Non-grazed Treatment Research Articles

Grazing and defoliation timing effects in Great Plains ponderosa pine woodland following a large summer wildfire

There remains a high level of ambiguity around post-fire grazing management. The Lodgepole Complex fire burned 109,346 ha in east-central Montana in July 2017, including areas previously burned in 2003 by the Bureau of Land Management for fuels mitigation. We hypothesized that herbaceous productivity and species composition in ponderosa pine woodlands are resistant to moderate grazing and timing of defoliation the first growing season after fire. Eight exclosures (25 × 15 m) were built, 4 on reburned area and 4 on wildfire area. To determine grazing effects, a 15 × 10 m sampling area in each exclosure was paired with a grazed section outside the exclosure. Additionally, four 5x10-m plots were established within each exclosure and were mowed to 10 cm in June, July, August, or not mowed to determine seasonal defoliation effects. Differences between grazed and nongrazed sites in 2019 were limited. There was a trend for less old dead biomass in grazed than nongrazed treatments. Species richness was greater on grazed sites than nongrazed or reburn sites. Native species richness was greater on grazed sites than nongrazed sites and reburn sites had an intermediate level of native species richness. Standing crop was reduced by defoliation, with current year and perennial grass biomass being unaffected. The reduction in standing crop was driven by less old dead biomass on defoliated plots. June defoliation reduced C3 grasses and increased C4 grasses, and June and July defoliation each reduced annual grasses. Simpson’s and native Simpson’s diversity were greater on defoliated plots. Results indicate ponderosa pine woodland is resilient to moderate post-fire grazing.

Grazing cover crops in a no‐till corn and soybean rotation

AbstractCover crop (CC) systems may offset expensive hay feeding in early spring. Field research evaluated the effect of CCs on forage mass, weed control, soil penetrometer resistance, and no‐till corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] grain yield (2003–2008) in upstate Missouri. The main plot was grazed and nongrazed CCs, corn and soybean were the subplots, and CCs [nonseeded control, cereal rye (CR, Secale cereale L.), wheat (Triticum aestivum L.), ‘King’ or ‘Marshall’ annual ryegrass (RG, Lolium multiflorum Lam.)] were the sub‐sub plot. Cover crops were grazed two to four times in the spring when the fastest growing species was 20–30 cm tall. Forage mass was collected from the nongrazed treatment. Cereal rye produced the greatest total forage mass in 3 and 4 of 6 yr following corn and soybean, respectively. Cereal rye forage mass (3,640 kg ha−1) was similar to wheat following corn and was 860 kg ha−1 greater than wheat following soybean. Winter annual weeds were controlled up to 92% with CCs. Cattle (Bos taurus) grazing CCs increased penetrometer resistance up to a 20‐cm soil depth compared with nongrazed CCs. Corn grain yield was reduced 0.4 Mg ha−1 following grazed CCs, while soybean yields following grazed CCs were similar to nongrazed treatments (4–6 yr). Corn yields following CCs were ranked nonseeded control = Marshall RG ≥ wheat ≥ King RG ≥ CR. Soybean yields following CCs were similar or less than the nonseeded control. Grazing CCs could add value over the absence of the CC and promote CCs in the U.S. Midwest.

Cattle Grazing Affects Peanut Root Characteristics in a Bahiagrass-Based Crop Rotation System

ABSTRACT The critical aspect of production agriculture in the southeastern US with increasing associated costs is to improve economic and agronomic sustainability. A four yr sod-based rotation system consisting of two yr of bahiagrass (Paspalum notatum Flueggé) (grazed or non-grazed) followed by a yr of peanut (Arachis hypogaea L.) and a yr of cotton (Gossypium hirsutum L.), each with winter cover crop (grazed or non-grazed) was established in Marianna, FL. The effect of grazing on root parameters (length, volume, surface area, and diameter) of peanut was observed using a mini-rhizotron. There were differences in several root parameters between grazed and non-grazed plots including: peanut root length (307 mm in grazed vs 167 mm in non-grazed), volume (50 mm3 in grazed vs. 23 mm3 in non-grazed), surface area (399 mm2 in grazed vs. 197 mm2 in non-grazed), and diameter (2.4 mm in grazed vs. 1.7 mm in non-grazed). Roots at the 45-60 cm and 60-75 cm depths had significantly greater length in the grazed than the non-grazed plots. Likewise, surface area was significantly greater in the grazed plots at the 30-45 cm, 45-60 cm and 60-75 cm depths. Grazed plots at the 40-65 cm depths showed significant increase in root diameter. No significant difference in peanut yield was observed for the grazed or non-grazed treatments. A more developed root system associated with cattle grazing in the sod-based rotation system may enable peanuts to be more resilient in adverse environmental conditions such as drought stress, enhance nutrient cycling without affecting yield, thereby improving long-term sustainability.

Phosphorus and potassium cycling in a long-term no-till integrated soybean-beef cattle production system under different grazing intensities insubtropics

Long-term integrated crop-livestock system enables constant and more efficient nutrient cycling because animal, pasture and crop residues release nutrients at different rates. Therefore, appropriate management of these systems is needed to maximize benefits from nutrient cycling. The objective of this study was to evaluate how grazing intensity affected the release rates of phosphorus (P) and potassium (K) in pasture, dung and soybean residues in a no-till long-term integrated crop-livestock system. The experiment was established in 2001 on a clayey Oxisol after soybean harvest. Treatments consisted of pasture with sward heights maintained at 1020, 30 and 40 cm by different cattle stocking rates and a non-grazed (NG) treatment. Decomposition and release rates of nutrients in the pasture and dung were determined using litter bags, which were installed at soybean seeding and pasture seeding during two pasture-crop cycles (2009–2011). Lighter grazing intensities resulted in greater P release rate from pasture and dung residues. Pasture and dung residues released K at a very high rate and were not influenced by grazing intensity. The P and K released from soybean residue were not affected by grazing intensity; however, decomposition of soybean leaves was greater than of stems. Greatest rates of total P and K released were from pasture and dung residues under lighter grazing intensities and in the NG areas. Large amounts of P (~25 kg ha−1) and K (~130–180 kg ha−1) were cycled in a complete soybean-beef cattle integrated system and must be considered in the fertilization management.

Grazing intensities affect weed seedling emergence and the seed bank in an integrated crop–livestock system

Integrated crop-livestock systems (ICLS) are widespread worldwide. Although weeds can result in several types of losses, essential information regarding weed seedling emergence and seed banks is lacking in these systems for integrated weed management planning. This study investigated the effects of different grazing intensities (no grazing and grazed swards maintained at 10-, 20-, 30- and 40-cm sward heights) on weed seedling emergence and seed banks in a winter cover crop during the 15th year of an ICLS experiment under no-tillage management in subtropical Southern Brazil. We hypothesized that low grazing intensities would reduce weed interference and weed seed banks size in an ICLS. We determined treatment effects on weed species richness, seed bank population density, and seedling emergence during winter and summer. Higher sward heights in the winter-grazed cover crop reduced the number of weed species, the density of emerged weed seedlings, and the weed seed bank size compared with the non-grazed control. With a sward grazing height of 10cm, the seed bank contained an average of 3151 seeds m−2, and the weed population densities during the summer and winter were 11.8 and 21.7 plants m−2 greater, respectively, compared with the sward grazing height of 40cm. Fifteen years after adopting low grazing intensities (30- and 40-cm sward heights) in the ICLS, the size of the weed seed bank was reduced by 42.1% compared with the non-grazed treatment. Decreasing the grazing intensity reduced the number of weed species, the density of emerged weed seedlings, and the weed seed bank density; therefore, integrated weed management strategies should consider minimizing grazing intensities in an ICLS.

Responses of chlorophyll and MDA of Vallisneria natans to nitrogen and phosphorus availability and epiphytic algae

It is common for submerged plants to become less abundant and disappear in eutrophic lakes in the world. To explore degradation mechanisms of submerged plants during the growing season, we determined chlorophyll and malondialdehyde (MDA) concentrations in Vallisneria natans (Lour.) Hara cultivated under four different nutrient concentrations (N/P, mg/L): T1 = 0.5/0.05, T2 = 2.5/0.25, T3 = 4.5/0.45, and T4 = 12.5/1.25) and under low and high density of epiphytic algae. With the increase in nutrients, the content of chlorophyll in the leaves of V. natans varied in the order T3 > T2 > T1 (T4) at low density (grazed) of epiphytic algae and varied in the order T3 > T2 > T1 > T4 in the high density (ungrazed) of epiphytic algae. With the increase in nutrients, the content of MDA in the leaves of V. natans varied in the order T4 >T1 (T2, T3) at the low density (grazed) of epiphytic algae and varied as T4 > T3 > T2 > T1 at the high density of epiphytic algae. At the same time, chlorophyll content of V. natans was significantly higher (p < 0.01 and the content of MDA was significantly lower (p < 0.01) in the grazed treatment than in the non-grazed treatment under the same nutrient concentrations. During the growing season, total nitrogen and total phosphorus concentrations were low (N < 4.5 mg N/L, P < 0.45 mg P/L) in Lake Taihu, a large shallow eutrophic lake in China; consequently water column nutrient loading benefited the growth of V. natans, but the high biomass of epiphytic algae and planktonic algae caused by the high nutrient loading of water column inhibited the growth of V. natans.

Soil carbon and nitrogen stocks and fractions in a long-term integrated crop–livestock system under no-tillage in southern Brazil

Managing grazing stocks in integrated crop–livestock (ICL) systems under no-tillage is a key variable for reaching equilibrium in soil C and N budgets. Understanding how different plant and animal residues affect soil C and N stocks in these systems goes beyond soil dynamics since these elements are crucial for the functioning of the soil–plant–atmosphere system. The objective of this research was to determine soil C and N fractions, stocks, budgets and the carbon management index as affected by nine years of ICL with grazing intensities under no-tillage conditions. The experiment established in May 2001 in a Rhodic Hapludult (Oxisol) of southern Brazil was composed of black oat (Avena sativa) plus ryegrass (Lolium multiflorum) pasture in winter and soybean (Glycine max) crop in summer. Treatments were regulated by grazing pressures to maintain forage at 10, 20, 30 and 40cm high (G10, G20, G30 and G40, respectively). Non-grazed (NG) treatment was the control. Changes in soil C and N stocks and fractions (particulate and mineral-associated) were assessed in the ninth year of the experiment. Moderate and light grazing intensities (G20, G30 and G40) resulted in similar increases in total organic C, particulate organic C, total N, and particulate organic N compared with NG treatment. Soil C additions ranged from 0.54 to 8.68Mgha−1 from NG to the other grazing treatments. The G10 led to a soil N loss of 1.17Mgha−1 due to soil organic matter degradation. The carbon management index (CMI) values, compared with native forest (NF) as a reference, indicated soil quality loss and degradation under high grazing intensity (G10). For a positive contribution to the soil system, ICL must be managed with moderate grazing intensities and adjustment of N additions through N fixation or fertilization.

Impact of an integrated no-till crop–livestock system on phosphorus distribution, availability and stock

Biophysical and socio-economic improvements of farming systems have been obtained with integrated crop–livestock systems (ICLS). Grazing in ICLS under no-tillage conditions may alter soil phosphorus (P) dynamics, because of changes in P budgets and distribution of P forms. The need to understand impacts on P dynamics is important because they can modify soil quality and, consequently, have implications for agriculture sustainability. The objective of this work was to compare the geochemical and biological forms of P (P-geo and P-bio) under long-term grazing, as well as to assess the mobility and lability of these nutrient forms in the soil profile. The experiment was established in 2001 in an Oxisol and consisted of soybean (Glycine max L.) rotated to a winter cover crop of black oat (Avena strigosa Schreb.) and Italian ryegrass (Lolium multiflorum Lam.) under no-tillage management. Treatments were two grazing heights (target of 20 and 40cm) with young cattle and a non-grazed (NG) treatment. The experimental design was set up in completely randomized blocks with three replicates. Soil was sampled at the beginning of the experiment (May 2001) and again after six years (May of 2007) to study P fractionation at depths of 0 to 2.5, 2.5 to 5.0, 5.0 to 7.5, 7.5 to 10, 10 to 15 and 15 to 20cm. After six years of ICLS, total P increased in the top 20cm soil layer in both the grazed and non-grazed treatments. The P-geo and P-bio concentrations also increased in all treatments in the 0 to 2.5-cm layer. Despite the greater P accumulation in the non-grazed no-till treatment, grazing maintained the P lability (labile+moderately labile) at the same level as in the long-term no-till treatment. The ICLS also promoted increases in labile P stocks, both in organic and inorganic forms. In spite of the higher P-geo stocks in the non-grazed treatment, P cycling was enhanced by grazing, such that soybean yields were similar in all treatments. Furthermore, the grazing treatment led to higher P budgets (surpluses) and more efficient P use, as represented by meat production in addition to the soybean production.

Grazing and Grazing Exclusion Along a Resource Gradient in Magellanic Meadows of Tierra del Fuego

Although Magellanic meadows have been subject to ungulate herbivory for a long time; they have suffered severe grazing by domestic sheep since the end of the 19th century. We hypothesized that, if protected from grazing, the present dominant dwarf forms would be replaced by taller competitive forms, mainly grasses. We then monitored floristic composition along a gradient of soils over periods of 5 to 16 on seven meadow sites under grazing and, in five, also on exclosures. Initial and final floristic samples were classified with TWINSPAN and ordinated with Detrended Correspondence Analysis (DCA). We also tracked cover changes in life-forms (grasses, dwarf sedges/rushes, rhizomatous helophytes, and cushion shrubs). At the end of the study, we assessed floristic similarity between paired grazed and exclosed treatments, and compared their life-form cover, diversity, biomass, and soil compaction with Student's t and Mann-Whitney's tests. Temporal displacements of composition in ordination space showed the effect of a long drought (DCA axis 1) and of grazing removal (DCA axis 2). Grazing exclusion had a much greater impact in wet meadows–where grasses became the dominant life-form–than in dry meadows. The effect of drought was shown in 1) the expansion of the cushion shrub Azorella trifurcata (Gaertn.) Pers. in both grazed and nongrazed treatments of the drier wetlands; 2) the decrease in Caltha sagittata Cav. in riparian meadows, and its encroachment under grazing in the wettest spring meadow; and 3) the large increase in Poa pratensis L. in exclosed wet meadows. Despite this strong response to environmental conditions, overall results supported the idea that these wetlands are equilibrium systems. Although the drier meadows have crossed an irreversible threshold, the Caltha-encroached wet meadows returned to a grass state in about 4 when protected from grazing. Their resilience suggests that they could be partly restored with rest-rotation grazing.

Spring cattle grazing and mule deer nutrition in a bluebunch wheatgrass community

Although domestic cattle and mule deer (Odocoileus hemionus) share bluebunch wheatgrass (Pseudoroegneria spicata) communities throughout western United States and Canada, no study has yet evaluated the effects of cattle grazing on the nutrient intake of mule deer and the nutritional carrying capacity in these semi-arid rangelands. We expected that spring cattle grazing would decrease plant biomass available to deer but increase the nutritional quality of forage by arresting the phenology of grasses, reducing the proportion of standing dead biomass of grasses and promoting forbs. Using 0.4-ha exclosures, we created 3 replicates of paired grazed and non-grazed treatments within 3 pastures ranging from 64 to 509 ha in bluebunch wheatgrass communities on dry-stony ecological sites in southeastern Washington, USA. After cattle had grazed 1 of each pair of 0.4-ha plots to 40% utilization from 10 April to 29 May 2009, we sampled the biomass and measured the digestible energy (DE; kJ/g) and digestible protein (g/100 g feed) of plants that spring and fall. Using hand-raised, tractable mule deer, bite count methods, and behavioral observations, we measured daily DE and digestible protein intake within each plot. When compared to non-grazed plots, grazed plots had half the total and 3 times less live plant biomass in spring, and 25% less biomass in fall, whereas DE content of bluebunch wheatgrass and mule deer diets did not differ between grazing treatments. However, daily DE intake of deer was 39% less in spring and 13% less in fall in grazed than non-grazed plots. Nutritional carrying capacity (deer days/ha) did not vary between grazing treatments except for 1 pasture in fall, indicating that spring grazing by cattle in bluebunch wheatgrass communities did not improve the nutritional quality of deer diets enough to offset the overall loss of live forage biomass. © 2013 The Wildlife Society.

Comparing warming and grazing effects on birch growth in an alpine environment – a 10-year experiment

Background : Tree encroachment of arctic tundra and alpine vegetation is a generally predicted response to climate warming. However, herbivory plays an important role in structuring these ecosystems and their responses to warming. Aims : To experimentally test how grazing and increased growing season temperature influence growth, physiognomy and stature of birch in the alpine zone. Methods: Trait responses of naturally regenerated birch saplings to warming (open-top chambers), and changed grazing regime (exclosures) were compared with those growing in unmanipulated conditions over a 10-year period (1999–2008). The effect of treatment over time and differences between treatments were analysed with repeated measures GLM (Generalised Linear Model) and simple contrasts in GLM. Results: Warming alone had no major effect on trait responses, however, significantly smaller leaves and an increased number of short shoots indicated warming-related growth constraints. Grazing showed a strong controlling effect on most traits, conserving low stature sapling stage characterised by fewer shoots and larger leaves, compared with non-grazed treatments. Conclusions: Although derived from one experimental site, the results point to a grazing-controlled response to environmental change, with climate (warming) as a secondary driver. This herbivore-driven masking of expected climate-driven tree expansion emphasises the necessity to consider changes in grazing regimes along with climate change, in order to avoid misleading interpretations regarding climate-driven tundra encroachment.

Delaying sheep grazing after wildfire in sagebrush steppe may not affect vegetation recovery

Although many land managers prohibit grazing for 2 years after a fire, little research has been conducted to determine the interaction of grazing with vegetation recovery after fire. In a study conducted in sagebrush steppe rangelands after a 2000 wildfire at the United States Sheep Experiment Station in Idaho, the influence of delay and season of sheep grazing on vegetation recovery was measured. A moderate level of sheep grazing was started 1, 2, or 3 years after fire in the autumn or 2 or 3 years after fire in the spring. Pseudoroegneria spicata, the dominant perennial grass, and Crepis acuminata increased in the autumn and non-grazed treatments, whereas perennial forb and Crepis acuminata cover declined in the spring grazing treatments beginning the second year after fire. There was no impact of sheep grazing on cover of Bromus tectorum, an important exotic winter annual grass, probably owing to the lack of autumn germination. In this study, moderate sheep grazing had subtle impacts on vegetation recovery after fire, with spring grazing having more negative consequences. Management decisions on when to start grazing livestock after fire should consider pre-fire ecological conditions, post-fire climatic conditions, and current knowledge of impacts of grazing on plant recovery.

Response of organic and inorganic carbon and nitrogen to long-term grazing of the shortgrass steppe.

We investigated the influence of long-term (56 years) grazing on organic and inorganic carbon (C) and nitrogen (N) contents of the plant-soil system (to 90 cm depth) in shortgrass steppe of northeastern Colorado. Grazing treatments included continuous season-long (May-October) grazing by yearling heifers at heavy (60-75% utilization) and light (20-35% utilization) stocking rates, and nongrazed exclosures. The heavy stocking rate resulted in a plant community that was dominated (75% of biomass production) by the C4 grass blue grama (Bouteloua gracilis), whereas excluding livestock grazing increased the production of C3 grasses and prickly pear cactus (Opuntia polycantha). Soil organic C (SOC) and organic N were not significantly different between the light grazing and nongrazed treatments, whereas the heavy grazing treatment was 7.5 Mg ha(-1) higher in SOC than the nongrazed treatment. Lower ratios of net mineralized N to total organic N in both grazed compared to nongrazed treatments suggest that long-term grazing decreased the readily mineralizable fraction of soil organic matter. Heavy grazing affected soil inorganic C (SIC) more than the SOC. The heavy grazing treatment was 23.8 Mg ha(-1) higher in total soil C (0-90 cm) than the nongrazed treatment, with 68% (16.3 Mg ha(-1)) attributable to higher SIC, and 32% (7.5 Mg ha(-1)) to higher SOC. These results emphasize the importance in semiarid and arid ecosystems of including inorganic C in assessments of the mass and distribution of plant-soil C and in evaluations of the impacts of grazing management on C sequestration.

Grazing Effects on Nutritional Quality of Bluebunch WheatGrass for Elk

We tested the hypothesis that nutrient content of bluebunch wheatgrass (Agropyron spicatum [Pursh] Scribn. &Smith) either cattle grazed in the spring, rested from cattle grazing for a full year, or given long term rest would all be equal during the given season at 1 location. A 3 pasture rest-rotation grazing system and an exclosure on the Mt. Fleece elk winter range in southwestern Montana were studied during 4 seasons over 3 years. Only nitrogen (N) and phosphorus contents were generally greater in the in the spring grazed regrowth pasture. However, regrowth from bluebunch wheatgrass grazed in the spring did not improve the species nutrient content for wildlife the following winter over nongrazed treatments. During winter when elk (Cervus elaphus nelsoni Bailey) are present, N, TDN, and IVDMD were not among the 3 treatments. Elk were determined unlikely to consume enough bluebunch wheatgrass to meet protein maintenance requirements during winter. Our findings resulted from analyses repeated over the 3 years for a complete cycle of a 3 pasture rest-rotation system: however, our hypothesis needs to be tested at other locations before assuming the same results elsewhere.

The Effect of Cattle Grazing on Ponderosa Pine Regeneration

During the summer and fall of 1982, a study was established on the Nez Percé National Forest in central Idaho to assess the effects of cattle grazing on the performance of a new ponderosa pine (Pinus ponderosa Laws.) plantation. Three study sites were located in a Douglas-fir/ninebark (Pseudotsuga menziesii/Physocarpus malvaceus) habitat type. Results after six years showed that livestock use can influence tree seedling establishment. Mortality and damage to tree seedlings from cattle resulted primarily from trampling rather than from browsing. Five and one-half percent of the seedlings were damaged by cattle, of which 3.6% died. Browsing by deer and elk caused the most growth loss. Overall damage was greatest from non-animal causes. Seedling quality at the time of planting, handling of the seedling, and droughty conditions during the first two years of the study contributed to this type of damage. Overall performance of the tree seedlings were relatively poor. Total mortality to tree seedlings resulting from all sources of damage was 43.6% for the grazed treatment and 25.3% for the non-grazed treatment.

Yield and Regrowth Characteristics of Alfalfa Grazed by Steers during Spring and Summer

AbstractCurrent recommendations for grazing alfalfa (Medicago sativa L.) include grazing for a few hours or days when plants are near a haycut stage. This study investigated the influence of continuous grazing of ‘Arc’ alfalfa by steers (Bos taurus L.) during spring (38 d), summer (26 d), or both on plant growth, potato leafhopper (Empoasca fabae Harris) population, and subsequent hay yields. Three grazing and one nongrazed treatments were imposed during 1986 and 1987 on Groseclose silt loam (clayey, mixed, mesic Typic Hapludults). Treatments included: (i) spring grazing started when alfalfa height was approximately 10 cm, followed by three hay harvests; (ii) hay harvested twice, followed by summer grazing that started when alfalfa height was approximately 15 cm, followed by one hay harvest; (iii) both spring and summer grazing as described above with hay harvested after the second and fourth growth cycles; and (iv) nongrazed with hay harvested four times. No grazing was imposed during 1988 when hay was harvested four times to measure the influence of previous grazing. In 1986 and 1987, hay yields following spring grazing (6.61 Mg ha−1) were not different than those for nongrazed (6.47 Mg ha−1). Fall hay yields following summer grazing (1.39 Mg ha−1) and spring plus summer grazing (1.34 Mg ha−1) treatments were lower than for spring grazing (1.85 Mg ha−1) or nongrazed treatments (1.80 Mg ha−1). Grazing had no influence on root total nonstructural carbohydrates at the end of Year 2. Grazing for 2 yr by any treatment had no effect on yields during Year 3 when alfalfa was harvested four times for hay. Potato leafhopper density increased during regrowth following summer grazing to a level of 11.5 insects per sweep that required insect control, whereas little buildup occurred following hay harvest during the same growth period. Our data show that grazing, begun in spring or summer at a young vegetative stage, can continue for a time equal to a growth cycle in either or both seasons of at least 2 yr with no detrimental effect on alfalfa hay yield in Year 3.

Comparisons of energy partitioning in contrasting age-structured populations of the limpet Patella vulgata L

Estimates of the consumption by two limpet populations of similar biomass at Robin Hood's Bay, northeastern England, were made by measuring the difference in organic or chlorophyll a content between grazed and non-grazed treatments. Limpets at the low level, bare rock, Site (1) consumed three times more per unit area than at a high level, barnacle, Site (2) despite higher densities at the latter site. The growth rates derived from marked individuals were lower at Site 2 than Site 1 and annually more variable at the latter site. Within standard shell-lengths the tissue mass and energy equivalents varied seasonally indicating changes in the “condition” of the limpets. The gonads matured in autumn but all did not release at the same time. There were significant variations between sites and years in total reproductive output per unit area. The energy proportion of gonad to total net production (reproductive effort) increased with age from 11–15% at first maturity (3 yr) to 65–96% at 10 yr and above. Somatic production also varied between sites and years, 1979-Site 1 being greatest (116 kJ · m −2). Values of P B were between 0.1 and 1.7 which indicates a high degree of intraspecific variation, but were consistent with general invertebrate relationships. The concept of exploiters and conservers is discussed to explain the population differences. Reproductive value was greater for males than females at Site 1 due to the effects of early mortality preventing the development of a large female population. The total mean reproductive value per individual was greater at Site 2 than Site 1 due to the lower mortality rate. The contrast regarding the greater annual gametic output of the population at Site 1 and the higher individual output at Site 2 is discussed.