Basal Grain Research Articles

Grain Weight Reductions in Wheat Associated with Semidwarfism: an Analysis of Grain Weight at Different Positions Within the Spike of Near‐isogenic Lines

AbstractSemidwarf genes have made a major contribution to grain yield gains in wheat during this century. These genes have frequently been reported to increase grain number but decrease grain size. To analyse possible causes of this negative association between number of grains per head and mean grain weight, grains from different positions within the spikes were individually weighed in three isolines (standard‐height, SH; semidwarf, SD; and dwarf, DD) of the spring wheat cultivar Maringa grown under field conditions.Dwarfing genes affected both grain number and grain weight. Increasing doses of Rht genes resulted in a decreased average grain weight (ca. 22 %) but in a greater increase in number of grains per m2 (ca. 40%). Since the isogenic lines did not differ in either the number of spikes per m2 or number of spikelets per spike, the frequencies of appearance of grains in the distal positions (florets 3 and 4) of the central spikelets and in the top spikelets of the spike was much higher for the SD and DD lines than for the SH line. Averaging across spikelets, basal grains (grains 1 and 2) of SH, SD and DD lines represented ca. 82, 73, and 69 %, respectively, of the total number of grains. On the other hand, grain 4 was almost absent in the SH line but represented around 10 % of the grains in the DD line.Averaging across lines and spikelets, basal grains weighed ca. 30 mg, which is approximately 20 % heavier than grain 3 (ca. 25 mg) and 85 % heavier than grain 4 (ca. 16 mg). Although there was a clear trend for a greater grain weight in the SH line (with no major differences between the SD and DD line) when basal grains were considered, grains 3 and 4 did not show any clear difference between lines.The fact that genotypes with Rht alleles had grains in the distal positions with a much higher frequency of appearance than in the SH line could be the main reason for their lower average grain weight. Therefore, the negative relationship between grain number and average grain weight might be not reflecting that competition for assimilates among grains of the DD and SD lines is stronger than that in the SH line.

Individual grain weight responses to genetic reduction in culm length in wheat as affected by source-sink manipulations

This paper describes the effects of Rht alleles in isogenic lines of Maringa spring wheat ( Triticum aestivum L.) on number of grains per spike, average grain weight and weight of individual grains from different positions within the spike. Plants were grown under five environmental conditions in the field (in the 1991, 1992 and 1993 growing seasons, with two sowing dates in the first two years), aiming to determine causes of lower average grain weight commonly observed in semi-dwarf (SD) compared with standard-height (SH) cultivars. The number of grains per spike was significantly greater in SD and dwarf (DD) lines than in the SH line due to differences among lines in number of grains per spikelet. Therefore, the relative contribution of proximal grains was affected negatively by Rht alleles while the contribution of distal grains was affected positively. Average grain weight was reduced with increases in the dose of Rht alleles, and this trait was negatively correlated to number of grains per spike. To understand the possible causes of this negative relationship, individual grains from specific positions within the spike were analysed. Basal grains in central spikelets were heavier than those in near apical and near basal spikelets. Within central spikelets, the ranking of individual grain weights were grains 2 > 1 > 3 > 4, numbered from the most proximal to the most distal positions. Proximal grains (1 and 2) were heaviest in SH, lightest in DD, and intermediate in the SD line. In general, the relative differences between the lines were smaller than that found for the average of all grains of the spike, in particular when grains of different positions within the central spikelets were considered. For all these grains, differences in weight due to Rht alleles were due to differences in rate of grain filling, with the effective duration of grain filling being almost unchanged. An increased source-sink relationship did not significantly modify the weight of any of the analysed grains in SD and SH lines (and just slightly increased rate of grain filling in the DD line). Therefore, the smaller grains in SD and DD lines could hardly have been due to an increased competition for assimilates. Alternatively, SD and DD lines had a greater proportion of grains from distal positions than the SH line, and these grains were always smaller than proximal grains. The reduction in average grain weight produced by Rht alleles therefore was due to a combination of effects on the potential size of each grain and on the contribution of grains from distal positions within the spike, with no effects attributed to an increased competition for assimilates.

Effects of adding barley hulls and linseed meal to wheat and hulless barley diets fed to growing pigs

Two high energy basal grains (wheat and hulless barley), three levels of barley hulls derived from hulless barley (0, 4 and 8%), two levels of linseed meal (LM) (0 and 5%) and two sexes of pigs (barrows and gilts) were compared in a factorial experiment involving 72 pigs over the weight range 23 to 100 kg. The animals were housed in groups of four but were fed individually. All diets were nutritionally adequate except for digestible energy (DE) which ranged from 12.6 to 13.5 MJ kg −1; the US National Research Council requirement level is 14.2 MJ kg −1. Average daily gain was unaffected by dietary treatments in either the grower or finisher stages. Efficiency of feed utilisation was improved with wheat diets by addition of hulls but hulls had no effect with hulless barley diets. Linseed meal depressed the efficiency of feed and DE utilization. The carcass dressing percentage declined with increasing hull level but only when LM was also fed. Backfat depth was reduced by adding hulls to hulless barley diets, especially when LM was included. The findings are discussed in relation to the possible effects of different levels and kinds of fibre.

Fruiting Efficiency in three Bread Wheat (Tritkum aestivum) Cultivars Released at Different Eras. Number of Grains per Spike and Grain Weight

AbstractIncreasing sink size (i.e. number of grains m−2) has been the way of the genetic improvement for obtaining higher grain yield of wheat. We have studied under field conditions the number of fertile florets and grains per spike and individual grain weight of three bread wheat cultivars (Klein Favorito, Eureka FCS and Buck Pucara) which were previously selected to represent cultivars released at different eras (1920, 1940 and 1980, respectively) in Argentina (Slafer and Andrade 1989).The three cultivars Buck Pucara, Eureke FCS, and Klein Favorito averaged 56.9, 48, and 34.5 grains spike−1. Differences were observed in both main shoot and tiller spikes. The number of fertile florets at anthesis was greater in the modern cultivar than in the other two cultivars; however, the intermediate and old cultivars did not significantly differ in this trait. Differences in number of florets and number of grains spike were independent of the number of spikelets spike−1; spikelet numbers did not vary among the three cultivars. Therefore, both the number of fertile florets produced per spikelet and the ability of the cultivar to set grains in those florets were responsible for the different number of grains spike−1 among cultivars. The modern, high yielding cultivar had a lower individual grain weight than the old, low yielding cultivar when the average of all grains were considered; but basal grains of its central spikelets were as heavy as those of the other cultivars.We suggest (i) that the modern cultivars of Argentina have increased their ability both to produce fertile florets and to set grains, and (ii) that the lower individual grain weight of the modern cultivars does not imply an increased competition for resources as a consequence of their higher number of grains.

Green Area Duration during the Grain Filling Period of an Argentine Wheat Cultivar as Influenced by Sowing Date, Temperature and Sink Strength

AbstractThe effects of sowing date, air temperature during the grain filling period, sink‐strength, and then‐interactions on the wheat green area (leaves, leaf sheaths and peduncles) duration from anthesis to total senescence were determined under field conditions at Buenos Aires (Argentina). The individual grain weight was determined as an average of either all grains or basal grains of the spikelets 7 and 8 through the treatments.The delayed sowing date reduced the period of leaf senescence when it was regarded as calendar time between anthesis to maturity, but it did not affect that period regarded as thermal time. Similarly, the increased temperature has also increased the rate of senescence. On the other hand, the sink strength did not affect the green area duration; since neither the rate nor the period of senescence was altered.Similarly, both sowing date and mean temperature decreased the individual grain weight, but the sink strength did not affected the weight of the basal grains, which could be interpreted as the effects of the other two factors were independent to the availability of assimilates per grain.The effect of increased grain filling temperature on reducing grain weight did not operate via reducing the availability of assimilates although it also reduced the total grain area duration. We observed that the increased temperature decreased the individual grain weight of the basal grains at any source sink ratio.

Effects of whole cottonseed and live yeast culture on ruminal fermentation and fluid passage rate in steers.

The effects of adding whole cottonseed (WCS) and live yeast culture (YC) to a diet of 60% alfalfa and 40% concentrate on ruminal fermentation and liquid dilution rate in steers were examined in two trials. Twelve ruminally cannulated Hereford steers (approximately 420 kg) were used. Concentrates were 1) a basal grain mix composed of ground corn, ground barley and rice bran, 2) basal mix with 10% WCS, 3) basal mix with 1.6% YC and 4) basal mix with 10% WCS and 1.6% YC. No differences were detected (P greater than .10) in ruminal pH of steers fed WCS or YC in either trial. Ruminal ammonia levels tended to be higher (P greater than .10) in steers fed WCS. Acetate was higher and butyrate was lower in steers receiving 10% WCS (P less than .10) compared with controls. Molar proportions of acetate and butyrate increased while propionate decreased (P less than .05) over time after feeding. Intake and apparent digestibility of dry matter was not influenced by treatments. Whole cottonseed and(or) YC appear to have minimal effects on ruminal fermentation, but they might alter ruminal liquid passage rate with diets containing 60% ground alfalfa and 40% concentrate.

Nutritive Value of a New Hull-less Barley Cultivar in Broiler Chick Diets

The hull-less barley mutant Franubet, derived from Betzes (CI 6398), has an unusual starch type, with granules that appear fractured when viewed with an electron microscope. Hull-less Franubet barley from three crops and a hulled barley, Ershabet (CI 15722) were used in a series of 21-day broiler chick feeding studies using 21% protein diets supplemented with 44% protein soybean meal, DL-methionine, minerals, vitamins, and an antibiotic. Barley-containing diets were compared with corn diets in varying proportions as basal grains.Supplemental beta-glucanase added to Franubet diets resulted in improvement in chick growth over that of birds fed diets unsupplemented with beta-glucanase. In some cases, chicks fed Franubet barley with or without the supplemental enzyme had weight gains equal to those fed corn. Feed:gain ratios corresponded to weight gain results. In one trial, Ershabet barley, a hulled cultivar, was compared with Franubet barley, with and without beta-glucanase, and corn as basal grains. Results from feeding both barleys were equal to those of feeding corn when supplemented with enzyme, although performance of the hull-less cultivar tended to be superior with respect to chick body weight gain and feed:gain ratio. Franubet barley appears to have high nutrient digestibility for poultry compared with other barleys.

Expression of the regulation of energy balance on the ingestive behavior of grazing beef cattle supplemented by grain

A change-over design was used to measure the effect of 0.0, 2.5 and 5.0 kg day −1 of ground corn ( Zea mays L.) kernels on the ingestive behavior of 12 tethered 2-year-old Angus ( Bos taurus) heifers receiving a basal grain ration of 2.5 kg day −1 and grazing summer regrowth pastures of ‘Boone’ orchardgrass ( Dactylis glomerata L.). The experimental design allowed the estimation of the immediate effects (direct effects) of grain on ingestive behavior, its carry-over effects expressed the next day (first residual effects), and the combined effects of direct and first residual effects (permanent effects). Grain had no direct effect on ingestive behavior. Heifers ingested herbage dry matter (DM) at 1.64 kg h −1, forming bites of 745 mg DM at 37 bites min −1. The first residual effect was significant for residual dry matter, utilization, intake and rate of biting. Five kg of grain depressed the rate of herbage DM intake from 1.67 to 1.50 kg DM h −1 on the day following its ingestion. This residual effect was not expressed in herbage intake per bite, but rate of biting was reduced from 39 to 35 bites min −1. The 2.5-kg treatment had a small positive residual effect on rate of herbage intake. Data indicate the operation of energy balance regulation mechanisms on the ingestive behavior of grazing beef cattle.

Investigations of Threonine Requirements of Broiler Chicks Fed Diets Based on Grain Sorghum and Soybean Meal

Three trials were conducted to estimate the threonine (Thr) requirement of male broiler chicks. Chicks 7 days of age were fed a grain sorghum-soybean meal diet containing .59% Thr, supplemented with 0, .05, .09, .15, .22, .27, or .28% L-Thr. Levels of basal and supplemental amino acids were confirmed by ion-exchange chromatography. Diets were fed for 18 or 20 days, depending upon the trial. A corn-soybean meal diet analyzed to contain .90% Thr was used for comparison. Results of the three trials were pooled for determination of Thr requirements.A significant improvement in gain was observed when the basal grain sorghum-soybean meal diet was supplemented with .09% supplemental Thr (total .68% Thr). No further improvements in gain were observed at higher levels of Thr supplementation. Body weight gain of chicks fed the Thr-supplemented diets did not differ significantly from that of chicks fed the corn-soybean meal control diet.For maximum feed utilization, the Thr requirement was .79%, as estimated by regression analysis. This is in agreement with the .80% requirement suggested by the National Research Council in 1984. Feed utilization of chicks fed the corn-soybean meal diet was significantly better than that of chicks fed any of the grain sorghum-soybean meal diets regardless of the level of Thr supplementation.

Fecal steroid excretion in chickens with hereditary hyperlipidemia.

Plasma lipids (cholesterol, triglycerides, and phospholipids; mg/dl) and the fecal excretion (mg/day) of neutral steroids and bile acids were studied in layers (L), hereditary nonlayer hens (NL), and roosters (R) fed a basal cholesterol-free grain diet ad libitum. Each group had significantly (P less than 0.05) different levels of plasma cholesterol, triglycerides, and phospholipids when compared to the other groups. The highest lipid values were found in the NL group (cholesterol, 798 +/- 89; triglycerides, 8914 +/- 679; phospholipids, 2458 +/- 112). There was no difference in the fecal excretion of neutral steroids between L and NL; however, fecal bile acid excretion by these two groups was significantly different (P less than 0.05) (L, 13.1 +/- 1.7 vs NL, 26.9 +/- 3.4). Fecal neutral steroid excretion by R was significantly greater (P less than 0.05) than that by either L or NL (L, 6.4 +/- 1.3; NL, 6.0 +/- 1.4; R, 14.4 +/- 1.2). While fecal excretion of bile acids by R (36.1 +/- 4.0) was also greater than that by either L or NL, only the difference between R and L was statistically significant (P less than 0.05). Since, in the steady state, fecal bile acid excretion is equal to its synthesis, these results suggest that bile acid metabolism in these animals can be affected by both sex and egg-laying status.

Relationships between floret size and grain weight in aneuploid lines of homoeologous group-5 chromosomes of common wheat (Triticum aestivum L.) cv. Chinese Spring

Various aneuploids of common wheat (Triticum aestivum L.) cv. Chinese Spring were used to study the dosage effect of group-5 chromosomes on spike morphology, size of floral organs, and grain weight. Reduced dosage resulted in smaller floral organs, smaller floret cavities, and lighter grains than in the euploid, while increased dosage had the opposite effects. Chromosome 5A was the most effective in inducing these changes, whereas in most cases 5B was the least effective. Based on the various lines studied, a high linear correlation was found between the weight of the basal grains in the central spikelets and the volume estimates of the florets in which they were developed. The different lines exhibited a considerable similarity between the size distribution of floral organs and the grain weight distribution in the main spike. Pleiotropic effects of genes located on group-5 chromosomes on size of floral organs and grain weight are suggested.Key words: Wheat, aneuploids, group-5 chromosomes.

Effects of pure and auto-oxidized forms of cholesterol on plasma, liver lipids and hepatic lipogenesis in chicks

1. 1. Pure cholesterol (PC), oxidized cholesterol (OC) and cholestane-3β,5α,6β-triol (CT) were fed to male Hubbard chicks for 3 weeks in a basal grain diet. 2. 2. Feed consumption and weight gain of chicks fed OC (1.0%) or CT (0.1%) were not significantly different from that of chicks fed PC (0.85%) or the basal diet alone. 3. 3. Plasma cholesterol level was significantly ( P < 0.05) higher in chicks fed PC compared to controls; however, compared to other dietary groups, differences were not significant. 4. 4. Measurement of hepatic lipogenesis, in vivo, from labeled substrates showed that [ 14C]acetate was primarily utilized for fatty acid synthesis in all dietary groups. The relative order was PC > PC + CT > OC > CT >; basal 5. 5. Conversely, [ 3H]mevalonate was preferentially used for cholesterogenesis and the relative effectiveness was basal > CT > OC > PC + CT > PC.

Effect of Post-anthesis Drought on Cell Division and Starch Accumulation in Developing Wheat Grains

Wheat plants (Triticum aestivum L., cv. Warigal) were subjected to 20 d of water deficit during the period of endosperm cell division. Drought accentuated the differences in final grain weight between spikelets and between grains within spikelets. The distal grains of top spikelets were most affected by drought. The maximum number of endosperm cells was, respectively, 30 and 40 per cent lower in basal grains and distal grains of draughted plants. In basal grains of middle spikelets, the number of large starch granules per cell was unaffected but the number of small starch granules per cell was 45 per cent lower in grains of draughted plants. The initiation of small starch granules was more affected than cell division because severe water deficit occurred earlier during the former process than the latter. Final dry weight appeared to correlate well with the maximum number of endosperm cells, but depended also on the number of starch granules per cell. Consequently, the amount of dry matter per cell was not constant in both treatments. The concentration of sucrose per endosperm cell was lower only in the droughted distal grains of top spikelets. The supply of sucrose to endosperm cells did not regulate the initiation of small starch granules.

Role of Changes in Cell Size in the Evolution of Wheat

The objective of the present work was to examine to what extent increase in cell size has contributed to the parallel increases in leaf and grain size in the course of evolution in wheat. Eighteen lines were chosen to represent wild and cultivated wheats at the diploid and tetraploid levels, cultivated hexaploid wheats, and the two Aegilops species likely to have contributed the B and D genomes. All plants were grown at 21/16�C under natural light. The penultimate leaves and the basal grains from central spikelets were selected for comparison. The projected area of separated mesophyll cells from leaves was 1.5–2 times larger in the cultivated tetraploid and hexaploid wheats than in the diploids, and correlated positively but weakly with leaf blade area (r = 0.50), and negatively with photosynthetic rate in Triticum species (r = -0.66). Cell size in the endosperm bore no relation to cell size in the aleurone layer or mesophyll, or to grain volume. Aleurone cell size, however, correlated positively with both grain volume (r = 0.82) and mesophyll cell size (r = 0.79). Increase in grain weight during evolution has not involved increase in either specific gravity of the mature grain or endosperm cell size. Presumably increase in endosperm cell number has been the major factor. With increase in grain size during evolution there has been a fall in percentage nitrogen in the grain.

Chronic toxicity and carcinogenicity in mice of the purified mycotoxins, luteoskyrin and cyclochlorotine

Long-term feeding studies in mice, initiated in 1960, have been carried out on luteoskyrin and cyclochlorotine, obtained in purified form from Penicillium islandicum Sopp, an infectant of yellowed rice. In the case of luteoskyrin, five different experiments were conducted to investigate the importance of strain, sex, dose level, frequency of dosage and type of basal grain diet on the development of acute, subacute and chronic toxicity, and especially on hepatoma induction. The results provide evidence that these two mycotoxins are responsible for the hepatotoxic and hepatocarcinogenic effects of yellowed rice. The effects of the mycotoxins differed in that necrosis was observed in the early stages with luteoskyrin, and fibrosis and cirrhosis with cyclochlorotine, but the chronic effects on the liver were similar. The tumorigenic effect of luteoskyrin on the liver was more marked in males than in females.

Hypothalamo-hypophysial and ovarian characteristics of gilts fed methallibure.

Twenty-eight crossbred gilts were assigned randomly in equal numbers to four subgroups in an experiment of 22 factorial design. All gilts were individually fed a basal grain ration each day. Treatment consisted of feeding gilts 125 mg of methallibure (1-α-methy-lallylthiocarbamoyl-2-methylthiocarbamoylhy-drazine) daily, mixed with ration, beginning on the first day of detected estrus. Autopsy was performed on day 8 or 16 of the estrous cycle (first day of estrus=day 0 of the cycle). Treatment caused an increase in pituitary FSH and LH activity at each stage of the cycle studied (FSH, P<.01; LH, P≅.07). Levels of pituitary FSH and LH activity were significantly greater at day 16 than day 8 of the estrous cycle. The stalk median eminence was heavier and contained significantly more LRF at day 16 than day 8 of the cycle. Feeding methallibure to gilts failed to affect the weight or luteinizing hormone releasing factor (LRF) activity of the stalk median eminence. These data suggest that methallibure does not alter levels of pituitary LH by interfering with LRF activity. Corpora lutea in both control and treated gilts weighed significantly more at day 8 than day 16 of the estrous cycle. Treatment tended to reduce the weight of corpora lutea at day 8 and increase the weight of these glands at day 16 but the differences were not significant statistically. Methallibure caused a reduction in luteal progesterone content and concentration at day 8 but an increase in luteal progesterone content and concentration at day 16 (day × methallibure interaction, P<.01, for each characteristic). In both control and treated gilts ovarian follicular fluid was significantly greater at day 8 than day 16 but less in treated than control gilts at each stage of the cycle studied (P<.01). Treatment reduced the number of ovarian follicles @?@ mm in diameter, particularly at day 16 of the cycle.

The partial replacement of soybean meal by amino acids in pig rations based on wheat and sorghum

In five experiments involving 104 pigs, rations consisting of grain and 15 per cent soyhean meal as the major components were compared with rations of grain plus 7.5 per cent soybean meal in combination with lysine and methionine. In three of the experiments, comparisons were also made between wheat and sorghum as basal grains. The results showed a response in growth rate and feed efficiency to the addition of 0.24 per cent L-lysine HCl.2H2O either with or without 0.20 per cent DL-methionine to rations of grain plus 7.5 per cent soybean meal. From weaning to 45 kg body weight the growth rates of pigs fed these amino acidsupplemented rations were consistently less than those obtained with pigs fed the grain plus 15 per cent soybean meal rations, although not always significantly so. From 45 kg to the final slaughter weight of approximately 90 kg, only in one experiment were the results obtained with the rations containing 7.5 per cent soybean meal plus amino acids significantly less than those obtained with the 15 per cent soybean meal rations. Where comparisons were made between wheat and sorghum, the wheat-based rations consistently produced better growth rates and feed efficiencies, and in one experiment better carcase measurements.

WHALE SOLUBLES AS A PROTEIN SUPPLEMENT IN RATIONS FOR FINISHING BEEF CALVES

Sixty-three Hereford steer calves were used in a feeding trial which comparade a simple basal grain ration [11.7 per cent total crude protein (TCP)] with the basal grain ration supplemented with either whale solubles (16.3 per cent TCP) or linseed meal (15.3 per cent TCP) to determine whether whale solubles could replace linseed meal as a source of protein in steer finishing rations. Forage (mixed grass-legume hay and grass-legume silage) and grain were fed in a ratio of approximately 1:1.25.Results for steers fed the basal ration, basal plus whale solubles and basal plus linseed meal were: average daily gain— 1.68, 1.72, 1.85; pounds of total digestible nutrients per pound of gain — 5.5, 5.4, 5.2; dressing percentage — 58.5, 58.7, 59.1; and per cent lean in rib-cut at last rib — 64.6, 65.5, 67.8.Steers fed the basal ration plus linseed meal gained significantly more (P &lt; 0.05) and required less feed per pound of gain than those fed either the basal ration or the basal ration plus whale solubles. Differences between groups for the other measured characteristics were not significant (P = 0.05). Carcasses of steers fed the basal ration or basal plus whale solubles graded slightly higher than those of steers fed the basal ration plus linseed meal.Whale solubles did not equal linseed meal as a source of supplemental protein for finishing steer calves under the conditions of this experiment.

NUTRIENT REQUIREMENTS FOR CANADIAN YORKSHIRE SWINE: I. A STUDY OF THE U.S. N.R.C. (1953) STANDARDS

A co-operative nutrition project involving eight institutions across Canada was undertaken in order to test some of the swine feeding standards in the 1953 revision of "Nutrient Requirements for Swine". Four stations (University of British Columbia, University of Alberta, University of Manitoba, and the Experimental Farm, Nappan, N.S.) conducted swine feeding trials. Four rations were fed at each station: A—a centrally formulated barley ration based on the N.R.C. requirements for protein, Ca, P, vitamins A and D; B—a formula chosen by the co-operator; C—a ration based on the standards but employing barley plus oats as the basal grains, all ingredients being from local supplies; D—similar to ration C, but with barley plus wheat as the basal grains.All feeds were analysed for crude protein, crude fibre, ether extract, ash, energy, Ca, P, weight/quart, riboflavin, niacin, choline and pantothenic acid. Carcasses were graded according to commercial grades and scored in detail according to Canadian Advanced Registry standards.Approximately normal gains and feed efficiencies were obtained during the growing period (40–110 lb.) despite riboflavin, T.D.N. and pantothenic acid levels below the standards. Similar results occurred in the finishing period (110–200 lb.) but carcass quality proved to be unsatisfactory. It is suggested that the gains during the finishing period represented too much fattening in relation to skeletal and muscle development.

Yield characters of selected oat varieties in relation to cereal breeding technique

1. Results obtained from two seasons' population studies of selected spring oat varieties in Scotland indicated that extent of tillering had little or no effect on yield. These results are in sharp contrast with those obtained by workers with wheat.2. The yield of individual plants under the conditions of the experiments became adjusted to differences in population density primarily by variation in number of grains per panicle. Spikelet weight tended to be negatively correlated both with population density (external competition) and with number of grains per panicle (intrapanicle competition). These factors tended to neutralize each other.3. The decrease in size of additional grains in spikelets with more than one grain was balanced by increased weight of the basal grains as compared with the weight of a single grained spikelet. Average grain weight per spikelet therefore tends to remain constant.