Longevity and lifetime productivity are important factors influencing profitability for the cowcalf producer. Heifers that conceive earlier in the breeding season will calve earlier in the calving season and have a longer interval to rebreeding. Calves born earlier in the calving season will also be older and heavier at weaning. Longevity data were collected on 2,195 heifers from producers in South Dakota Integrated Resource Management groups. Longevity and weaning weight data were collected on 16,549 individual heifers at the U.S. Meat Animal Research Center (USMARC). Data were limited to heifers that conceived during their first breeding season. Heifers were grouped into 21-d calving periods. Heifers were determined to have left the herd when they were diagnosed not pregnant at the end of the breeding season. Heifers that left the herd for reasons other than reproductive failure were censored from the data. Heifers that calved with their first calf during the first 21-d period of the calving season had increased (P < 0.01) longevity compared with heifers that calved in the second 21-d period, or later. Average longevity for South Dakota heifers that calved in the first or later period was 5.1 ± 0.1 and 3.9 ± 0.1 yr, respectively. Average longevity for USMARC heifers that calved in the first, second, or third period was 8.2 ± 0.3, 7.6 ± 0.5, and 7.2 ± 0.1 yr, respectively. Calving period as a heifer influenced (P < 0.01) unadjusted weaning BW of the first 6 calves. Estimated postpartum interval to conception as a 2-yr-old cow was greater for females that calved in the first period as heifers but did not differ between heifer calving periods in subsequent calving seasons. In summary, heifers that calved early in the calving season with their first calf had increased longevity and kilograms weaned, compared with heifers that calved later in the calving season.

Comparisons of stocking rates across sites can be facilitated by calculating grazing pressure. We used peak standing crop and stocking rates from six studies in the North American Great Plains

Following grazing big bluestem in mid-May, animals can be returned to the cool-season pasture to make full Big bluestem (Andropogon gerardii Vitman) has a rapid growth use of the cool-season forage before returning to the phase that begins in early to mid-June in eastern Nebraska. During this rapid growth phase, rate of biomass accumulation exceeds intake big bluestem pasture in mid-to late June. Even without rate of grazing livestock, resulting in low levels of harvest efficiency. cool-season pasture, grazing big bluestem in May canTo delay the rapid growth phase, big bluestem pasture can be grazed be used to extend the length of the grazing season. in mid-to late May without affecting herbage yields for the remainder The objective of this study was to determine the effect of the growing season. A pasture experiment was conducted in 1999, of timing and frequency of grazing big bluestem pasture 2000, and 2001 near Mead, NE. The objective was to determine the following a May grazing period on (i) herbage availabileffect of timing and frequency of grazing big bluestem pasture, followity and herbage disappearance for the remainder of the ing a May grazing period, on cumulative pregrazing yields, cumulative grazing season and (ii) stand persistence. Results of herbage disappearance, resulting harvest efficiency, leaf/stem ratio, this study should guide the development of an effective and stand persistence. Yield and morphological characteristics were summer grazing strategy for big bluestem pasture folobtained immediately before and after each grazing period, and basal cover of big bluestem was estimated annually. May grazing had no lowing early-season grazing. effect (P Ͻ 0.1) on cumulative pregrazing yields and resulted in an increase of cumulative herbage disappearance (3638 vs. 2673 kg ha Ϫ1 ) MATERIALS AND METHODS and leaf/stem ratio (2.02 vs. 2.83) compared with paddocks with no May grazing. Grazing at the vegetative stage in June compared with Study Site the elongation stage resulted in an increase in cumulative pregrazing The study was conducted in 1999, 2000, and 2001 at the yields (10774 vs. 9510 kg ha Ϫ1 ), cumulative herbage disappearance University of Nebraska Agricultural Research and Develop-(4116 vs. 3194 kg ha Ϫ1 ), and leaf/stem ratios (2.57 vs. 1.98). Grazing ment Center near Mead, NE (41Њ11Ј N, 96Њ33Ј W; 315 m at the elongation stage in June followed by a grazing period in early elevation). The continental climate of the area is characterized August is not an advisable management strategy.

Quantifying root structure response to multiple defoliation events in a grazing situation is critical in developing management plans for warm‐season tallgrasses. A pasture experiment was conducted in 1999, 2000, and 2001 near Mead, NE. The objective of the experiment was to determine the effect of timing and frequency of grazing on big bluestem (Andropogon gerardii Vitman) etiolated tiller growth and root and rhizome structure. Paddocks were grazed at a stocking rate of 9.9 Animal Unit Month (AUM) ha−1 in two to four cycles from mid‐May to early‐September. In April 2002, five 6.6‐ × 132‐cm soil cores were extracted from each paddock. Soil cores were subsampled at 30‐cm depth increments for estimates of root mass, root surface area, and root volume. Etiolated tiller tents were used to estimate organic reserves of big bluestem in each paddock in spring 2002. Mean number and weight of etiolated tillers were reduced by up to 40% and 50%, respectively, in paddocks grazed in a sequence of June after internode elongation, early August, and early September. Root structure in the top 30 cm of the soil profile was affected most by multiple defoliation events with <40 d of recovery between grazing periods. Root mass decreased by 25%, while mean surface area and volume of roots declined 10 and 15%, respectively, in the upper 30 cm of the soil profile in paddocks grazed in the sequence of post‐internode elongation in June, early‐August, and early‐September. To maintain vigorous big bluestem pastures, grazing management should concentrate on the elongation and postelongation periods. Grazing at the elongation stage should be rotated among paddocks in successive years and the recovery period following grazing at internode elongation should be >40 d.

The headwaters of the mighty Missouri River flow out of the western states of Montana, Wyoming, Nebraska, and North and South Dakota. The streams, reservoirs, ponds, and other water bodies that are part of the headwaters system are heavily depended upon by these states for their economic and ecological health. These waters support and provide for agriculture, recreation, tourism, wildlife habitat, irrigation, drinking water, power generation, and livestock throughout these states. The state weed coordinators from each of these states attest to the importance of these water systems for the states' livelihoods: