Base temperature and thermal time requirements for germination and emergence of temperate pasture species

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1 New Zealand Journal of Agricultural Research ISSN: (Print) (Online) Journal homepage: Base temperature and thermal time requirements for germination and emergence of temperate pasture species D. J. Moot, W. R. Scott, A. M. Roy & A. C. Nicholls To cite this article: D. J. Moot, W. R. Scott, A. M. Roy & A. C. Nicholls (2) Base temperature and thermal time requirements for germination and emergence of temperate pasture species, New Zealand Journal of Agricultural Research, 43:1, 1525, DOI: 1.18/ To link to this article: Published online: 17 Mar 21. Submit your article to this journal Article views: 238 View related articles Citing articles: 4 View citing articles Full Terms & Conditions of access and use can be found at

2 New Zealand Journal of Agricultural Research, 2, Vol. 43: //43115 $7./ The Royal Society of New Zealand 2 15 Base temperature and thermal time requirements for germination and emergence of temperate pasture species D. J. MOOT W. R. SCOTT A. M. ROY A. C. NICHOLLS Field Service Centre P.O. Box 84 Lincoln University Canterbury, New Zealand Abstract The base temperature (T b ) and thermal time (Tt) requirements for germination and emergence of temperate herbage species were defined using a linear model of development rate against temperature. A T b of 4 C was found for all species. The Tt requirement for germination was lower for legume than grass species and generally lowest for small seeded species. The poor relationship (R 2 =.42) between Tt requirements for germination and 5% field emergence indicated that germination results could not be extrapolated to predict the rate of field emergence. The Tt for 5% field emergence was highest for the smallseeded grass species cocksfoot (22 Cd) and timothy (2 Cd). The times (days and Tt) for emergence of white clover and perennial ryegrass were similar from five autumn sowing dates but 57 days after sowing on 21 March 16 ryegrass seedlings were 12 times heavier. The implication of these results for establishment of pasture mixtures is discussed together with the need for accurate measurement of soil temperatures to assist the prediction of field emergence. Keywords base temperatures; chicory; cocksfoot; emergence; germination; Italian ryegrass; lucerne; pasture establishment; perennial ryegrass; phalaris; prairie grass; red clover; subterranean clover; tall fescue; temperatures; thermal time; timothy; white clover A25 Received 3 May 19; accepted 3 December 19 INTRODUCTION Laboratory conditions have frequently been used to determine optimum temperatures for germination of herbage species. For example, Charlton et al. (16) reported the number of days to germination of eight temperate grass species across a 5 to 3 C range. Similarly, Hampton et al. (17) reported the germination responses of five temperate legumes between 5 and 2 C. In most cases cultivar differences were minimal and the germination rate declined as temperature moved away from the speciesspecific optimum. A common approach for expressing the relationship between temperature and plant development is to calculate the thermal time (Tt) (also known as heat units or growing degreedays ( Cd)) required between two development stages (Arnold & Monteith 14). In its simplest form Tt is calculated as the mean temperature minus the base (Tb) or threshold temperature below which no development takes place. Angus et al. (11) used this approach to define T b and Tt required for field emergence of a range of tropical and temperate crops. Both T b and Tt were species dependent and unaffected by cultivar. For temperate species T b was below 4 C compared with 1 to 14 C for tropical crops. Similar analyses for perennial warmseason forage grasses indicated a T b of 1 ± 1 C (Hsu & Nelson 16) compared with 4.7 C for a temperate legume Lotus corniculatus (Hur & Nelson 15). Despite the routine use of the thermal time approach for describing phenological development of annual crops, T b and Tt requirements for common New Zealand pasture species have not been determined. A recent book published about temperate legumes makes no mention of the concept (Frame et al. 19), preferring to report trie germination results from Hampton et al. (17). Laboratory studies commonly use constant temperature regimes. However, the soil temperatures that sown seeds experience for germination and emergence fluctuate on a daily and seasonal basis. Thus, the initial aim of this study was to define and compare the T b and Tt requirements for

3 16 New Zealand Journal of Agricultural Research, 2, Vol. 43 germination and emergence of temperate herbage species, using data from laboratory, field, and published experiments. The second aim was to use these results to assess the implications for pasture establishment of species mixtures. In all experiments and the subsequent discussions, moisture was assumed to be nonlimiting and temperature was considered the main factor influencing germination and emergence. MATERIALS AND METHODS Germination Experiment 1 For Experiment 1, three replicates of 1 seeds per species (white clover Trifolium repens cv. Grasslands Huia, perennial ryegrass Lolium perenne cv. Embassy, chicory Chichorium intybus cv. Grasslands Puna) were placed on wetted blotting paper in petri dishes and germinated in unlit cabinets at either a constant 5, 1, 15, 2, 25, or 3 C, or a 12 h fluctuating cycle of 1 and 2 C. White clover seeds were scarified but no other preconditioning treatments were used. Normal seedlings (International Seed Testing Association 16) were counted and removed twice daily, for a duration of 36 days, to facilitate counting of germinated seedlings. Distilled water was added as required to ensure moisture was nonlimiting for germination. Germination results were used to determine the final germination % and the number of days to reach 75% of the final germination, as determined by a cumulative percentage fit of germination against days. Reanalysis of published data: Experiments 2 and 3 In Experiment 2, the data published by Charlton et al. (16, table 3) were reanalysed to determine the T t, and Tt requirements for 75% germination for nine grass species germinated at 5, 5/1, 1, 15, 2,25, and 3 C. Similarly, in Experiment 3, results published by Hampton et al. (17, table 3) were reanalysed for five legume species germinated at 5, 5/1, 1, 15, or 2 C. In both experiments the fluctuating temperature was 16 hours at 5 C and 8 hours at 1 C giving the mean temperature of 6.7 C used in the analyses. Field emergence In a randomised complete block field experiment, 8 herbage species were sown in four replicates on five dates in 16 (8 March, 21 March, 4 April, 24 April, 8 May). Plots were 1.3 x 5. m drilled with an Oyjoord cone seeder at 15mm row spacing and a target depth of 2 mm. Prior to sowing, the experimental area had contained greenfeed oats which were sprayed with glyphosate and ploughed and the area then worked into a fine, firm seedbed using conventional cultivation implements and techniques. Before the second and subsequent sowings, glyphosate was sprayed to kill the weeds and the dead herbage was removed from the experimental area. Directly after sowing, soil temperature probes (KTY8311) supplied by Cooltronic Services, Christchurch, New Zealand were placed in seven plots, at a depth of 2 or 5 mm. Seven probes were used to monitor soil temperatures. Three were placed in bareground plots that were to be sown on the first sowing date. One probe was randomly placed in each of the other four sowing date treatments. Probes were successively removed from the plots immediately after emergence of the sown species and were then reallocated to a bareground treatment for a later sowing date. Air temperature was recorded from a single probe placed in a Stevenson screen in the middle of the experimental area. Temperatures were recorded every minute and integrated every hour to determine daily mean temperatures. After sowing, the middle metre of the centre two drill rows was marked. The number of seedlings which emerged within the area was counted daily until seedlings ceased to emerge. Emergence was considered to have occurred when the coleoptile of monocotyledonous species was visible and when both cotyledons of dicotyledonous species were visible. The mean seedling weight of individual plants from each species was measured on 17 May from a single destructive harvest of 1 plants taken from each plot of the 21 March and 4 April sowing dates. Data analysis Data for each species were plotted as the reciprocal of the duration (in days) to 75% germination or 5% field emergence against the mean temperature. The inverse of duration (I/days) represents the development rate. A linear relationship indicates that the thermal time procedure is appropriate for the data. Least squares regression analysis was used for the positive linear portion of the line whereby: b o + b)x. (1)

4 Moot et al. Thermal time requirements of temperate pasture species The regression coefficients can then be related to T b and Tt (Angus et al. 11) as: T b =b /b (2) and Tt = 1/bi. (3) Where the germination rate deviated from a linear model at the low or high temperatures, data were excluded from the analysis on the basis that these were outside the species optimal thermal range (Angus et al. 11). Regression was also used to analyse the relationship of air temperature against 2mm and 5mm soil temperatures and 5mm soil temperature against 2mm soil temperature. Maximum standard errors are reported for each measured variable. Multiple range tests were not performed due to the increased risk of type II errors when the number of values within the range increases (Sokal & Rohlf 11). This approach was consistent with that followed by Angus et al. (11) who reported standard errors for each species but did not use mean separation tests and Mohamed et al. ( 18) who reported the mean standard error and temperature range for the determination of cardinal temperatures. Data for the dry weight analysis of seedlings sampled in Experiment 4 were log transformed, due to unequal variances, before calculation of the standard error. RESULTS Germination Experiment 1 The final germination percentage of each species differed across temperatures. For white clover, germination was between 89 and % from 5 to 3 C but for ryegrass germination decreased from % at 15 C to about 7% at 25 and 3 C (Fig. 1). At 5 and 1 C the final germination for ryegrass was above 8% but there was a delay in the start of germination. For chicory, the maximum germination was about 8% from 15 to 3 C but this declined to 6% at 1 C. At 5 C chicory did not begin to germinate until after 12 days and it took a further 15 days to reach 75% of the maximum, which was only 3%. The number of days to 75% of the maximum germination was highest at 5 C for all species but decreased exponentially as temperature increased (Fig. 2A). This led to a linear increase in the rate of germination (Fig. 2B) that enabled T b and Tt to be r Time (days) Fig. 1 Cumulative germination of A, 'Huia' white clover; B, 'Embassy' perennial ryegrass; and C, 'Grasslands Puna' chicory, at 5 ( ), 1 (O), 15 ( ), 1/ 2 ( ), 2 (A), 25 (A), and 3(+) C. Error bars are the maximum standard error for the final germination percentage. estimated for white clover ( C, 57 Cd), chicory (3.7 C, 57 Cd), and perennial ryegrass (1.2 C, 84 Cd). Reanalysis ofcharlton et al. (16): Experiment 2 A linear increase in germination rate with temperature was found for all grass species. Within species there was consistency in T b and Tt values so cultivar results were averaged to enable standard errors to be calculated. Representative data for the relationship between development rate and mean temperature are presented in Fig. 3. The temperature range used for thermal time calculations differed among species (Table 1). The 3 C treatment was excluded from all analyses and 5 C was also excluded for phalaris and tall fescue because it was outside the linear range (Fig. 3). For all species T b was <3.5 C

5 18 New Zealand Journal of Agricultural Research, 2, Vol. 43 ou O B * O A.35.3 lo.25 S S o.2o B B B O I Temperature ( C) Fig. 3 Germination rate of 'Kahu' timothy ( ), 'Mt. Barker' subterranean clover (D) 'Maru' phalaris ( ), and 'Roa' tall fescue (O), at different temperatures Temperature ( C) Fig. 2 Number of days to 75% of final germination (A) and germination rate (B) of ' Huia' white clover ( ), 'Embassy' perennial ryegrass ( ), and 'Grasslands Puna' chicory (O) at different temperatures. and Tt ranged from 7 (timothy) to 24 Cd for Apanui cocksfoot. For Wana cocksfoot, analysis using 5 to 2 C resulted in a Tt of 319 Cd with T b = C compared with 28 Cd when only 5, 6.7, and 1 C were included (Table 1). Results for cocksfoot were not averaged due to cultivar differences reported in the original germination data (Charlton et al. 16). Reanalysis of Hampton etal. (17): Experiment 3 The 52 C range was used for thermal time calculations and cultivar results were averaged across all species with the exception of subterranean clover (Table 2; Fig. 3). In all cases T b was <1. C and the Tt requirement of the legume species was lower than for the grasses. Lucerne and white clover had a Tt requirement of about 4 Cd compared with 66 Cd for red clover and 45 or 78 Cd for the subterranean clover cultivars (Table 2). Field emergence Rainfall occurred at regular intervals throughout the sowing period which provided adequate moisture for germination and emergence. During the experimental period the mean soil temperature decreased from 16 to 4 C (Fig. 4). Across this period the range in the number of days to 5% emergence increased among species from about 9 days for the first three sowing dates to 29 days for the 8 May sowing (Table 3). The reciprocal of this was used to determine the rate of field emergence for each species. There were differences in the mean air and soil temperatures during the emergence period and this influenced the estimated values of T b and Tt (Table 4). Regression analysis of 5mm soil temperature against 2mm soil temperature showed a strong relationship (R 2 =.), so only results from the 2mm probes are presented. There was comparatively poor agreement (R 2 =.82) between soil and screen temperatures, particularly for early sowing dates when the soil temperatures were up to 4 C wanner than screen temperatures (Fig. 4). Based on soil temperatures, T b ranged from to 3.6 C and Tt from 12 Cd to 22 Cd (Table 4). Based on air temperatures, T b was about 1.9 C higher and Tt was 5 Cd lower. The ranking of the species was similar from both analyses with a high T b for tall fescue and slow rate of emergence for timothy and cocksfoot (Fig. 5). In most cases, species with low seed weights also had a low Tt requirement for germination, a high requirement for emergence, and produced the

6 Moot et al. Thermal time requirements of temperate pasture species Table 1 Estimates of the base temperature (Tb) and the thermal time (Tt) requirement for germination of herbage grass species (reanalysis of Charlton et al. 16). Potential temperature range was 5, 6.7, 1, 15, 2, 25, and 3 C, giving n = l when all temperatures were included in the regression. Embassy Perennial ryegrass results are from the germination Experiment 1, in this paper. Cocksfoot data were not averaged because of cultivar differences and variability in the original data for Wana cocksfoot. Species Perennial ryegrass Italian ryegrass Hybrid ryegrass Westerworlds Perennial ryegrass Cocksfoot Phalaris Prairie grass Tall fescue Timothy Cultivar Nui Ruanui Paroa Moata Manawa Ariki Tarna Mean s.e. Embassy Apanui Wana Wana Maru Matua Roa Kahu T b ( C) Tt ( Cd) R 2 (%) Excluded temperatures ( C) 3 3 2, ,3 25,3 15,2,25,3 5,3 3 5, Table 2 Estimates of the base temperature (Tb) and the thermal time (Tt) requirement for germination of dicotyledonous herbage species (reanalysis of Hampton et al. 17). Potential temperature range was 5,6.7,1, 15,and2 C, giving n = 5 when all temperatures were included in the regression. *Huia clover and chicory results are from the germination Experiment 1 in this paper. Subterranean clover data were not averaged because of cultivar differences. Species White clover Red clover Subterranean clover Lucerne Chicory Cultivar Huia Pitau Tahora Kopu Mean s.e. *Huia Hamua Turoa Pawera Mean s.e. Mt. Barker Tallarook Woogenellup Oranga Wairau Rere Mean s.e. *Puna Tb ( C) Tt ( Cd) R 2 (%) Excluded temperatures ( C) _

7 2 New Zealand Journal of Agricultural Research, 2, Vol n O 15 a 1 I. 4 3 Soil temperature ( C) Fig.5 Field emergence rate of'kahu'timothy ( ), 'Mt. Barker' subterranean clover ( ) 'Wana' cocksfoot ( ), and 'Nui' perennial ryegrass (O) at different 2mm mean soil temperatures. "S CE 1 _JJ 9/3 28/3 17/4 7/5 27/5 16/6 6/7 26/7 Day/month (16) Fig. 4 Mean 7day screen ( ) and 2mm soil ( ) temperatures, and sowing dates(j) (A) and daily rainfall (B) for the field experiment at Lincoln University in 19%. lightest seedlings 43 and 57 days after sowing (DAS). For example, white clover (Table 2) germinated faster than perennial ryegrass (Table 1) and seedlings emerged at about the same time (Tables 3 and 4), but 57 DAS on the 21 March ryegrass seedlings were 12 times heavier than white clover seedlings (Table 5). To enable direct comparison of the Tt requirements for germination and emergence within and between species, additional regression analyses of rate against temperature were performed with the intercept, and therefore T^ set at zero. Species means highlight the rapid germination of legume species compared with grasses and the slow emergence of timothy and cocksfoot (Table 6). Over all species there was a poor relationship (R 2 =.42) between Tt requirements for germination and time to emergence. Table 3 Number of days to 5% field emergence of species sown on different dates in 16. Maximum s.e. is the maximum standard error calculated for any of the species at each sowing date. Species Italian ryegrass Subterranean clover Perennial ryegrass Red clover Tall fescue Timothy Cocksfoot White clover Cultivar 8 Mar Moata 1 Mt. Barker 8 Nui 11 Pawera 8 Advance 13 Kahu 17 Wana 17 Huia 1 Maximum s.e..3 sowing date 21 Mar 4 Apr Apr May

8 Moot et al. Thermal time requirements of temperate pasture species 21 DISCUSSION Germination The germination rates of the temperate herbage species tested increased linearly with temperature up to an optimum. The decline in germination rate at low and high temperatures (Tables 1 and 2; Fig. 2, 3) was consistent with a proposed sigmoid or logistic response of development to temperature (Angus et al. 11). However, for subterranean clover and cocksfoot there was a distinct decline in germination rate at supraoptimal temperatures (Fig. 3). Mohamed et al. (18) reported a similar linear decline from the optimum to a threshold temperature below which germination ceased. The temperature ranges used in the present study were too narrow to define accurately whether an asymptotic or negative linear response was appropriate for describing the supraoptimal response of most species. Therefore, the discussion focuses on utilising the positive linear portion of the response to determine thermal time requirements for each species. The linear response is indicative that temperature was the major factor limiting germination and consequently that the thermal time concept was appropriate for all species (Hur& Nelson 15). Results from Experiment 1 highlighted the effect of temperature on the final germination % and pattern of germination for different species. The advantage of the thermal time approach is that it summarises the array of individual temperature responses (Fig. 1) into repeatable coefficients that can be used for all temperatures in the linear range (Fig. 2). Temperatures between those defining the lower and upper limits of the linear range (detailed in Tables 1 and 2) can be expected to accelerate developmental progress through a stage at a Table 4 Estimates of base temperature (Tb) and the thennal time (Tt) requirement for 5% field emergence of herbage species calculated from 2min soil and screen (air) temperatures. Maximum s.e. is the highest standard error recorded for all species. Species Cultivar White clover Huia Red clover Pawera Subterranean clover Mt. Barker Perennial ryegrass Nui Italian ryegrass Moata Cocksfoot Wana Tall fescue Advance Timothy Kahu Maximum s.e. T b ( C) Soil Tt red) R T b ( C) Air Tt ( Cd) R Table 5 Seed fresh weight (SW) and shoot dry weight per plant of herbage species 43 days after sowing (DAS) for the 4 April 16 sowing and 57 DAS for the 21 March 16 sowing. Species Italian ryegrass Subterranean clover Perennial ryegrass Red clover Tall fescue Timothy Cocksfoot White clover Cultivar Moata Mt. Barker Nui Pawera Advance Kahu Wana Huia s.e. SW (mg) Shoot dry weight (mg) 43 DAS DAS

9 22 New Zealand Journal of Agricultural Research, 2, Vol. 43 predictable rate that differs among species. For example, at 8 C white clover is predicted to have germinated in 5 days compared with 13 days for chicory, which is predicted to accumulate only 4.3 Cd each day due to its T b of 3.7 C (Table 1). For all species in the germination experiments, values of Tb were between and 4 C. This range was consistent with those reported for other temperateadapted annual (Angus et al. 11) and perennial (Hur & Nelson 15) species. The influence of a Tb at the high end of this range will be greatest at low temperatures leading to slow germination rates and reduced final germination percentages, as shown for chicory (Fig. 2). Germination results for timothy (T b = 3.5 C) showed a similar pattern with a reduction (P <.5) in final germination from about 8% at 15 C to 28% at 5 C (Charlton et al. 16). The base temperatures determined in Experiment 1 for white clover and ryegrass were within the range of values calculated among cultivars of the same species from Experiments 2 and 3 (Tables 1 and 2). Some variation in values can be expected from different definitions of "germination". In Experiment 1, att of 57 Cd was estimated for white clover compared with 41 Cd for the mean of four cultivars in Experiment 3. The difference probably resulted from a delay in defining germination in Experiment 1 caused by the seed coat remaining around the cotyledons. In the field this would be removed by the soil as cotyledons emerge. This was Table 6 Summary of thermal time to 75% germination and 5%. emergence of herbage species used in Experiments 1, 2, and 3 in this paper. Analysis assumes a base temperature of C and mean temperatures within the linear response range for each species. Maximum s.e. is the highest standard error recorded for all species. Species Germination ( Cd) Perennial ryegrass 9 Italian ryegrass 9 Tall fescue 15 Timothy 85 Wana Cocksfoot 21 Phalaris 14 Prairie grass 18 White clover 45 Red clover 65 Lucerne 4 Subterranean clover 45 Chicory 75 Maximum s.e. 5.9 Emergence ( Cd) not a problem for grass species where emergence of the coleoptile and seminal roots are easily defined. In most cases, the Tt requirement for germination ranged from about 4 to 16 Cd and was lower for legumes than grasses. The exception was cocksfoot, with Apanui having a Tt of 24 Cd. According to the raw data for Wana (Charlton et al. 16), germination took 22 days at 1 C (or Tt = 22 Cd, when T b = C), 51 days at 5 C (Tt = 255 Cd), and 17 days at 15 C (Tt = 255 Cd). It follows that the mean value (~24 Cd) is justified on the basis of cultivar consistency, as seen for other species, rather than the calculated value of 319 Cd (Table 1). Reasons for the high Tt calculation from the current data are unclear. Additional germination tests are required for Wana to confirm which value of Tt is most appropriate. Field emergence Support for retesting germination of Wana is also implied from field emergence results. Across sowing dates the Tt requirement calculated for Wana (22 Cd), using soil temperatures, was less than that determined for germination. As expected, when soil temperatures were used the Tt for emergence for all other species was greater than those calculated for germination (Table 4). In this study, results from the soil measurements were assumed to be a more accurate predictor of thermal time requirements than those from screen temperatures. This assumption was based firstly on the greater agreement between germination and soil measurements in estimates of Tb, which were not expected to vary between germination and emergence. Secondly, the soil probes are located closer to the seedling apical growing points, the major site of temperature perception by the plant, and such placement has been recommended for accurate prediction of crop development rates (Jamieson et al. 15). This is important during the spring and autumn in temperate environments when the difference in soil and air temperatures is greatest, particularly if the soil is dry (Ross et al. 15), or there are ground frosts. As the mean soil temperature decreased the number of days to emergence at least doubled for all species (Table 3). In contrast, the Tt required for emergence was constant across sowing dates for each species. Thus, provided the soil temperature is measured and within the linear response range, emergence from 2 mm can be predicted from these thermal time results. The small range of

10 Moot et al. Thermal time requirements of temperate pasture species 23 soil temperatures experienced in this study limits extrapolation to a wide range of environments, but the germination results indicate that a consistent Tt requirement can be expected for soil temperatures up to 2 C. Unfortunately, the poor correlation of Tt requirements between germination and emergence results prevents extrapolation of laboratory results into the field. Specifically, the lighter seeded species, which tended to germinate rapidly, did not emerge any faster than other species (Fig. 5). Thus, results from this study do not substantiate the conclusion of Hampton et al. (17) that because herbage legumes germinate more rapidly than grasses "the time of sowing for seed mixtures will depend on the grass component." Implications for pasture establishment. The inference from the present study is that the time of sowing and composition of pasture seed mixtures should depend on the thermal time requirements of different species to emergence rather than extrapolation of germination results. In isolation, emergence results can also be misleading if they take no account of seedling competition and plant mortality. For example, despite rapid germination, emergence of white clover occurred at about the same time as perennial ryegrass (Table 3). By 43 DAS (sown 4 April 16), the white clover plants averaged 3.6 mg compared with 24.8 mg for perennial ryegrass (Table 5). This advantage was accentuated by 5 7 DAS (sown 21 March 16) and was greater for annual than perennial ryegrass. The rapid seedling growth rate of ryegrass confers a substantial competitive advantage over white clover in the establishment phase when competition for resources, particularly light (Brougham 1953), is intense. The rapid germination by white clover did not translate into rapid emergence or vigorous seedling growth and supports the description by Brougham (1954) of white clover as a slow establishing species. The difference in seedling growth was probably caused by the heavier ryegrass seed weight. Of the legumes tested, only the heaviest seeded subterranean clover showed emergence and seedling growth comparable with the ryegrass species (Table 5). The Tt, and Tt requirements for emergence (Table 4) provide an explanation for the traditional classification of timothy and cocksfoot as slow establishing pasture species (Sangakkara & Roberts 11; Stevens et al. 13). Both species had high Tt requirements and low seed weights and produced the smallest seedlings. Tall fescue had the highest Tb but its heavier seed weight contributed heavier seedlings than for timothy and cocksfoot, although this was still lower than for ryegrass (Table 5) despite the similarity in Tt requirements (Table 4). Brock et al. (12) attributed the difference in seedling vigour of tall fescue and ryegrass to differences in their ability to mobilise and utilise endosperm reserves. Cullen (1958) recognised the implications of sowing mixtures of species with different establishment rates and concluded that low (41 kg ha') rates of ryegrass were needed if slow establishing species (cocksfoot, timothy, and white clover) were to survive. Similar conclusions have been drawn by other authors (e.g., Culleton et al. 16; Praat et al. 16). However, the impact of soil temperature and thermal time requirements for different species in relation to these recommendations was not addressed. It is the combination of thermal time requirements for emergence and seedling growth rates that should form the basis for recommendations on the time of sowing and compatibility of species within a mixture. It is likely that, as soil temperatures decline in the autumn (Fig. 4), the low thermal time requirements and rapid seedling growth of ryegrass will favour its establishment at the expense of other species. It follows that the components of pasture mixtures will change if sowing is delayed in the autumn, for example, by a lack of moisture or late crop harvest. Species that emerge slowly and have low seedling growth at low temperatures should be replaced by faster establishing species. Thus, as autumn temperatures decline, it seems prudent to sow Italian ryegrass rather than permanent pasture mixtures that contain slow emerging species such as cocksfoot, timothy, tall fescue (Table 6), or species like white clover which have low seedling growth (Table 5). To provide recommendations for pasture establishment it is important to define the soil temperature regimes that will enable successful establishment of different herbage species. This would then enable experimental results to be extrapolated to other sites and seasons compared with current literature that reports the time of year (e.g., HamiltonManns et al. 15) which has little relevance outside the region of study. Soil temperatures at.1 m are readily available from meteorological stations and have been reported in relation to emergence studies (Charlton et al. 16). However, that recorded temperature is about 8 mm from the apical growing point, depending on the

11 24 New Zealand Journal of Agricultural Research, 2, Vol. 43 sowing depth, and bears little relation to the actual temperature the plant is experiencing. In the present study, consistent results were obtained from analyses using 2 or 5mm soil temperatures compared with analyses using air temperatures. CONCLUSIONS The thermal time concept can be applied to germination and emergence results to define base temperatures and thermal time requirements for herbage species. A linear model was appropriate for all species and showed T b ranged from to 4 C across several temperate grass and legume species. Germination results could not be extrapolated to predict Tt requirements for field emergence. This was because species, like white clover, that germinated quickly, did not always have correspondingly rapid field emergence. Species with high Ti, and Tt requirements for emergence (cocksfoot and timothy) had relatively light seed weights and slow seedling growth. Therefore, the thermal time requirements provided an explanation for the previously recorded observation that these are slow emerging species recommended for spring or summer establishment. Germination results for chicory indicated that a similar classification might apply although this species was not tested in the field. Predicting emergence results and decisions for establishing multispecies pastures should be based on 2 or 5mm soil temperatures in preference to screen or 1mm soil temperature readings. ACKNOWLEDGMENTS We thank B. E. Smith for technical assistance with monitoring temperatures for the field experiment and M. J. Robertson for his constructive review of the manuscript. REFERENCES Angus, J. F.; Cunningham, R. B.; Moncur, M. W.; Mackenzie, D. H. 11: Phasic development in field crops. I. Thermal response in the seedling phase. Field Crops Research 3: Arnold, S. M.; Monteith, J. L. 14: Plant development and mean temperature in a Teesdale habitat. Journal of Ecology 62: Brock, J. L.; Anderson, L. B.; Lancashire, J. A. 12: 'Grasslands Roa' tall fescue: seedling growth and establishment. New Zealand Journal of Experimental Agriculture 1: Brougham, R. W. 1953: Seeding rates of short rotation ryegrass. Proceedings of the New Zealand Grassland Association 14: Brougham, R. W. 1954: Pasture establishment studies. 1. New Zealand Journal of Science and Technology A36: Charlton, J. F. L.; Hampton, J. C.; Scott, D. J. 16: Temperature effects on the germination of New Zealand herbage grasses. Proceedings of the New Zealand Grassland Association 47: Cullen, N. A. 1958: Pasture establishment studies at lnvermay Research Station. Proceedings of the New Zealand Grassland Association 2: Culleton, N.; Murphy, W. E.; O'K.eeffe, W. F. 16: The role of mixtures and seeding rates in ryegrass productivity. Irish Journal of Agricultural Research 25: 236. Frame, J.; Charlton, J. F. L.; Laidlaw, A. S. 19: Temperate forage legumes. Wallingford, CAB International. HamiltonManns, M.; Ritchie, W. R.; Baker, C. J.; Kemp. P. D. 15: Effects of sowing date on ryegrass and tall fescue establishment by direct drilling. Proceedings of the Agronomy Society of New Zealand 23: Hampton, J. G.; Charlton, J. F. L.; Bell, D. D.; Scott, D. J. 17: Temperature effects on the germination of herbage legumes in New Zealand. Proceedings of the New Zealand Grassland Association 48: Hsu, F. H.; Nelson, C. J. 16: Planting date effects on seedling development of perennial warmseason forage grasses. I. Field emergence. Agronomy Journal 78: Hur, S. N.; Nelson, C. J. 15: Temperature effects on germination of birdsfoot trefoil and seombadi. Agronomy Journal 77: International Seed Testing Association 16: Seed Science and Technology 4: 118. Jamieson, P. D.; Brooking, I. R.; Porter, J. R.; Wilson, D. W. 15: Prediction of leaf appearance in wheat: A question of temperature. Field Crops Research 41: Mohamed, H. A.; Clark, J. A.; Ong, C. K. 18: Genotypic differences in the temperature responses of tropical crops. Journal of Experimental Botany 39: Praat, J. P.; Ritchie, W. R.; Baker, C. J.; Hodgson, J. 16: Target populations for directdrilled ryegrass and tall fescue. Proceedings of the New Zealand Grassland Association 55: Ross, P. J.; Williams, J.; McCown, R. L. 15: Soil temperature and the energy balance of vegetative mulch in the semiarid tropics. II Dynamic analysis of the total energy balance. Australian Journal of Soil Research 23:

12 Moot et al. Thermal time requirements of temperate pasture species 25 Sangakkara, R.; Roberts, E. 11 : Competition between 'Nui' ryegrass, 'Matua' prairie grass, and 'Apanui' cocksfoot during establishment and early growth. Proceedings of the New Zealand Grassland Association 43: Sokal, R. R.; Rohlf, F. J. 11: Biometry. New York, W.H. Freeman and Company. Stevens, D. R.; Turner, J. D.; Baxter, G. S.; Casey, M. J.; Miller, K. B. 13: Quality pasture for animal performance. Proceedings of the New Zealand Grassland Association 55: 4144.