Grazed Swards Research Articles

Seasonal variations in nutritive and botanical composition properties of multispecies grazing swards over an entire dairy grazing season

AbstractBackgroundRecently, there has been increasing interest in the use of multispecies (MS) swards in dairy grazing systems.MethodsA plot grazing study was established to investigate the potential contribution of different sward species (perennial ryegrass, legumes and herbs) sown in grazing pastures. Ten sward mixtures were sown, ranging in complexity from a perennial ryegrass (PRG) monoculture to binary mixtures including either forage legumes or herbs to a five‐species mixture; four different nitrogen (N) application rates of 0, 100, 150 and 200 kg N ha−1 were applied to each sward.ResultsSpecies abundance for all sward species components was associated with the interaction of time point and sward mixture; species abundance for all sward components, except PRG, was associated with the interaction of N application and sward mixture. The interactions of time point and sward mixture, and time point and N application rate, were associated with all analysed sward chemical components, except for sward digestibility.ConclusionsIncreased N application rates reduced the level of clover in all sward mixtures. Increased levels of white clover led to increased sward crude protein levels; the inclusion of ribwort plantain led to some seasonal variation in the nutritive value of swards.

Sward type alters the relative abundance of members of the rumen microbial ecosystem in dairy cows

The performance of ruminant livestock has been shown to benefit from the enhanced nutritive value and herbage yield associated with clover incorporation in the grazing sward. However, little research to date has been conducted investigating the effects of mixed swards containing white clover on the composition of the rumen microbiome. In this study, the rumen microbial composition of late lactation dairy cows grazing perennial ryegrass only (PRG; n = 20) or perennial ryegrass and white clover (WCPRG; n = 19) swards, was characterised using 16S rRNA amplicon sequencing. PERMANOVA analysis indicated diet significantly altered the composition of the rumen microbiome (P = 0.024). Subtle shifts in the relative abundance of 14 bacterial genera were apparent between diets, including an increased relative abundance of Lachnospira (0.04 vs. 0.23%) and Pseudobutyrivibrio (1.38 vs. 0.81%) in the WCPRG and PRG groups, respectively. The composition of the archaeal community was altered between dietary groups, with a minor increase in the relative abundance of Methanosphaera in the WCPRG observed. Results from this study highlight the potential for sward type to influence the composition of the rumen microbial community.

Morphogenetic and structure characteristics of marandu grass subjected to grazing management strategies

ABSTRACT The objective of this study was to evaluate the morphogenetic and structural characteristics of Brachiaria brizantha cv. marandu subjected to grazing management strategies. The treatments corresponded to four grazing management strategies (25-10; 25-15; 35-15; and 45-15 pre and post grazing swards heights, respectively). Grazing management strategies were allocated to the experimental units (100 m2 paddocks) in a completely randomized blocks design with four repetitions. There was no effect of the grazing management on leaf elongation rate and leaf senescence rate, with average values of 11.22 mm/day and 2.17 mm/day, respectively. There were significant differences (p < 0.05) of leaf appearance rate, leaf elongation rate and stem elongation rate among months, where higher values were seen in months with greater rainfall. The evaluated management strategies showed a small effect on the morphogenetic and structural characteristics and can consequently be utilized for the management of marandu grass. In addition, during rainy periods, farmers can use management with higher defoliation frequencies (25-15), while the opposite (45-15) may be used during months with low rainfall in order not to exhaust the organic reserves of the plant.

The fatty acid profile and stable isotope ratios of C and N of muscle from cattle that grazed grass or grass/clover pastures before slaughter and their discriminatory potential

Abstract Consumption of grazed pasture compared to concentrates results in higher concentrations, in beef muscle, of fatty acids considered to be beneficial to human health. Little information is available on the influence of the type of grazed forage. Our objectives were to determine 1) the effect of inclusion of white clover in a grazing sward on the fatty acid profile of beef muscle and 2) the potential of the fatty acid profile and stable isotope ratios of C and N to discriminate between beef from cattle that grazed grass-only or grass/clover swards before slaughter. A total of 28 spring-born Charolais steers grazed from March until slaughter in October, either on a perennial ryegrass (Lolium perenne L.) sward that received approximately 220 kg N/ha or a perennial ryegrass–white clover (Trifolium repens L.) sward that received 50 kg N/ha. The longissimus muscle from cattle finished on grass/clover had a higher (P &lt; 0.05) proportion of C18:2 and C18:3 but a lower (P &lt; 0.05) proportion of conjugated linoleic acid and δ15N value than animals finished on the grass-only sward. Discriminant analysis using the fatty acid data showed that, after cross-validation, 80.7% of grass/clover and 86.1% of grass-only muscle samples were correctly classified. Discriminant analysis using the stable isotope data showed that, after cross-validation, 95.7% of grass/clover and 86.5% of grass-only muscle samples were correctly classified. Inclusion of white clover in pasture is likely to have little effect on healthiness of meat for consumers. However, changes in fatty acids and stable isotopes can be used to distinguish between grass/clover-fed and grass-only-fed beef.

軽種育成馬の放牧管理 : 3.固定放牧地での追肥

軽種育成馬の放牧管理 : 3.固定放牧地での追肥

Complementary effects of red clover inclusion in ryegrass–white clover swards for grazing and cutting

AbstractIncreasing plant species diversity in grasslands may improve productivity and stability of yields. In a field experiment, we investigated the herbage dry‐matter (DM) yield and crude protein content of two‐species swards of perennial ryegrass–white clover (Lolium perenne L.–Trifolium repens L.) and three‐species swards of ryegrass–white clover with red clover (Trifolium pratense L.). Five different managements represented cutting, grazing and combinations thereof, with different slurry fertilization treatments in 1‐ to 4‐year‐old swards. The three‐species mixture out‐yielded the two‐species mixture in years 1 and 2. Across all 4 years, yields were 8–10% higher in cut swards. Inclusion of red clover increased the yields of clover across the 4 years by 51% without fertilizer and by 90% when fertilized. Responses to slurry fertilization were similar in both mixtures and were mainly independent of sward age. There was a complementary effect over the season and across managements. Red clover dominated in the first and third cuts; white clover dominated in the second and fourth cuts. Red clover dominated in cut swards and white clover in grazed swards. Future prospects of the inclusion of red clover in sown swards are discussed. These may include higher nitrogen‐use efficiency in ruminants, increased soil fertility and improved sward flexibility to cope with changing managements. The findings also suggest positive yield effects of alternating between cutting and grazing within the season or between years.

Short-term foraging dynamics of cattle grazing swards with different canopy structures1

The objective of the present experiment was to describe the sward canopy structures of 3 different wheat (Triticum aestivum L.) pastures and relate them to short-term herbage intake rate and foraging dynamics by steers. Pastures were sampled for leaf and stem fractions at the bottom, middle, and top canopy strata. Sward surface heights and tiller and bulk densities were measured. Herbage was separated into stem and leaf, and leaves were then ranked phenologically. Three steers grazed (grazing sessions) 3 different pastures in a Latin square design. Ruminal contents were emptied and weighed before and after grazing sessions to assess herbage intake rate and bite mass. All grazing sessions were video recorded and analyzed for feeding stations (eating steps demarking the potential area of herbage consumption), bites per feeding station, and feeding stations per minute. Bite depth, bite area, and area grazed per feeding station were calculated. Morphological components and tiller density did not differ (P > 0.05) between the pastures, but sward surface height (P < 0.05) and leaf proportions in the middle and top canopy strata did differ. The herbage intake rate, bite mass, and bite area differed between treatments (P < 0.05). Steers grazing the tallest pasture with the greatest leaf accessibility ate faster, navigated slower, and grazed more efficiently (P < 0.05). The area grazed per feeding station differed between treatments (P < 0.05), being 87% for the tallest pasture with the greatest leaf accessibility and the least, 31%, for the pasture with the least leaf accessibility. Pastures with greater leaf accessibility may lead grazing cattle to reach the same herbage intake amount in less time while grazing more efficiently per unit area. Therefore, it may be logical to reduce the area and time allocations in pastures with taller swards where a leafy upper canopy stratum is found.

A review of research on the interactions between dugongs (Dugong dugon Müller 1776) and intertidal seagrass beds in Indonesia

Research on dugong-seagrass interactions in Indonesia was done during the period 1990 until 2005 in respectively East Aru, Maluku Province and East Kalimantan, Indonesia. This research investigated intensive rotational grazing by dugongs in intertidal inshore Halodule univervis seagrass meadows, dugong movements in relation to these grazing swards, and analyses of parameters explaining the temporal and spatial patterns of grazing in these meadows. In this paper, we report the findings of this long-term study. The patterns of movement and the results of snorkelling surveys confirmed a practice of regular recropping of restricted grazing swards by small feeding assemblages of dugongs. Dugong grazing showed a significant correlation with carbohydrate content of the below-ground biomass and no significant relation with total N. The timing of dugong grazing in these intertidal meadows coincides with high below-ground biomass and high carbohydrate content in the rhizomes of H. uninervis in the upper 0-4 cm sediment layer. Our findings support the hypothesis that temporal dugong grazing is ruled by carbohydrate content in below-ground biomass. The mechanisms of rotational grazing in restricted grazing swards are not yet well understood, and the maximisation of carbohydrates does not fully explain this phenomenon. Our research confirms that intertidal H. univervis seagrass meadows form a crucial resource for dugong survival. These relatively unknown biotopes need therefore more attention in research and conservation programmes.

Agronomic methods: Evaluation by multiple‐species pasture mixtures

Two trials, of 13 and 24 pasture species respectively, illustrated the utility of using the methodology of an initial multiple species mixture of contending accessions as a means of determining those most suitable to particular environment and management systems. Grazing trials on a developed lowland site and an initial development of a rangeland site showed rapid convergence in pasture composition from different initial sowing patterns. The option of sowing individual accessions as rows within mixed grazed swards gave the opportunity of comparing species or cultivars at early evaluation stages when seed supplies may be limited.

The effects of varied grazing management on epigeal spiders, harvestmen and pseudoscorpions of Nardus stricta grassland in upland Scotland

A hypothesis that epigeal arachnid assemblages benefit more from greater vegetation structure than botanical species composition in upland grasslands was tested. The test was carried out within a grazing experiment, initiated in 1991, to investigate vegetation dynamics in response to stocking with mixed livestock at varied rates. The experimental treatments comprised: no livestock, sheep only or sheep with cattle. Livestock treatments were grazed to maintain either 4.5 or 6.5 cm average sward heights between tussocks. Two replicates of each treatment were used and allocated to 10 plots across 22 ha of Nardus stricta-dominated grassland. The effects on epigeal arachnids (excluding acarines) of the botanical and structural differences of the grassland between treatments during April–October 1993 and 1994 were assessed. Epigeal arachnid species composition was estimated using continuous pitfall trapping and the densities of mainly money spiders (Araneae: Linyphiidae) were estimated from monthly suction sampling and visual counts of spider webs in micro-habitats. These data were later compared with stocking rate, botanical species composition and vegetation structure. Forty of the 84 sampled species occurred in all experimental treatments. There was a significant effect of treatment on the number of arachnid species in suction but not in pitfall samples. There was also a significant effect of treatment on the relative abundance of 26% of these arachnid species. For most species of spider, harvestmen and pseudoscorpion, abundance was greater in the ungrazed and taller, grazed swards although a few species were captured in greater numbers in the treatments with shorter swards. Botanical composition, mean vegetation height and grazing intensity accounted for 48.5–53.2% of the variability in the species composition/relative abundance of these arachnids, calculated by direct gradient analysis. Almost half of the species were randomly distributed across the experimental treatments and are recorded as widespread in upland heathland or grassland habitats and lowland grassland. More spider webs were counted during July–September 1993–1994, with greater numbers (dominated by the linyphiid species, Lepthyphantes mengii) counted in tall, ungrazed swards compared with taller grazed swards created by sheep alone or sheep with cattle. In the treatments with fewer webs, these were occupied by more linyphiid species. Suction sampling detected greater diversity of arachnids in the ungrazed N. stricta. This was related to increased plant litter below the leaf stratum where webs were counted. Vegetation structure and not botanical species composition within the N. stricta plant community determined arachnid species composition and abundance. Furthermore, no single grazing treatment supported the total number of arachnid species represented across the entire grazing experiment. It is concluded that varied grazing management, including some temporary ungrazed areas, is necessary to maintain the structural variability of grassland patches so as to maintain a spatial mosaic that favours the optimum arachnid fauna of upland grasslands.

Mineral Composition of Swards Containing Forage Chicory

Chicory (Cichorium intybus L.), a highly productive forage under midsummer conditions in the eastern USA, often has higher concentrations of minerals relative to grasses and legumes. Low concentrations of minerals in herbage could reduce production efficiency or create metabolic disorders in livestock. Information on the mineral composition of chicory is limited, but it is needed to improve our understanding of nutritive value and inputs required to sustain chicory production, especially where high rates of N are applied. We conducted field experiments for 3 yr on a Ramsey soil (Loamy, siliceous, subactive, mesic Lithic Dystrudept) in southern West Virginia to determine (i) response to increasing fertilizer N and (ii) production of chicory as a component of swards, including grass and legume as a function of clipping frequency. Mineral concentrations and uptake in available herbage generally increased as N rate increased and were influenced by the botanical composition of the sward. As chicory content decreased in the sward, so did mineral concentrations and uptake. Changes in sward composition associated with chicory influenced important mineral ratios such as N/S, Ca/P, and K/(Ca + Mg) cation equivalent ratio that have significant bearing on livestock health and production efficiency. Clipping frequency and N inputs influenced chicory persistence and ultimately the mineral composition of the sward. Active accumulation of minerals indicates the need for high nutrient input to sustain production, especially on soils with marginal fertility. Health problems associated with mineral nutrient concentrations probably would be minimal in livestock grazing swards that include chicory.

Movement and Home Ranges of Dugongs Around the Lease Islands, East Indonesia

Abstract. Four individual dugongs Dugong dugong, MÜLLER, 1776 were tracked with buoyant, tethered. conventional and satellite radio transmitters. The dugongs three adult females and one immature male were encircled with a net at a catch site near Haruku Island, East Indonesia, and tracked for between 41 and 285 days. The animals showed individualistic patterns of movement, moving between 2 and 3 core areas, travelling between 17 and 65 km from the site of capture. One adult female spent most of the time at two distinct inshore seagrass beds separated by about 17 km; she made five trips between the two sites. Two other females made separate trips to two distinct sites, one of them returning to the catch site at Haruku Island. The immature male journeyed between two areas about 65 km apart. completing the journey in four days. The patterns of movement confirmed a practice of regular recropping of restricted grazing swards by small, loose feeding assemblages rather than fixed herds with a strong social bond. Mean home ranges covered 4.1 km2 50% harmonic mean and 43.4 km2 95% harmonic mean.

The spatial pattern of vegetation in cut and grazed grass/white clover pastures

AbstractThe spatial patterns of white clover and sward surface height (SSH) that developed In established perennial ryegrass (Lolium perenne)/white clover (Trifolium repens) pastures undercutting lent every 4 weeks to 5 cm) and gracing (continuously grazed with sheep to 5 cm) were measured. While clover cover was recorded in 1000 contiguous 5 × 5‐cm quadrats down 50‐m permanent transects from early spring to late autumn. Measurements of SSH were made at 10‐cm intervals down the same transect. Spatial pattern was analysed using two‐term local quadrat variance and patch‐gap analysis. At least two scales of spatial pattern existed for white clover when defoliation treatments began. White clover was not distributed at random but found in patches (mean size = 1.1 m) where it was finely intermixed with grass. Patches, separated by gaps (regions of no clover) (mean size = 2.3 m), were in turn aggregated into ‘patches of patches’, separated by larger gaps (mean size = 4.1 m). Under grazing the pattern of patches and gaps did not alter. Under cutting, patch size increased and gap size decreased, explaining in part the greater mass and cover of white clover that arose in cut than grazed swards during the experiment. No new patches of white clover due to seedling establishment or clonal growth were observed in either cut or grazed swards. The intensity of pattern increased in both cut and grazed swards, but the increase was greater m cut swards. The initial single scale of spatial pattern of SSH of tall patches (mean size = 1.2 m) separated by short patches (mean size = 2.7 m) did not change under grazing. SSH became uniform under cutting. It is suggested that the response of plants to selective (spatially heterogeneous) grazing is a crucial factor in the development and maintenance of spatial pattern in grasslands. The importance of spatial pattern to our understanding and interpretation of plant‐plant and plant‐animal interactions and to the composition of temperate grasslands is considered.

Influence of irradiance on branch growth of white clover stolons in rejected areas within grazed swards

AbstractDevelopment of white clover stolons (Trifolium repens) was compared when grown in rejected areas and in adjacent defoliated areas over 28 days in May ‐ June in continuously stocked grass/white clover swards. In the cut areas more branches were borne on stolons with more but shorter internodes and shorter petioles than in the rejected areas. Red (R)/far red (FR) ratio at the base of the cut sward was significantly higher than in the rejected sward.Five experiments were carried out in which red light‐emitting diodes (LEDs) supplied supplementary irradiation at the node of the youngest fully expanded leaf on white clover stolons growing in association with dense canopies of perennial ryegrass. Axillary bud/branch length was the only significantly affected aspect of development that was measured, irradiation enrichment increasing length by about three‐fold over a 2–4 week period. One of the experiments involved identification of the site of perception of the R/FR effect. Natural and supplementary light were excluded from the stolon (by covering with a 2‐cm layer of black beads), petiole (by wrapping in aluminium foil), both or neither and showed that exposure of the stolon/ petiole base to supplementary light was necessary to stimulate axillary bud growth.It is concluded that the relative content of red light, and possibly absolute level, in irradiance at the base of the canopy in rejected areas is implicated in the young branch growth and that release of the buds can be achieved by defoliation, presumably due to increasing the red light content; however, the effect of increased photosynthetically active radiation (PAR) reaching stolons and young leaves cannot be discounted as an additional factor.

White Clover Morphology and Physiology in Response to Defoliation Interval

Yield response of white clover (Trifolium repens L.) to defoliation varies with leaf size. Our objective was to determine how specific aspects of plant morphology and physiology are responsible for differences in growth of diverse white clover cultivars in response to defoliation interval. Individual plants of ‘Aberystwyth S184’ (small‐leaf), ‘Grasslands Huia’ (medium‐smali‐leaf), and ‘Osceola’ (large‐leaf) were clipped to remove all fully expanded leaves and petioles 55 d after sowing in the greenhouse and then clipped every 7, 14, or 28 d for the next 28 d. Plant fraction dry matter (DM) accumulation and morphological and physiological characteristics were measured 1, 3, 7, 14, and 28 d after the final clipping. Stolon growth rate was inverse to leaf size classification (Aberystwyth S184, 7.6 cm d−1 Grasslands Huia, 5.8 cm d−1 &gt; Osceola, 3.6 cm d−1). In contrast, maximum stolon weight/length ratio of Osceola increased 31% vs. a 1% increase for Grasslands Hula and Aberystwyth S184 as defoliation interval increased to 28 d, which resulted in greater stolon DM accumulation by Osceola at the 14‐ and 28‐d intervals. Stolon and root carbohydrate concentration of Osceola was greater than and leaf carbohydrate concentration was less than the other cultivars at all defoliation intervals, indicating greater reliance on stored carbohydrates for regrowth by Osceola. Although plant leaf area of Oseeola was greater than that of Grasslands Hula and Aberystwyth S184 at the 28‐d interval only, forage DM accumulation of Osceola was greatest at all intervals due to petiole length. In grazed swards, however, greater petiole length would likely increase accessibility to and removal of leaf tissue, subsequently reducing new leaf and stolon growth.

Studies on alfalfa mosaic virus infection of burr medic (Medicago polymorpha) swards: seed-borne infection, persistence, spread and effects on productivity

During 1988-90, burr medic (Medicago polymorpha) cvv. Circle Valley, Serena and/or Santiago were grown in field trials in which plots were sown with healthy seed or seed that carried varying levels of infection with alfalfa mosaic virus (AMV). Seed-infected plants were the primary source for subsequent virus spread by aphids. Levels of AMV infection normally reached 80-100% in swards sown with infected seed whether they were mown, grazed or not defoliated, most spread occurring late in the growing season. Infection of harvested seed was often less than in seed sown. However, in self-regenerated grazed swards AMV spread earlier and infection in harvested seed was up to 10 times greater than in seed before regeneration. In two trials sown in 1988, plots were mown or left uncut; AMV infection decreased herbage yields by 13-35% while seed yields were decreased significantly (by 7-30%) in one of the two trails. When one of these trials was cropped with barley in 1989 but allowed to regenerate in 1990 and grazed, herbage yield fell by 5-15% and seed yield by 2-29% due to AMV infection. In a trial sown in 1989 in which some plots were grazed and others not, AMV infection decreased herbage and seed yields by 27-32% and 21-29% respectively. In a similar grazing trial sown in 1990, AMV infection decreased herbage daily growth rates by 16-42% and seed yield sometimes by as much as 32%. Seed harvested from plots sown with infected seed was normally smaller than seed from plots sown with healthy seed. AMV-infected swards were established in 1987, allowed to regenerate in 1988-91 and grazed. With cvv. Circle Valley and Serena, AMV was readily detected each year in foliage and was also found in seeds.

Short‐term fasts and the ingestive behaviour of grazing cattle

AbstractTwelve tethered Angus cows (Bos taurus)weighing 471 ± 18 kg were used in a balanced changeover design to estimate the direct (D), residual (R), and permanent (P) effects of 1,2, and 3 h fasts following 2 h grazing sessions on the ingestive behaviour of cattle grazing swards of endophyte‐(Acremonium coenophialum Morgan‐Jones and Gams)‐free tall fescue (Festuca arundinaceaSchreb. cv Kentucky 31) (Exp I) and lucerne (Medicago sativaL. cv Arc) (Exp II). Herbage dry‐matter (DM) mass (&gt; 5 cm) and individual herbage DM allowances were 1‐16 t ha‐1 and 50 kg per hour in Exp I and 1‐861 ha‐1 and 78 kg per hour in Exp II, respectively. In Exp I, P effects of fasting followed trends of R effects since the D effects were small. Mean DM intake rates (P) were 2‐2., 1‐8, and 1‐3 kg h‐1 for cows given 1, 2, and 3 h fasts, respectively. Mean intakes (DM) per bite (P) were 082, 071, and 045 g bite‐1 for cows assigned 1, 2, and 3 h fasts, respectively. Rates of biting were not affected by fasting and averaged 45 bites min”’. In Exp II, P effects followed trends in D effects since R effects were small. Mean DM intake rates (P) were 20, 22, and 3‐0 kg for cows subjected to 1,2, and 3 h of fast, respectively. Rates of biting (P) were 24, 27, and 32 bites min‐1 for cows after 1, 2, and 3 h fasts, respectively. DM intakes per bite (P) were not affected by fasts and averaged 1‐46 g bite‐1. The behavioural responses of cattle to interval between meals were attributed to differences in flow characteristics of ingesta of lucerne and tall fescue in the gastrointestinal tract.

Field investigations into hypomagnesaemia in dairy cattle, with particular reference to changes in the concentration of blood constituents during the early grazing period

1. In five experiments, groups of milking cows were changed abruptly from winter-stall feeding to graze a number of different swards, and changes in the concentration of blood-serum magnesium and of other blood constituents have been studied.2. Wide variations in the severity of hypomagnesaemia in individual cattle were found in all experimental groups, due possibly to such factors as individual variations in the intake of herbage, the requirement and body reserves of magnesium and the capacity to absorb magnesium from the gut.3. The degree of hypomagnesaemia observed was independent of the level of milk production, but was generally less severe in Friesian than in Shorthorn and Guernsey cattle. The onset of hypomagnesaemia was delayed for a few days in cattle with a high serum-magnesium concentration at the commencement of grazing.4. In two out of three experiments in which the effect of fertilizer treatment was studied, the incidence and severity of hypomagnesaemia was increased by the application to the sward of heavy dressings of nitrogenous fertilizer. In the third experiment, severe hypomagnesaemia occurred on a plot which had received no nitrogen fertilizer, due, it is thought, to a restricted intake of herbage magnesium, since the sward was extremely sparse. The feeding of supplements of flaked maize, crushed barley, crushed dredge corn, molassed sugar-beet pulp or a concentrate mixture balanced for milk production, to grazing cattle did not reduce the incidence of hypomagnesaemia.5. The cutting of grass and feeding it in the stall did not prevent the development of hypomagnesaemia.6. Blood pH and the concentrations of bloodserum calcium, sodium, potassium and blood glucose in cattle were unaffected by a change from winter feed to spring grazing, but a marked change in blood-serum non-protein and urea nitrogen and blood ammonia nitrogen, but not in any other nitrogenous constituents of the serum, was observed. The concentrations of serum non-protein and urea nitrogen and blood ammonia nitrogen were highest in groups of cattle grazing swards which had received a heavy dressing of nitrogen fertilizer and had a high nitrogen content.7. In the two experiments in which the severity of hypomagnesaemia was increased by the application of nitrogenous fertilizer, there was a close group association between high concentrations of blood serum non-protein and urea nitrogen and blood ammonia and low concentrations of blood-serum magnesium. High levels of serum urea and blood ammonia during grazing are thought to reflect a high ammonia production in the gut, which may be responsible for the disturbance in magnesium metabolism which produces hypomagnesaemia.