Agricultural and Food Science in Finland, Vol. 10 (2001): 121–131 121 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 10 (2001): 121–131. © Agricultural and Food Science in Finland Manuscript received November 2000 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 10 (2001): 121–131. Yield trends of temperate cereals in high latitude countries from 1940 to 1998 Gustavo A. Slafer Departmento de Produccion Vegetal and IFEVA (Agricultural Plant Physiology and Ecology Research Institute), Facultad de Agronomía, Universidad de Buenos Aires, Av. San Martín 4453, 1417 Buenos Aires, Argentina, e-mail: slafer@ifeva.edu.ar Pirjo Peltonen-Sainio Department of Plant Production, University of Helsinki, PO Box 27, FIN-00014 Helsinki, Finland. Current address: MTT Agrifood Research Finland, Plant Production Research, FIN-31600 Jokioinen, Finland, e-mail: pirjo.peltonen-sainio@mtt.fi Wheat is the only temperate cereal for which yield trends have been exhaustively analysed on both global and national bases. This paper aims (i) to compare global yield trends of wheat, barley, oat and rye for the last five decades, (ii) to analyse their yield trends in Canada, Denmark, Norway, Sweden and Finland, the northernmost limits for extensive agriculture, and (iii) using case studies, to assess the relative contribution to yield gains made by cereal breeding. Average global yield data from FAO were regressed against years using linear or bilinear regressions. Yield gains in absolute and relative terms were calculated for comparison among countries and cereals. Data from the literature were used to assess the estimated contributions made by breeding to yield gains. Global yield trends were not standard throughout the 1950–1998 period: rye exhibited a constant yield gain (c. 28 kg ha–1 y–1), while barley and oat showed marked increases until around 1970 (c. 38 and 32 kg ha–1 y–1, respectively) but quite modest increases (c.19 and 5 kg ha–1 y–1, respectively) over the last 30 years. Wheat also showed a bilinear trend with only limited yield gains until the 1960s, followed by a more than 3-fold increase in rate of yield gain from then on (16 and 40 kg ha–1 y–1, respectively). However, during the 1990s wheat yield gains have been less than previously. Hence, global yields of barley, oat and wheat have increased very slowly lately. Trends for each combination of cereals and countries indicated consistently higher yields during the 1990s than at mid-century. In general, wheat yield tended to increase at a faster rate than yield of the other cereals. There was a trend in the last decade of low rates of yield increase compared with those of previous decades. This was clear for oat and barley, and a similar trend is emerging for wheat. This suggests that genetic and/or management improvements have had less effect in recent times. Furthermore, we found preliminary evidence to suggest that with the exception of wheat in Canada, genetic contributions in northern areas were smaller than those reported for wheat and bar- mailto:slafer@ifeva.edu.ar mailto:pirjo.peltonen-sainio@mtt.fi 122 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Slafer, G.A. & Peltonen-Sainio, P. Yield trends of cereals in high latitude countries ley at lower latitudes. Therefore, alternative approaches must be sought for future breeding work under these high latitude conditions. Key words: barley, breeding, northern agriculture, oat, rye, wheat, yield Introduction Cereals represent the major contribution to total world food production. In total their production is substantially greater than that of all other crops combined (Slafer et al. 1994). A share of that production comes from countries in which agri- culture is practised at high latitudes. Under high latitude growing conditions most cereal produc- tion involves the temperate species, barley (Hor- deum vulgare L.), oat (Avena sativa L.), rye (Se- cale cereale L.) and wheat (Triticum aestivum L.). The major factor limiting crop productivity in these marginal areas is the length of the grow- ing season. However, these countries of these regions may be expected to make a dispropor- tionately large contribution to future increases in crop production as they will benefit more from expected global warming than countries at low- er latitudes (Kettunen et al. 1988, Carter and Saarikko 1996). Wheat is the only temperate cereal for which yield trends have been exhaustively analysed on both global and national bases (Calderini and Slafer 1998). During the 20th century, the pro- duction of wheat increased more than 6-fold (Slafer et al. 1994). This increase was initially due to the enlargement in harvested area that occurred during the first half of the century, fol- lowed from the 1950s onwards by an increase of c. 150% in global average yield per unit land area (from 1 to 2.5 Mg ha–1; Slafer et al. 1996). However, a closer inspection of the records of average yields for recent years suggests a possi- ble levelling off of cereal yields (Slafer et al. 1996). Although the apparent levelling off should be regarded cautiously, it may indicate that new strategies for maintaining rising yields are need- ed, as a continued increase in productivity must be achieved to match the ongoing increase in world population (Mann 1999, Slafer and Satorre 1999). The degree to which other cereals exhib- it similar decreases in yield gains during the re- cent decades, has not been determined. Despite their physiological-ecological similarities, trends among temperate cereals may be different, as the pressure for increasing yields and the amount of research effort expended on this has always been greater for wheat than the other temperate cere- als. One of the objectives of this paper was to compare global trends for wheat, barley, oat and rye during the last five decades, as most cereals did not exhibit clear yield gains during the first half of this century (Slafer and Satorre 1999, Slafer and Otegui 2000). Global yield averages may not clearly reflect the situation in particular countries or regions, and they could either highlight similarities or mask differences among them. Calderini and Slafer (1998) reported some notable coincidenc- es in general trends for wheat yield among coun- tries with different environments, cultures and economic/agricultural policies. However, they concentrated on major production regions rath- er than on particular growing conditions that define a region. In their analysis, only Canada and Sweden represented countries with virtual- ly all production under northern growing condi- tions. Furthermore, only wheat was analysed and other cereals might exhibit quite different trends in specific regions. Therefore, an additional ob- jective of this paper was to analyse yield trends for the four major temperate cereals in Canada and Nordic countries including Denmark, Nor- way, Sweden and Finland. Yield statistics used in this study reflect changes in several socio-economic forces (e.g. grain prices, market demands, environmental issues) that determine modifications in both crop management and cultivar selection. When future perspectives are being sought, it is relevant to 123 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 10 (2001): 121–131. estimate the relative contribution of plant breed- ing to the total gain in yield (Slafer and Andrade 1991). As management and genetic factors in- teract to produce the final yield gains, recorded as averaged yield trends for a particular region, it is difficult to separate these contributions ac- curately (Harper 1983). Again this has been done for wheat in several regions. Genetic contribu- tions to the total gains in wheat yield have been estimated to be c. 50% in USA, UK and Argen- tina and c. 30% in Australia and Mexico (Slafer et al. 1994, Bell et al. 1995). In barley approxi- mately one third of the yield increase recorded in the UK between the 1940’s and the 1980’s has been attributed to genetic improvement (Silvey 1986). For neither wheat nor barley in the north- ernmost growing conditions, nor for other cere- als anywhere, have the genetic contributions to yield gains been quantified. Hence, we also aimed to assess the relative contribution of ge- netic improvement to yield gains in some cases. Material and methods Data for this study were obtained from FAO year- books (the 1947 issue carried forward data from 1940 collected by the preceding organisation, The International Institute of Agriculture in Rome, and published in their Annuaires del In- stitut International D’Agriculture), the FAO’s World Crop and Livestock Statistics published in 1987, and the FAO’s web-site (www.fao.org). Data comprised the global yield records from 1951 to 1998 and from 1940 to 1998 for each of the countries included in the analysis. Global data for the World War II period were not avail- able for many countries and hence the trends for averaged yield were estimated from 1950 on- wards. The northern countries analysed includ- ed Canada, Denmark, Finland, Norway and Swe- den. In these countries most of the cereals are grown at northern latitudes. Countries with re- gions at high latitudes but also with temperate regions that make important contributions to the average yields of the country were not included, as both edaphic and climatic conditions, and consequently yield levels, varied widely within the north-south axis of the country. A simple model was used to characterise the yield trend in each case. Although in some cir- cumstances more complex models might provide a better statistical description of the actual trends, we restricted the analyses to simple models with agronomically meaningful parameters that allow comparisons to be made among countries and/ or cereals. Average global yields were regressed against years using linear or bilinear regressions with an optimisation model that was fitted itera- tively to the data using curve-fitting software (Jandell 1991). The regression models were y = a + bx (linear) and y = a + bc + d(x–c) (bi-line- ar); where ‘y’ represents yield, ‘a’ the intercept, ‘b’ the rate of yield gain during the first period (unique in the linear model), ‘c’ the year at which the inflection point occurred, ‘d’ the rate of yield gain during the second period, and ‘x’ the year. The model finally accepted for each case was that exhibiting the highest adjusted coefficient of determination, with the lowest fitted stand- ard error. For each country and cereal species combined simple linear regression was used to estimate the yield gains (slope) throughout the 59 years ana- lysed. Relative yield gains were the yield gains expressed as a percentage of yields averaged for the whole period in each country-cereal combi- nation (Slafer and Andrade 1991). Available data from the literature on yield increases due to genetic improvement (from ex- periments comparing cultivars released at dif- ferent times for particular combinations of coun- tries-cereals, Calderini et al. 1999) were used to assess the estimated contributions made by breeding to the exhibited trends in yield. In these cases both total (from the data of the country- cereal under consideration) and genetic gains were assumed to be constant through the period considered in analyses and calculated in relative terms to allow for comparison (for details see Slafer and Andrade 1991). 124 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Slafer, G.A. & Peltonen-Sainio, P. Yield trends of cereals in high latitude countries Results and discussion Global trends Yields of the four cereals analysed exhibited some general similarities during the 1950–1998 period. The yield of each species was greatly increased during this period. Current yields (average 1990–1998) are far greater than those generally found in the mid 1950s (average 1950– 1959). The relative increase was assessed as the simple ratio between these decade averages and was 47% for oat, 83% for barley, 99% for rye, and 131% for wheat (Fig. 1). However, yield trends were not similar throughout the study period. Only for rye were yields from the sec- ond half of the century best described by a line- ar model (Fig. 1, Table 1). The other cereals showed yield trends represented by contrary bi-linear dynamics. In wheat there were only modest yield gains until as late as the1960s, in accordance with the finding of Slafer et al. (1996) that virtually no yield gains occurred during the first half of this century. This was followed by a more than 3-fold increase in rate of yield gains by the end of the century (Fig. 1, Table 1). Yields of barley and oat exhibited marked increases until around 1970, followed by a strong reduction in yield gains, to the point that they became virtually negligible in oat (Fig. 1, Table 1). Fig. 1. Trends in global averaged yields for wheat, barley, oat and rye from 1951 to 1998. Percentages indicate the ratio of 1990–1998 to 1951–1959 yields, expressed as percentage of the latter. 125 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 10 (2001): 121–131. In relative terms yield gains, due to both management and breeding improvements, aver- aged only 0.9 and 0.3% y–1 for barley and oat, respectively. These rates are considerably lower than the rate of population growth (1.75% for the 1961–1998 period), and show no signs of recovery in the long term (Fig. 1). The situation seems to be much better for wheat, as its yield has been increasing on a global basis at a faster rate than the population growth during recent decades (Table 1). However, a more detailed analysis of data from the 1990s provides evi- dence of a slowing rate of yield increase during the last decade (Slafer et al. 1996, Slafer and Satorre 1999). We only used linear and bi-linear models in this paper, but the data for wheat were actually better represented by a tri-linear model (with an adjusted r2 greater than that of the bi- linear model highlighted in Table 1). The tri-lin- ear model for 1950–1998 confirms the hypothe- ses from a previous analysis, using only the data for 1980–1995. This work showed that wheat yields might be asymptotically approaching a ceiling (Calderini and Slafer 1998). The tri-lin- ear model shows a significant second breakpoint at 1990 (± 2.5 y) in which the slope represent- ing yield gains switched from a markedly high value of 47.0 (± 1.8) kg ha–1 y–1 (for the 1964– 1990 period) to a much lower (and statistically non-significant) value of 22.7 (± 10.9) kg ha–1 y–1 (for 1990–1998). As yields of the three most important tem- perate cereals show signs that a ceiling might be reached, there is an urgent need to acquire a much more comprehensive understanding of yield generation. This will allow new strategies to be developed to further increase yield, at least under the prevailing socio-economic conditions, either through management, breeding or combi- nation of the two. A rate of yield increase is need- ed that at least keeps pace with population growth (Slafer and Satorre 1999). This is increas- ingly urgent (Mann 1999) if we are to meet the growing requirements for food of a population estimated to reach 8–10 billion people during the early decades of the 21st century (Rasmuson and Zetterström 1992, Evans 1998). One of these cereals, wheat, is (together with rice) the most important crop for feeding the more than 6 bil- lion that already inhabit the planet. Yield trends for cereals grown at high latitudes Although for some specific combinations of ce- real crops and countries other regressions may explained the yield trends better than the sim- Table 1. Outputs [coefficient of determination (r2), years when the change in slope occurred (breakpoint), and slopes (yield gains; kg ha–1 y–1)] of the regression models used to analyse yields (global averages) against years for the period 1951–1998 (inclusive) for each of the 4 major temperate cereals included in the study. Standard errors of the breakpoint (for the bi-linear model) and slopes are shown in parentheses. Cereal Model r2 Adj. r2 Breakpoint Yield gains 1st period 2nd period Wheat Linear 0.965 0.963 --------- 38.4 (± 1.09) --------- Bi-linear 0.982 0.980 1963 (± 1.56) 12.9 (± 5.63) 43.6 (± 1.27) Barley Linear 0.913 0.909 --------- 26.5 (± 1.21) --------- Bi-linear 0.942 0.936 1971 (± 2.90) 38.1 (± 3.49) 18.8 (± 2.39) Oat Linear 0.729 0.717 --------- 14.0 (± 1.26) --------- Bi-linear 0.872 0.860 1969 (± 1.87) 31.6 (± 3.56) 5.01 (± 1.88) Rye Linear 0.924 0.921 --------- 27.8 (± 1.17) --------- Bi-linear 0.924 0.917 1958 (± 15.4) 22.3 (± 17.7) 28.2 (± 1.57) 126 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Slafer, G.A. & Peltonen-Sainio, P. Yield trends of cereals in high latitude countries plest linear model (y = a + bx; see below), there was at least a very strong linear component in all cases, with adjusted coefficients of determi- nation ranging from 0.63 to 0.92 (with 57 de- grees of freedom, Table 2). All combinations of cereals and countries analysed, had consistently higher yields during the 1990s than at mid-century (Table 2). Yields for the 1950–1959 decade were quite similar to those of the 1940–1949 period, and conclusions from Table 2 are not based on particularly low yields that could presumably be expected as a consequence of World War II. In all cases yields during the 1990s were also far higher than dur- ing the 1950s. Despite the large differences in average yield among countries and cereal crops, reflecting profound differences in the growing conditions, no case was found in which yield had not increased substantially. In general, the small- est absolute increases were for Canada while the highest rates of yield gain were for Sweden (Ta- ble 2). This most likely reflected the differences in environmental conditions in which cereals are grown, although some other factors such as dif- ferences in specific requirements for grain qual- ity, could have played a role. In fact, there was a positive relationship between the rate of increase in yields in absolute terms and the average yield during the period analysed (Fig. 2, top panel). This trend, however, was not evident for indi- vidual cereals but was a general phenomenon. Barley, for example, did not exhibit a signifi- cant relationship and for the other cereals the relationship was strong but not linear (Fig. 2, top panel). By virtue of the general positive influ- ence of the environment on the trends in yield, Table 2. Average yields for the 1940–1949 and 1990–1998 periods and outputs of linear regressions (coefficient of determi- nation (r2), and slope (yield gains)] of yield (averages for each cereal in each country) against years for the period 1940– 1998 (inclusive, 57 degrees of freedom). Means are followed by standard errors (in parentheses). Cereal Country Yield 40/49 Yield 90/98 r2 Yield gains (1940–1998) kg ha–1 kg ha–1 y–1 % y–1 Barley Canada 1.30 (± 0.28) 2.96 (± 0.13) 0.850 33.7 (± 1.9) 1.65 Denmark 3.11 (± 0.31) 5.02 (± 0.71) 0.688 35.7 (± 3.2) 0.91 Finland 1.24 (± 0.19) 3.30 (± 0.43) 0.812 40.2 (± 2.6) 1.84 Norway 1.80 (± 0.49) 3.61 (± 0.43) 0.754 37.2 (± 2.8) 1.31 Sweden 1.92 (± 0.20) 4.01 (± 0.51) 0.860 42.8 (± 2.3) 1.45 Oat Canada 1.19 (± 0.25) 2.40 (± 0.16) 0.844 23.8 (± 1.4) 1.33 Denmark 2.85 (± 0.34) 4.96 (± 0.91) 0.632 37.4 (± 3.8) 1.01 Finland 1.28 (± 0.23) 3.30 (± 0.33) 0.823 39.4 (± 2.4) 1.78 Norway 1.76 (± 0.42) 3.82 (± 0.58) 0.734 44.3 (± 3.5) 1.53 Sweden 1.45 (± 0.14) 3.71 (± 0.69) 0.779 49.5 (± 3.5) 1.84 Rye Canada 0.79 (± 0.16) 1.93 (± 0.10) 0.792 23.1 (± 1.6) 1.70 Denmark 2.06 (± 0.27) 4.80 (± 0.55) 0.905 56.5 (± 2.4) 1.70 Finland 1.23 (± 0.14) 2.47 (± 0.49) 0.674 25.3 (± 2.3) 1.38 Norway 1.68 (± 0.47) 3.36 (± 0.70) 0.664 38.9 (± 3.7) 1.40 Sweden 1.71 (± 0.26) 4.66 (± 0.44) 0.897 59.3 (± 2.7) 1.97 Wheat Canada 1.13 (± 0.24) 2.23 (± 0.09) 0.638 19.9 (± 2.0) 1.22 Denmark 3.02 (± 0.58) 6.99 (± 0.42) 0.894 79.8 (± 3.6) 1.65 Finland 1.32 (± 0.17) 3.45 (± 0.52) 0.821 43.1 (± 2.7) 1.91 Norway 1.67 (± 0.42) 4.59 (± 0.57) 0.863 62.5 (± 3.3) 2.00 Sweden 1.83 (± 0.41) 5.84 (± 0.37) 0.916 85.3 (± 3.4) 2.25 127 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 10 (2001): 121–131. the relationship between yield gains and aver- age yield is lost if the former is expressed in rel- ative terms (Fig. 2, bottom panel). With the exception of Canada, wheat yield has increased at a faster rate than that of the oth- er cereals (Table 2), which is likely to have re- sulted from the greater effort directed toward improving yields of this cereal, not only in these countries but also at international centres and world-wide. The cereal species with the second- fastest rate of yield gains varied among coun- tries, being rye in Sweden and Norway, but oat and barley in Finland, and barley in Canada (Ta- ble 2). These trends were independent of yield gains in absolute or relative terms, and differ- ences in yield gains among countries and cere- als were not related to patterns of changes in growing areas (data not shown, available in the FAO yearbooks). In this paper we analysed the data with line- ar models, if they produced reasonable adjust- ments, because they facilitated comparison be- tween cases and calculation of genetic contribu- tions to yield gains (see below), while provid- ing an estimate of the average rates of yield in- crease during the whole period analysed. How- ever, this analysis prevented the identification of any indications of yields levelling off, which have been reported to be starting in most wheat growing areas of the world (Calderini and Slafer 1998). The global trends in cereal yields discussed above reflect a levelling off with the exception of rye. The trend to lower rates of yield increase than in previous decades was clear for oat and barley and is emerging in wheat. Whether this is also the case for particular agricultural regions cannot be answered if the analysis is confined to linear trends. In some of the cases analysed above, a clear departure from linearity was evi- denced by fitting the data to different models. This is, in part, due to the lack of clear trends of continued increasing cereal yields during the last decade. With the exception of oat in Denmark no clear positive yield trends were observed in any case for the last decade (Fig. 3). The coeffi- cients of determination for the positive yield trends for 1990–1998 ranged from 0.001 to 0.226 (and average yields of some cereal crops in some of these countries have tended to decline; e.g. rye in Finland; Fig. 3). This suggests that there has been a striking inability to improve yields through either genetic or management improve- ments in most combinations of cereal crops and countries analysed. All trends for the last dec- ade must be regarded cautiously as they are based on a relatively short period. If this incipient lev- Fig. 2. Relationship between the yield gains due to both genetic and management improvements and the averaged yields from 1940 to 1998 for wheat, barley, oat and rye (symbols inside bottom panel) in Canada, Denmark, Fin- land, Norway, and Sweden. Line in top panel was fitted by linear regression. 128 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Slafer, G.A. & Peltonen-Sainio, P. Yield trends of cereals in high latitude countries elling off is confirmed, however, in agreement with findings for wheat trends in most of grow- ing regions of the world (Calderini and Slafer, 1998), and if the prevailing socio-economic sit- uations are not strongly modified, new strate- gies for further increasing yields will be needed (Araus 1996, Richards 1996, Slafer et al. 1996, Mann 1999). Genetic contributions to yield gains The influence of breeding on the yield gains dis- cussed above (Table 2) may be sought by com- paring them (in relative terms) with the mag- nitude of the relative genetic gains in yield calculated from experiments in which cultivars released at different times have been grown together (Slafer and Andrade 1991). We found only a few of these studies in the literature. Those that do exist help to draw only a preliminary, and clearly incomplete, picture of the genetic improvement contributions to yield gains in cereal production made at the highest latitudes. They were two independent Canadian studies on barley, two independent evaluations of oat improvement effects carried out in Finland, and two estimates of breeding effects on wheat yield, Fig. 3. Yield trends for wheat, barley, oat and rye during the last decade in Canada (�), Denmark (�), Finland (�), Norway (�) and Sweden (�). 129 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 10 (2001): 121–131. one for Canada and the other for Sweden (Ta- ble 3). The cases of barley in Canada and oat in Fin- land clearly illustrate that estimates of genetic gains are strongly related to the conditions of the study. The estimated genetic gains for bar- ley in Canada were 13 or 25 kg ha–1 y–1, depend- ing on the studies that reported results from two regions clearly differing in weather, particular- ly water availability. The difference was not ap- parent if the relative genetic gain was calculat- ed (Table 3). The same was true for oat in Fin- land (Table 3). Genetic gains in wheat yields in Canada and Sweden appeared to have been sim- ilar (c. 25 kg ha–1 y–1, Table 3). However, as wheat growing environments in Sweden and in Canada are quite different (not only for their average national yields shown in Table 2, but also in their specific experimental conditions), the relative genetic gains were far greater in Cana- da than in Sweden (Table 3). Consequently, there seems not to be a simi- lar genetic contribution to yield gains across these conditions (cereal crops and high latitude countries). It ranges from c. 15% for oat in Fin- land and wheat in Sweden to c. 65% for wheat in Canada. Intermediate relative contributions were made by barley breeding in Canada (c. 25%). With the exception of wheat in Canada, these contributions are clearly smaller than those reported in the literature for wheat and barley in temperate areas such as USA, UK, Argentina, Australia and Mexico (where c. 30–60% of the on-farm yield gains were attributed to breeding; Silvey 1986, Slafer et al. 1994, Bell et al. 1995). It may be considered that either (i) the condi- tions under which breeding is done in these high- latitude countries results in a relatively low con- tribution in terms of improving yield potential (e.g. by focussing the breeding programs on ad- aptation and yield stability rather than on yield potential), or (ii) the efficiency of breeding it- self has been markedly lower than at other sites, indicating that new alternatives must be sought for future breeding in these conditions. This is particularly critical if future gains in yield are expected, due to economic and environmental reasons, to come more from breeding than from management improvement (Slafer et al. 1996). Conclusions Results from the present study confirmed the absence of any significant yield gain in wheat during approximately the last decade when ana- lysed globally. They also showed that the global yields for the two other major temperate cere- Table 3. Genetic gains in yield assessed in absolute (slopes of the relationships between yield and the year of introduction of the cultivars) and relative (genetic gains as a percentage of the averaged yield of the study) terms for some cereals in some northern agricultural systems. Only data for cultivars released after 1940 in each study were included in the analysis. Cereal Country Genetic gains in yield Source kg ha–1 y–1 % y–1 Barley Canada 25.0 0.59 Bulman et al. 1993 12.9 0.30 Jedel and Helm 1994 Oat Finland 13.4 0.29 Rekunen 1988 7.6 0.19 Peltonen-Sainio 1990 Wheat Canada 25.0 0.82 Hucl and Baker 1987 Sweden 23.1 0.33 Ledent and Stoy 1988* * Only the experiment conducted in the normal year was included in the analysis 130 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Slafer, G.A. & Peltonen-Sainio, P. Yield trends of cereals in high latitude countries als, barley and oat, have been increasing very slowly during a longer period. Only rye exhibit- ed a trend of increased yields equivalent to the increase in world population, even during the last decade. Trends in cereal yields for the five nations producing them, virtually exclusively at very high latitudes, showed that in general the better the environmental conditions the faster the rate of yield improvement. Comparing cereals grown in these high latitude environments, wheat showed the greatest yield gains in virtually all conditions, while the ranks of the other cereals varied among the countries. A more detailed analysis of the latter period (from 1990 to 1998) revealed that in virtually all these regions yields of most cereals tended to level off. All analyses were done when the data were available until 1998, but even using the latest data for 1999, the trends were unchanged. 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Verrattaessa eri viljalajien satoisuuden muutok- sia tutkitulla ajanjaksolla (1950–1998), havaittiin, että rukiin hehtaarisadot kasvoivat tasaisesti noin 28 kg vuodessa. Sen sijaan ohran (38 kg/ha/vuosi) ja kauran sadot (32 kg/ha/vuosi) kasvoivat ensin huo- mattavasti 1970-luvun paikkeille saakka, ja kasvu ta- saantui voimakkaasti viimeisen 30 vuoden aikana. Myös vehnän satoisuuden kehitystä kuvasi parhaiten bi-lineaarinen malli; vähäistä satoisuuden kasvua 1960-luvulle saakka seurasi satoisuuden kolminker- taistuminen 1990-luvulle mentäessä tosin tästä edel- leen kasvun tasaantuminen. Tutkittaessa satoisuuskehitystä niin pohjoismaissa kuin Kanadassakin voitiin havaita, että yleisesti ot- taen vehnän satoisuus kehittyi muita viljoja nopeam- min. Tämä kuvastanee vehnän arvostusta ja kehitys- työhön käytettyjen tutkimusvarojen runsautta. Toi- saalta kauran ja ohran, mutta vähissä määrin myös vehnän, osalta voidaan havaita satoisuuskehityksen hidastumista viime vuosikymmenenä. Alustavat tu- loksemme antoivat myös viitettä siitä, että kasvinja- lostuksen rooli tutkittujen viljalajien satoisuuden pa- rantamisessa oli pohjoisilla viljelyalueilla pienempi (poikkeuksena vehnän tuotanto Kanadassa) kuin mitä on raportoitu viljeltäessä vehnää ja ohraa alhaisem- milla leveysasteilla. 132 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Slafer, G.A. & Peltonen-Sainio, P. Yield trends of cereals in high latitude countries Title Introduction Material and methods Results and discussion Conclusions References SELOSTUS