BW daily. Periods were 13 d long, with 7 d for adaptation and 6 d for collection. Steers were in metabolism crates for total collection of urine and feces. Jugular infusion of (15)N(15)N-urea, followed by determination of urinary enrichment of (15)N(15)N-urea and (14)N(15)N-urea was used to determine urea kinetics. Forage and N intake increased (linear, P < 0.001) with increasing DIP. Retention of N was negative (-2.7 g/d) for steers receiving no DIP and increased linearly (P < 0.001; 11.7, 23.0, and 35.2 g/d for 59, 118, and 177 mg of N/kg of BW daily) with DIP. Urea synthesis was 19.9, 24.8, 42.9, and 50.9 g of urea-N/d for 0, 59, 118, and 177 mg of N/kg of BW daily (linear, P = 0.004). Entry of urea into the gut was 98.9, 98.8, 98.6, and 95.9% of production for 0, 59, 118, and 177 mg of N/kg of BW daily, respectively (quadratic, P = 0.003). The amount of urea-N entering the gastrointestinal tract was greatest for 177 mg of N/kg of BW daily (48.6 g of urea-N/d) and decreased (linear, P = 0.005) to 42.4, 24.5, and 19.8 g of urea-N/d for 118, 59, and 0 mg of N/kg of BW daily. Microbial incorporation of recycled urea-N increased linearly (P = 0.02) from 12.3 g of N/d for 0 mg of N/kg of BW daily to 28.9 g of N/d for 177 mg of N/kg of BW daily. Provision of DIP produced the desired and previously observed increase in forage intake while also increasing N retention. The large percentage of urea synthesis that was recycled to the gut (95.9% even when steers received the greatest amount of DIP) points to the remarkable ability of cattle to conserve N when fed a low-protein diet.

Effect of a wide range in the ratio of supplemental rumen degradable protein to starch on utilization of low-quality, grass hay by beef steers

Hereford×Angus steers were used in a 14-treatment, 2-period, crossover design experiment to examine effects of a wide range in the ratio of supplemental starch to rumen degradable protein (RDP) on low-quality forage utilization and ruminal characteristics. Steers were given ad libitum access to grass hay (4.9% CP, 42.4% ruminally degradable) and supplemented in a 2×7 factorial arrangement of treatments. All supplements were administered directly into the rumen and delivered one of two levels of ruminally degradable starch (cornstarch grits; 0 and 0.3% of initial BW) and one of seven levels of RDP (sodium caseinate; 0, 0.015, 0.051, 0.087, 0.123, 0.159, and 0.195% of initial BW). Supplementation with RDP increased consumption of forage OM, total OM, NDF, and digestible OM in a quadratic (P<0.01) fashion (intake increased and then declined). Starch supplementation depressed (P<0.01) forage OM and NDF intakes. In general, RDP supplementation elicited a positive quadratic response on NDF digestion (P=0.02). However, an interaction between supplemental starch and RDP level was observed (P<0.01) for NDF digestion. At the four lowest levels of supplemental RDP, starch supplementation substantially reduced NDF digestion, although for steers receiving the three highest levels of supplemental RDP, starch supplementation had little effect on NDF digestion. In contrast, neither the starch×RDP interaction nor the starch main effect was significant for OM digestion, which increased linearly (P<0.01) with supplemental RDP. Supplementation with RDP altered passage rate of acid detergent insoluble ash in a quadratic (P=0.05) manner that paralleled the intake response. However, liquid passage rate was not affected significantly. A decline in ruminal pH was associated (P=0.02) with increasing supplemental RDP and tended (P=0.07) to be associated with increasing starch, but it was not requisite for starch-induced depressions in NDF digestion. Ruminal NH3 concentration increased in response to increasing RDP, although the increase when starch was supplemented was less than that observed without starch (P=0.03). Supplemental starch generally elicited negative effects on low-quality forage intake and fiber digestion, but the effects on fiber digestion were overridden by adequate supplemental RDP. Supplemental RDP exerted a highly positive effect on consumption and digestion of this low-quality forage.

A collaborative study comparing an in situ protocol with single time-point enzyme assays for estimating ruminal protein degradability of different forages☆

Abstract Seven institutions conducted a collaborative trial to compare three methods for estimating effective degradability (ED) of forage protein from alfalfa ( Medicago sativa ; 2.9% N), bermuda ( Cynodon dactylon ; 1.4% N), brome ( Bromus inermis ; 0.9% N), forage sorghum ( Sorghum bicolor ; 0.8% N), and prairie (mixture of native tallgrass prairie species; 0.9% N) hays. To facilitate the collaborative nature of this study, an adaptation of an abbreviated in situ procedure proposed by Broderick [Quantifying forage protein quality. In: Fahey, G.C., Collins, M., Mertens, D.R., Moser, L.E. (Eds.), Forage Quality, Evaluation, and Utilization. ASA, CSSA, and SSSA, Madison, WI, USA, pp. 200–228] and subsequently validated by Vanzant [J. Anim. Sci. 74 (1996) 2773] was adopted for comparison with other procedures. The abbreviated in situ procedure (conducted at four institutions) assumed first-order degradation kinetics and entailed not incubating (0xa0h) as well as ruminally incubating (2, 8, and 96xa0h) duplicate polyester bags ( 10 cm ×20 xa0cm) of each forage (5xa0g) in the rumens of two steers per location during each of two periods. Rinsing, drying, and microbial correction procedures were standardized. The in situ estimates of ED were compared with two single time-point enzyme assays (using Streptomyces griseus protease) derived from the work of Roe [Techniques for measuring protein fractions in feedstuffs. In: Proceedings of the Cornell Nutrition Conference, Department of Animal Science, Cornell University, Ithaca, NY, pp. 81–88] and Krishnamoorthy et al. [Br. J. Nutr. 50 (1983) 555] with adaptations based on the work of Abdelgadir et al. [J. Anim. Sci 75 (1997) 2215] and Coblentz et al. [J. Diary Sci. 82 (1999) 343]. For the protease assays, duplicate forage samples (15xa0mgxa0N) were incubated (at seven institutions) in 40xa0ml of a buffer solution for 1xa0h. Subsequently, 10xa0ml of a S. griseus protease solution (0.33xa0U of activity/ml) were added to the buffer plus forage and incubated for 48xa0h. In addition, a shorter procedure was evaluated (at six institutions) wherein an increased concentration (33.0xa0U of activity/ml) of S. griseus concentration was used to compensate for shorter incubation time (4xa0h). Average ED values obtained from both the 48 and 4xa0h S. griseus assays explained a large portion of the variation observed for the average ED values from the in situ technique ( r 2 =0.87 and r 2 =0.88, respectively). However, there did appear to be some evidence that the S. griseus assays tested overestimated in situ ED in forages with low degradability. The 4xa0h/high-concentration and 48xa0h/low-concentration S. griseus assays were closely related ( r 2 =0.99) with little bias evident in the relationship. These assays were also less variable than the abbreviated in situ approach. In conclusion, single time-point S. griseus assays with appropriately balanced incubation lengths and enzyme concentrations appear promising for estimating ED under routine laboratory conditions.

Effect of frequency and amount of rumen-degradable intake protein supplementation on urea kinetics and microbial use of recycled urea in steers consuming low-quality forage.

We evaluated the effect of frequency and amount of rumen-degradable intake protein (DIP) on urea kinetics in steers consuming prairie hay. Five ruminally and duodenally fistulated steers (366 kg of BW) were used in a 5 x 5 Latin square and provided ad libitum access to low-quality prairie hay (4.7% CP). Casein was provided daily in amounts of 61 and 183 mg of N/kg of BW (61/d and 183/d) and every third day in amounts of 61, 183, and 549 mg of N/kg of BW per supplementation event (61/3d, 183/3d, and 549/3d). Periods were 18-d long with 9 d for adaptation and 9 d for collection. Steers were in metabolism crates for total collection of urine and feces. Jugular infusion of (15)N(15)N-urea followed by determination of urinary enrichment of (15)N(15)N-urea and (14)N(15)N-urea was used to determine urea kinetics. Treatment means were separated to evaluate the effects of increasing DIP supplementation and the effects of frequency at the low (61/d vs. 183/3d) and at the high (183/d vs. 549/3d) amounts of DIP provision. Forage OM and total digestible OM intakes were linearly (P < or = 0.05) increased by increasing DIP provision but were not affected by frequency of supplementation at either the low or high amounts. Production and gut entry of urea linearly (P < or = 0.006) increased with DIP provision and tended to be greater (P < or = 0.07) for 549/3d than 183/d but were not different between 61/d and 183/3d. Microbial N flow to the duodenum was linearly (P < 0.001) increased by increasing DIP provision. Additionally, 183/d resulted in greater (P = 0.05) microbial N flow than 549/3d. Incorporation of recycled urea-N into microbial N linearly (P = 0.04) increased with increasing DIP. Microbial incorporation of recycled urea-N was greater for 549/3d than 183/d, with 42 and 23% of microbial N coming from recycled urea-N, respectively. In contrast, there was no difference due to frequency in the incorporation of recycled urea-N by ruminal microbes at the low level of supplementation (i.e., 61/d vs. 183/3d). This study demonstrates that urea recycling plays a substantial role in the N supply to the rumen and to the animal, particularly in steers supplemented infrequently with high levels of protein.

Effect of undegradable intake protein supplementation on urea kinetics and microbial use of recycled urea in steers consuming low-quality forage

We evaluated the effect of undegradable intake protein (UIP) on urea kinetics and microbial incorporation of urea-N in ruminally and duodenally fistulated steers (n 4; 319 kg) provided ad libitum access to grass hay in a 4 x 4 Latin square. Casein was continuously infused abomasally in amounts of 0, 62, 124 and 186 mg N/kg body weight per d to simulate provision of UIP. Periods were 13 d long with 7 d for adaptation and 6 d for collection. Jugular infusion of [15N15N]urea followed by determination of urinary enrichment of [15N15N]urea and [14N15N]urea was used to measure urea kinetics. Forage and N intake increased (quadratic, P<0.02) with increasing UIP. Urea synthesis was 27.1, 49.9, 82.2 and 85.8 g urea-N/d for 0, 62, 124 and 186 diets, respectively (linear, P<0.01). The proportion of urea synthesis that entered the gastrointestinal tract was 0.96 for steers receiving no UIP and decreased linearly (P=0.05) to a low of 0.89 for steers receiving 186. The amount of urea entering the gastrointestinal tract was least for 0 (26.3) and increased (linear, P<0.01) to 48.7, 77.2 and 76.6 g urea-N/d for 62, 124 and 186 diets, respectively. Microbial incorporation of recycled urea-N increased quadratically (P=0.04) from 13.9 for 0 to 47.7 g N/d for 124. The proportion of microbial N derived from recycled urea increased (quadratic, P=0.05) from 0.31 to 0.58 between 0 and 124 and dropped to 0.44 for 186 mg N/kg body weight per d. UIP increased intake of hay and provided a N source for ruminal microbes via urea recycling.

Stocking rate effects on intensive-early stocked Flint Hills bluestem range.

Stocking rate effects on intensive-early stocked Kansas Flint Hills range were studied from 1982 tbrougb 1987. Rates were 2X, 2.5X, and 3X normal season-long stocking rates for 200-225 kg steers. Study design was a randomized complete block witb 2 replicates. Grass and forb standing crop (kg/ha) were estimated at the time of livestock removal (mid July) and again in early October. Plant bas81 cover 8nd composition were taken in e8rly June the year prior to the study and annuaily thereafter. Overall growing season precipitation during tbe study period was below norm81, with late-summer precipit8tion much below normal in the second and third years of the study. Grass standing crop (GSC) in mid July decreased witb increased stocking rate, but by early October GSC was similar under the 2.5X and 3X stocking rates, but continued to be lower than that under the 2X rate. There was no consistent response in mid July forb standing crop (FSC) with respect to stocking rate. In early October, FSC was either not affected by stocking rate (1983, 1986, and 1987) or was greater under tbe bigbest stocking rate (1982,1984, and 1985). Tbe major changes in botanical composition and basal cover were a reduction in Indiangrass (Sorgh&run# nutuns Nash) and an increase in Kentucky bluegrass (Poa prutennb L.) as &eking rate increased. Botanical composition of big bluestem (Anhopogon gerar& Vitman) increased under tbe 2X rate but did not change under the bigber rates. Individual steer gains were similar under the different stocking rates, but livestock breed appeared to affect magnitude of the gain. Since individual gains did not differ, gains per ba were substantiaiiy increased by tbe bigber stocking rates.

Radiometry for predicting tallgrass prairie biomass using regression and neural models.

Standing forage biomass (SFB) and the percent of standing biomass composed of forbs (PCTF) were modeled across the growing season. Samples representing stages of plant maturity from early vegetative to dormant were collected from grazed and ungrazed native tallgrass paddocks using a 0.5 X 0.5 m quadrat. Total biomass was measured during all years of the study (1992-1995). Grass and forb biomass were measured separately during 1995. Height of canopy closure also was measured during 1995. Before clipping, plots were scanned with a multispectral radiometer. Models were prepared using simple regression, multiple regression (MR), or a commercial neural network (NN) computer program. Potential inputs to MR and NN models of SFB and PCTF included Julian day of harvest (JD), range site, canopy closure height (CH), incident radiation, spectral reflectance values (RFV) at 8 discreet bandwidths, and the normalized difference vegetation index (NDVI). The NDVI alone accounted for little variability (R2 = 0.13) in SFB during all years of the study. The optimal MR model for the same data set (SFB = 3.5[JD] - 43.7[460 nm RFV] + 1099[NDVI] - 992; R2 = 0.62) accounted for a greater amount of the variability in SFB. The capacity to describe variation in SFB for the 1995 data with MR was improved when CH was included as a variable (R2 = 0.58 versus 0.78). A NN model accounted for the most variation in SFB across the entire study (R2 = 0.76). During 1995, the capability of a NN to account for variation in SFB within the training data was similar whether or not CH was included as an input (R2 = 0.86); however, prediction of SFB from validation data using the same NN was improved by using CH as an input variable. Little variation in PCTF was accounted for by a MR model (R2 = 0.23); however, a considerably larger proportion of the variation in PCTF was accounted for when an NN was used (R2 = 0.59). Seasonal changes in SFB and PCTF were described with an acceptable degree of accuracy by forage reflectance characteristics that were adjusted for time of season and canopy complexity. Moreover, when provided with the same potential inputs, NN predicted SFB and PCTF from validation data more accurately than MR models.

Methodology for concurrent determination of urea kinetics and the capture of recycled urea nitrogen by ruminal microbes in cattle.

We measured the incorporation of recycled urea-nitrogen (N) by ruminal microbes, using five ruminally and duodenally fistulated steers (237 kg) fed low-quality grass hay (47 g crude protein/kg dry matter (DM)). Three received 1 kg/day of soybean meal (SBM) and two received no supplemental protein (control). The experiment was 15 days long. Background enrichments of 15N were measured on day 9 and continuous jugular infusion of 0.12 g/day [15N15N]urea began on day 10. Daily samples of urine, feces, ruminal bacteria and duodenal digesta from days 10 through 14 were used to determine plateaus in 15N enrichment. Duodenal and bacterial samples collected on day 15 were used to measure duodenal N flows. Bacterial N flow was calculated as duodenal N flow multiplied by duodenal 15N enrichment divided by bacterial 15N enrichment. Bacterial N from recycled urea-N was calculated as bacterial N flow multiplied by bacterial 15N enrichment divided by urinary urea 15N enrichment. Urinary enrichment of [15N15N]urea plateaued within 24 h, whereas 14N15N urea plateaued within 48 h of [15N15N]urea infusion. Bacteria reached a plateau in 15N enrichment within 24 h and duodenal samples within 48 h. Urea production was 17.6 g of urea-N/day for control and 78.0 g/day for SBM. Gut entry was 0.99 g of urea-N/g of urea-N produced for control and 0.87 g/g for SBM. Incorporation of recycled N into microbial N was 9.0 g of N/day for control and 23.0 g/day for SBM. Recycled urea-N accounted for 0.33 g of N/g of microbial N at the duodenum for control and 0.27 g/g for SBM. Our methods allowed measurement of incorporation of recycled urea-N into ruminal microbial N.