The Effect of Irrigation System and Crop Load on the Vigour of Barlinka Table Grapes on a Sandy Soil, Hex River Valley *

Transcription

1 The ffet of Irrigation System and Crop Load on the Vigour of Barlinka Table Grapes on a Sandy Soil, Hex River Valley * D. Saayman and J.J.N. Lambrehts Department of Soil and Agriultural Water Siene, University of Stellenbosh Stellenbosh. South Afria Submitted for publiation: June 1995 Aepted for publiation: Otober 1995 Key words: Grapevine, Vitis vinifera, irrigation, rop load The effet of drip and miro-sprinkler irrigation, as well as rop load, on the vigour of Barlinka table grapes was studied in a field trial in the Hex River Valley over a 12-year period. Regulating soil water by means of tensiometers alone proved to be ineffetive in the ase of drippers, ausing redued vigour ompared to miro-sprinklers. This ould be retified by using a fixed 2-day shedule and evaporation data. Inreased bunh numbers per vine were found to have a pronouned and onsistently depressive effet on shoot mass. It was proposed that an approah be followed of deiding on an aeptable vigour and then alloating bunhes aordingly, using a formula developed from data obtained in this experiment. Signifiant seasonal variation in vigour aused by rop load and indiations of similar effets due to alulated water defiits, were obtained. Combining these two fators in a regression model, shoot mass data were realulated, revealing no onsistent effet systems on shoot growth. Mean seasonal water requirements were found to be 569 mm for miro-sprinklers and 411 mm for drippers. The more than 25% saving with drippers was mainly due to a redued wetted soil volume. Barlinka, a late-maturing, blak grape variety, brought to South Afria in 1910 (Perold, 1926), is still an important ultivar in the South Afria range of table grapes. Aording to U nifruo statistis for the 1994 season, Barlinka omprised about 19% of the national export rop of 18,75 million 5 kg artons and 34% of the Hex River Valley export of 9,53 million artons. Barlinka is mainly grown in the Hex River Valley (92% of the national Barlinka export), where table grape prodution is virtually a monoulture. This region has a total mean annual rainfall of 284 mm, of whih only 84 mm ours in summer, giving it a "very arid" aridity index of 640 mm, i.e. the differene between 0,4 standard USA lass A pan evaporation and seasonal rainfall (Smart & Dry, 1980). The sandy soils (less than 5% silt and lay) and boulderbeds, whih are ommon in the Hex River Valley, together with the aridity, make irrigation of table grapes essential. However, beause of relatively low spring temperatures, the Hex River Valley falls into Region III aording to the Winkler et al. (1974) lassifiation. Consequently it is a late region, with the late-ripening Barlinka maturing towards the end of Marh to early April, thus putting it in the high demand/low supply marketing period of the Northern Hemisphere. Additionally, during the maturation period there is a 16,1 C mean night/day temperature differene, ompared to 13,7 C in the oastal Paarl region, enhaning the olouration of Barlinka, a variety known for its tendeny towards inferior olouring (Saayman, 1988). Consequently more than 90% of all Barlinka vineyards in the RSA are situated in the Hex River Valley, making it unique in this respet. stimated water requirements of wine grapes in South Afria were proposed by Saayman & Van Zyl (1975). Van Zyl & Weber (1977) subsequently found that on high potential soils of the oastal region, judiiously applied supplementary irrigation of about 170 mm inreased the growth and yield of Chenin blan without loss of wine quality. Optimum response was obtained with an irrigation of about 90 mm at veraison. Lysimeter studies showed that the table grape ultivar Waltham Cross (Regina, Dattier de Beyrouth), grown in sandy soil (98% sand), was very sensitive to variations in soil matrix potential and that this should not be allowed to drop below -10 kpa (Van Rooyen, Weber & Levin, 1980a). Contrary to popular belief, it was found that a wet soil water regime (85% available soil water) during the budbreak to veraison (vegetative) phase, aused negative effets and that a wet regime during the ripening phase was not detrimental to grape quality. As a ompromise between negative (berry raking, redued sugar/aid ratio) and positive (inreased growth and yield) effets of a wet regime during the vegetative phase, a mean minimum soil moisture regime of 70% available soil water (-15 kpa) was proposed for this period. For the ripening phase an 85% soil water regime (-5 kpa) was proposed. valuating the data of Van Rooyen et al. (1980a), Terblanhe (1981) agreed with a regime of 70% plantavailable water during the vegetative phase but reommended a soil water regime of 50% plant available water during the maturation phase. He also proposed maximum soil matrix potentials of respetively -15 kpa and - kpa on light-textured soil for these two growth phases. Corresponding values for medium-textured soils were - kpa and -30 kpa and for heavy-textured soils -30 kpa and -40 kpa. Van Rooyen, Weber & Levin (l980b) onluded that the vine is not a drought-resistant plant as generally aepted but that it merely has a low water onsumption and extensive root system. The mean onsumptive water use over three seasons of lysimeter-grown Waltham Cross in the oastal region was 226 mm and rop fators varied from 0,1 to 0,7 over the season for a relatively wet * Pan ofa Ph. D. (Agri.) dissel1ation by the senior author to be submitted to the University of Stell en bosh and panl)' presented at the SASV Congress November 1994, Cape Town. Aknowledgements: Sinerest appreiation to Nietvoorij Institute for Vitiulture & Oenology forfunds and use of infrastruture and personnel and in PUl1iuiar to Ms A.. Theronfor invaluable tehnial assistane and data proessing. Also to the fanner FFTRI (presently Infruite) and Dr i.h. Terblanhefor establishing the experimental vineyard. S. Afr. J. nol. Viti., Vol. 16, No.2,

2 ffet of Irrigation and Crop Load on Barlinka 27 soil water regime. Van Zyl & Van Huyssteen (1980a) studied irrigated Chenin blan in the Robertson area and also ame to the onlusion that vines use water more sparingly than most other rops. They found a steady derease in rop fators from 0,48 three days after a midseason irrigation, to as low as 0,1 after 45 days. Beause of shading, large, slanting trellis-trained vines reated a ooler mirolimate and as a result did not neessarily use more water than smaller bush wines (Van Zyl & Van Huyssteen, 1980b). For Colombar wine grapes in the Robertson area, Van Zyl (1984) onluded that irrigation an be a powerful tool to ontrol unwanted growth and to improve quality, provided that the rooting volume is limited, as in the ase of drip irrigation. Shoot growth should be suppressed during the period budbreak to flowering, whereas the vine should be well watered during flowering and Phase I of berry growth (ell division and rapid enlargement). The growth of berries during Phase II (slower ell enlargement, pip development) was found not to be very sensitive to water stress, therefore shoot growth an again be urbed at this stage by redued irrigation. During Phase III (maturation), limited irrigation was found to inrease sugar onentration and to lower aidity, without dereasing yield. High rop loads inreased water onsumption and water stress started at - 64 kpa soil matrix potential (42% of plant available water), orresponding to a pre-dawn leaf water potential of -315 kpa (Van Zyl, 1987). Crop fators for vines in South Afria were regularly updated (Van Rooyen, 1980; Terblanhe, 1981; Van Zyl, 1981; Van Zyl & Fourie, 1988; Myburgh, 1992). During the early eighties, for a growing season from Otober to April, rop fators of 0,3 for the months Otober, Marh and April and 0,4 for the remaining months were onsidered to be appropriate for Barlinka in the Hex River Valley (J.L. van Zyl, personal ommuniation). Using these values, a onsumptive use of 634 mm was alulated, whih was onsiderably lower than the more than 1000 mm onsidered neessary by most produers. The rop fators later proposed by V ln Zyl & Fourie (1988) for Barlinka in the Hex River Valley were more luxurious and implied a onsumptive water use of 880 mm, whih was 0% higher than that of supplementary irrigated wine grapes in the oastal (Stellenbosh) region. During the early part of the season, these rop fators for Barlinka are very similar to those proposed for intensively irrigated, high-yielding' wine grapes in the interior regions, but are about 30% higher towards the end of the season (Van Zyl & Fourie, 1988). Traditionally, under-vine sprinklers were used in the Hex River Valley, but during the late sixties Israelideveloped trikle or drip irrigation beame popular, rapidly replaing the under-vine sprinklers. Problems with the water distribution ability of some soils and a general lak of managerial skills soon led to some disillusionment with drip irrigation and to the introdution of loally developed miro-sprinkler systems towards the end of the sixties. This system is very similar to the layout of drip irrigation, but requires a less elaborate filtering system and instead of drippers, small, inert, plasti sprinklers are srewed into the plasti laterals. At that time the merits or demerits of these two miro-irrigation systems were highly ontroversial, as they still are today. In an effort to resolve this ontroversy, the effets of drip and miro-sprinkler irrigation were studied in a fertilisation trial with Barlinka on a sandy soil in the Hex River Valley. This paper reports in the experiene gained with the two systems, the atual water applied, as well as the effet of rop load on the vigour of Barlinka. MATRIALS AND MTHODS This investigation was done in a Barlinka (lone 47) table grape vineyard, grafted on Ramsey, on the experimental farm of the Nietvoorbij Institute at De Dooms in the Hex River Valley. The soil was lassified as Fernwood 1110 (Soil Classifiation working Group, 1991), i.e. a pale, sandy topsoil, underlain by a leahed, TABL 1 Mean physial properties of the Fernwood* soil used for an irrigation/fertilisation experiment with BarlinkaJRamsey; Nietvoorbij xperimental Farm, De Dooms, Hex River Valley. Depth Sand (%) Silt Clay Bulk Water retention (VoL %) (mm) Coarse Medium Fine (%) (%) density ,0-0,5 0,5-0,25 0,25-0,05 (kgm') (kpa) (mm0) ,9 52,5 23,9 2,5 2, ,5 4,6 2,5 2,2 1, ,2 52,3 22,9 2,3 2, ,6 4,4 2,6 2,3 1,7 * Soil Classifiation Working Group, (1991). S. Afr. J. nol. Viti., Vol. 16, No.2, 1995

3 28 ffet of Irrigation and Crop Load Oil Barlinka white, sandy, eluvial horizon without lamellae. The physial harateristis of the soil are summarised in Table 1. Before planting the soil was delve ploughed to a depth of about 900 mm. After this operation, blak plasti sheets (50 miron) were suspended vertially in trenhes to a depth of 1 m from steel wires, whih were strung on the soil surfae midway between the future vine rows, in suh a way that the rows of all future plots were isolated from one another between rows. This eliminated the need for buffer rows, a onsiderable saving in surfae area. Beause of a shortage of plant material, only three repliates were planted during the winter of 1978, whilst the fourth was planted during 1979, all reeiving 45 kg N ha-l during their first seasons and all bunhes being removed. Vines were planted at a spaing of 3 xl,s m, developed to six, short permanent arms on a 2,4 m slanting trellis and renewal pruned to one 6-8 bud bearer on eah arm. ah autumn a ereal over rop was sowed between rows, killed with a glyphosate herbiide and flattened before budbreak. The experimental design onsisted of ten randomised fatorial treatments, arranged in four bloks, with two further treatments inorporated on a split plot basis (72 treatment ombinations). Irrigation system treatments were separated between rows by the vertial plasti sheets and in rows by two border vines plus a 1,5m border path. There were five data vines per plot and treatments were as follows: 1. Drip and miro-sprinkler irrigation. 2. Three levels of nitrogen (N) fertilisation, applied through the irrigation systems, at 35, 70 and 105 kg N hal. As from 1985/86 these levels were inreased to 60, 1 and 180 kg N hal. 3. Two seasonal patterns of N appliation, viz. 67% of total seasonal N applied fortnightly in even inrements over the period budbreak till veraison and the balane in the same way over a period of four weeks after harvest. The other pattern was 50% of total seasonal N during the budbreak to veraison preharvest period and 50% during the four weeks after harvest. 4. Three rop loads: 15,22 and 29 bunhes per vine. 5. On a split plot basis: A 'stok' P + K fertilisation during soil preparation to inrease the phosphourus (P) and potassium (K) onntrations of the soil to a level of 50 mg kg- l and 100 mg kg- l respetively, against a ontrol of no P + K stok fertilisation. Crop load was gradually inreased from the seond season, starting at 4, 6 and 9 bunhes per vine and inreasing it to 6, 8 and 10 bunhes per vine during 1981/82, to 8, II, 14 bunhes per vine during 1982/83, 11, 16 and 22 bunhes per vine during 1983/84 and finally to the design number of 15, 22 and 29 bunhes per vine in 1984/95. Drip-irrigated plots reeived water through 2 L ha-l emitters, plaed at 0,5 m intervals on the 3 m spaed laterals. Until the end of the season, water was applied in suh a way that tensiometer readings stayed below 15- kpa. From the 1982/83 season onward, estimated evapo-transpiration (T) water losses were replaed every seond day, using Amerian lass A-pan evaporation and rainfall (whih was measured daily on the farm), rop fators as shown in Table 2 and a water appliation effiieny of 95%. The alulated T was further empirially redued by 25% to ompensate for the fat that the drippers did not wet the entire soil volume. Crop fators for Barlinka as proposed by Van Zyl & Fourie (1988) were used from the 1990/91 season. Miro-sprinklers (32 L h-l) were plaed at 1,5 m intervals on the 3 m spaed laterals. Water appliation was initially also sheduled to keep tensiometer readings below 15- kpa. As from 1982/83, T water losses were replaed twie weekly, using the rop fators as for drip irrigation. For the miro-sprinklers a water appliation effiieny of 85% was used and a omplete wetting of the rooting volume was assumed. Soil water flutuations were monitored by two randomly plaed sets of merury manometer tensiometers for eah irrigation system. ah set onsisted of two tensiometers plaed on the vine row, 0,5 m from a vine and 125 mm from an emitter, at depths of 300 mm and 600 mm respetively. Tensiometer readings were reorded daily. During 1982 impeller type water meters were installed for eah irrigation system, allowing the amounts of water applied dming eah irrigation to be measured diretly. TABL 2 Crop fators used for Barlinka/Ramsey irrigation/fertilisation trial, Nietvoorbij xperimental Farm, De Dooms, Hex River Valley. Month Crop fator * ** May-Aug. (dormany) 0, 0, September (budburst) 0, 0, Otober 0,30 0,30 November (flowering) 0,40 0,36 Deember 0,40 0,46 January (veraison) 0,40 0,49 February 0,40 0,60 Marh (harvest) 0,30 0,60 April (postharvest) 0,30 0,39 " Adapted from fators proposed by Van Zyl (1981) for intensively ropped wine grape vineyards. ** Van Zyl & Fourie (1988). S. Afr. J. nol. Viti., Vol. 16, No.2, 1995

4 ffet of Irrigation and Crop Load on Barlinka 29 The mass of winter prunings was used as indiation of vigour, and yield was measured at harvest. Beause of an inreasing inidene of a phenomenon alled "Red Leaf', whih has a very negative effet on the growth and grape quality of Barlinka (Saayman & Lambrehts, 1993), only data from vines without a history of Red Leaf were eventually used. As from 1990/91 the experiment was drastially redued by seleting only one vine without a history of Red Leaf per plot and eliminating the 50%-50% seasonal N appliation pattern and 15 and 29 bunhes per vine rop load treatments. Were appliable, data were statially analysed using Genstat and Statgraphis software. RSULTS AND DISCUSSION The effet systems on vine vigour is shown in Fig. 1. It is evident that the approah of maintaining a soil matrix potential above -15 kpa to - kpa by using only tensiometers (until the end of the 1982 season) inhibited the vigour of drip-irrigated vines signifiantly ompared to miro-sprinkler irrigated vines. Tensiometer readings during the 1979/80 season, whih an be regarded as representative for this period, showed that for extended periods, lower soil matrix potentials developed under drippers than under miro-sprinklers (Fig. 2a, 2b). With drippers, matrix potentials lower than -30 kpa often ourred in the subsoil during the early season, in spite of a total of 25 appliations, ompared to the 18 irrigations with miro-sprinklers. It appeared that the long (for this sandy soil) and irregular intervals between irrigations (3-6 days during peak T periods) often resulted in the development of too low soil matrix potential peaks, espeially in the ase of drip-irrigated vines. This may have aused inadequate horizontal wetting and exessive deep perolation losses beause of the large amonts of water that had to be applied in order to lower tensiometer readings to about 10 kpa. By hanging to a fixed, two-day irrigation frequeny, replaing alulated T losses over the previous two days during eah irrigation, the vigour of drip-irrigated vines ould be brought to the level of miro-sprinklerirrigated vines within two seasons ( ; Fig. 1). Table 3 shows the quantities of water atually applied (sine 1982/83 when diret measurements ommened) as well as relevant limati parameters (T alulated from Amerian lass A-pan data and the rop fators presented in Table 2) and the deviations of atually applied water from those estimated to be needed by the vines. From this it would appear as if the vines were over-irrigated up till about 1985 and subjeted to various degrees of water defiits in subsequent seasons, dripirrigated vines generally being less deprived. 2,5 2,0 [] Drip Miro Full beari seasons Redued layout NS *** -Q).s; 1,5 0) - rn _ 1,0 o. Cf) 0,5 o Season FIGUR 1 The effet of drip and miro-sprinkler irrigation systems on the vigour of BarlinkalRamsey on sandy soil; Nietvoortbij xperimental Farm, De Dooms, Hex River Valley. NS: non-signifiant; ** p:':; 0,01; *** p:':; 0,0001). s. Afr. J. nol. Viti., Vol. 16, No.2, 1995

5 30 ffet o(lrrigation and Crop Load on Barlinka Ci! a e. 60 Cl 'g 50 "* (a) 30 em Depth t Budbreak Commenement t Bloom Drip Miro 21 mm 90 (b) em Depth 70 a. Ci! t e. 60.I:I.... Cl t ii.. 50 t t t Veraison Harvest * 0 w Commenement Bloom 21 mm Veraison..Harvest t Drip Miro FIGUR 2 Soil matrix potentials in drip and miro-sprinkler irrigated plots during 1979/80 at (a) 30 m and (b) 60 m depths: BarlinkaiRamsey irrigation/fertilisation experiment, Nietvoorbij xperimental farm, De Dooms, Hex River Valley. Fig. 3 shows tensiometer readings that were registered during the 1983/84 season, when both dripand miro-irrigated vines reeived lose to the estimated quantity of water needed (Table 3). Irrigation ommened towards the end of Otober 1983 when tensiometer readings reahed kpa. From that time, in the ase of drippers subsoil matrix potentials attained values lower than - kpa in the topsoil around veraison (Fig. 3a). The matrix potential of the soil irrigated by miro-sprinklers generally remained in the region of -10 kpa at both depths up till harvest (Fig. 3a, 3b). Tensiometer readings during the low-vigour 1987/88 season revealed a short period of possibly growninhibitory water stress in the subsoil of drip-irrigated plots during Deember (Fig. 4b). A water defiit of 52 mm was alulated for dippers during this season (Table 3). In the ase of miro-sprinklers, for whih a water defiit of 117 mm was alulated (Table 3), low, flutuating matrix potentials in the topsoil wete often reorded from bloom to veraison (Fig. 4a) and throughout the season in the subsoil (Fig. 4b). TABL 3 Water appled in a BarlinkalRamsey irrigation/fertilisation trial, relevant limati elements and deviations from alulated water appliation needs*; Nietvoorbij xperimental Farm, De Dooms, Hex River Valley. Water applied stimated T- xessldefiit ** Season Drip Miro (mm) T 0,75x Drip Miro (mm) (mm) (mm) (mm) (mm) (mm) 1982/ / / / / / / / / Mean Measured or alulated from beginning of Otober to end Marh. ** Assuming 95% and 85% appliation effifieny for Drip and Miro-sprinkler irrigation respetively and subtrating 25% of estimated T for a smaller wetted soil volume in the ase of Drip. S. Afr. J. nol. Viti., Vol. 16, No.2, 1995

6 ffet of Irrigation and Crop Load on Barlinka 31 Although tensiometer readings during the 1983/84 and 1987/88 seasons seem to onform to alulated water defiits, neither parameters appear to adequately explain the seasonal differenes in vigour enountered, as well as differenes in vigour apparently aused by the irrigation system (Fig 1). Apart from the possibility that tensiometer monitor sites were inadequate and/or that rop fators were inaurate, resulting in debatable water defiit alulations, other fators, apart from water stress, were obviously also involved in the observed variation in vigour. Fig. 5 shows, apart from an obvious seasonal effet, the well doumented but in this ase almost spetaularly onsistent negative effet of rop load per vine on shoot growth during the full bearing period. Regression analyses revealed a distint high-vigour group of seasons as well as a low-vigour group, both showing a highly signifiant depressive effet of rop load per vine and per season on vigour (Fig. 6). Although rop load was regulated and a mean of 22 bunhes per vine allotted sine 1984/85, the desired goal was not always obtained for all seasons. In aordane with redued shoot growth, the 1987/88 season had a high mean rop load. However, this was not the ase for the almost equally low-vigour 1988/89 season when rop load was below average (Table 4), thus also not satisfatorily explaining all seasonal variation in vigour. From Fig. 6 it is also evident that the rates of vigour redution were very similar for high- and low-vigour seasons, implying that this was relatively independent from other fators that also suppressed shoot growth. 90r----( DriP , em Depth Miro 56mm m 70 i;8 9;5mm "s,fmm 60Ht.s: '" Budbreak -g5q r j : {!!. Commenement Bloom 2,2 mill l Veraison t r 'est t.!ii i\... : '---(b) &------Drip ' Miro em Depth m 70 4;8"9;5 mm a/min 60 t. t '", : Budbreak 50 t $ 40 CD o CD 30 : {!!. Commenement Bloom 2,2"mm t Veraison Harvest 56mm.; r.;,#i "",,:-'Ii FIGUR 3 Soil matrix potentials in drip and miro-sprinkler irrigated plots during 1983/84 at (a) 30m and (b) 60m depths: BarlinkalRamsey irrigation/fertilisation experiment, Nietvoorbij xperimental Farm, De Dooms, Hex River Valley.., 70 a.. 60 g> 50 -* OO,----(a-) DriP---, em Depth 10 ;' q.c?.;:.':l:.-:rt iii1 i \. :... r!......, "1"e"mm"1"S;iii:5 lh... f:,' it Veraison Miro Harvest 90,----( Drip---, 80., 70 a.. 60 '" : 50 -* : {!! em Depth... n Miro 3mm 18mm15,67,5mm 39,8..e.nt... ttrn t}tir H H it... '1 1 " ihl4! Veraison + Harvest t rl/...,.:.. l... u. : : :. '. '...,.,. \, FIGUR 4 Soil matrix potentials in drip and miro-sprinkler irrigated plots during 1987/88 at (a) 30m and (b) 60m depths: Barlinka/Ramsey irrigation/fertilisation experiment, Nietvoorbij xperimental Farm, De Dooms, Hex River Valley. S. Afr. J. nol. Viti., Vol. 16, No.2, 1995

7 32 ffet of Irrigation and Crop Load on Barlinka Van der Merwe, Geldenhuys & Bates (1991) proposed 6 ') bunhes per m2 trellis area for Barlinka. Trellis area tyer vine in this trial was 3,6 m2, ditating 23 bunhes or a 15,2 kg rop load per vine aording to this guideline (assuming a mean bunh mass of 650 g). The general reommendations of Uys (1991) for moderate vigour were 4,0-6,5 bunhes (i.e. 2,6-4,2 kg) per m 2 trellis area or respetively a 12: 1 to 10: 1 bunh number: shoot mass ratio for a vigour of 0,50 and 0,55 shoots per m 2 trellis area. For this trial the mean shoot mass per vine was 1,822 kg, i.e. 0,506 kg m 2 trellis area. Using the 0,50 kg shoots per m 2 norm of Uys, the mean optimum rop load for this vineyard an be alulated as 18 bunhes or 11,7 kg rop load per vine. The data presented in Fig. 6 suggest that an aeptable vigour should first be deided on, from whih the rop load an then be alulated using the formula: Number of bunhes = (2,8 - desired shoot mass in kg vine!) 0,068 x bunh mass in kg A regression model was subsequently developed (R 2 = 0,30; n = 16) in whih the highly signifiant effet of rop load (Fig. 6) and indiations of negative effets of alulated water defiits (shoot mass = 1,94-0,003 x water defiit; R2 = 0,17, n = 16) were ombined, resulting in: Shoot mass = 3,669-0,138 (Crop mass) - 0,0017 (Water defiit) U sing this model, shoot mass data were "purified" and the effets system realulated for the period that the vines had a full rop load (1984/85 to 1991/92). This resulted in a muh redued seasonal variation over this period and only for the 1988/89 season in a signifiantly better performane of miro-sprinklers (Fig. 7). Over the last ten-year period of the trial, the mean gross quantity of water applied was 411 mm for drippers and 569 mm for miro-sprinklers. About 28% less water was -Cl).:; 2,5 2,0 0> U) 1,5 U) tl +-' (f) 15 Bunhes vine-1 22 Bunhes vine-1 29 Bunhes vine-1 *** 1,0 0, Season FIGUR 5 ffet of sed son and rop load on the vigour of Barlinka/Ramsey on sandy soil; Nietvoorbij xperimental Farm, De Dooms, He\ River Valley (NS: non-signifiant; * p 0,05;** p 0,01; *** p 0,0001). s. Afr. J. nol. Viti., Vol. 16, No.2, 1995

8 ffet of Irrigation and Crop Load on Barlinka 33 TABL 4 Mean number of bunhes alloated per vine: BarlinkaJRamsey irrigation/fertilization trail; Nietvoorbij xperimental Farm, De Dooms, Hex River Valley. Season 1984/ / / / / / / Mean Mean bunh number 22,23 b 22,90 d 22,13 b 23,97 d 21,73 b 21,49 b 24,05 d 17,41 a 21,99 soil volume but presumably also beause of a smaller surfae and appliation evaporation loss omponent, ompared to that of miro-sprinklers. Progressive drying of the subsoil during high summer (Jan-Feb.), as registered by the tensiometers, was often experiened in the ase of miro-sprinklers (data not shown) and water appliation sometimes had to be inreased by 25% for short periods. It was assumed that these temporary inreased water needs were the result of a too optimisti water appliation effiieny fator that was used for miro-sprinklers, resulting in a gradually inreasing water defiit in the subsoil, espeially during high summer when evaporation an be expeted to playa more prominent role in dereasing irrigation effiieny. 2,4 r I-=---:1-=S"""B-un-.,-he-s II = 22 Bunhes 2, := e!.l.!)c.i:1.ll to 90 = Full bearing seasons CD 2,0 r = 0,871 R"= 0,759. 1,8 :2 1,6 y = '2;:oa -0,0061;'...I.. (5 1,4 r.::.:ii6:i... o R"=0,745.s::; (J)... 1, , ,8 L-...L.-...l..l.- -L ---l Crop mass (kg vine 1) FlGUR6 Relationships between rop load and shoot mass of Barlinka/Ramsey on sandy soil: Nietvoorhij xperimental Farm, De Doorns, Hex River Valley. ffet of drip and miro-sprinkler irrigation systems on the shoot mass of BarlinkalRamsey, adjusted for rop load and estimated water defiit, on sandy soil; Nietvoorbij xperimental Farm, De Dooms, Hex River Valley. S. Afr. J. nol. Viti., Vol. 16, No.2, 1995

9 34 ffet of Irrigation and Crop Load on Barlinka CONCLUSIONS Based on the strong and onsistent depressive effet of rop load on shoot growth of Barlinka, it is proposed that bunh alloation should not simply be made on the basis of shoot mass per vine or per m 2, as is the general pratie for table grapes. In order to maintain a speifi vigour, the relationship established in this trial implies that in the ase of strong vigour, fever bunhes should be alloated than for low vigour. A logial approah would therefore be to deide on a vigour that is aeptable for a given situation and then to alulate the number of bunhes that will be in balane with that hosen vigour, using: number of bunhes vine! = (2,8 - desired shoot mass in kg vine! / (0,068 x bunh mass in kg). The dominant effet of rop load on the shoot mass of Barlinka, together with indiations of water defiits that aused onsiderable variation over seasons, ould largely be retified by adjusting shoot mass with a rop load + alulated water defiit regression model, revealing no onsistent effet system on vigour during the full bearing period of the trial. It would seem that there is little to hoose between the two systems in terms of effiieny on this medium-sand soil and assoiate limate, provided they are orretly managed. Although not measured in terms of atual onsumption by vines, it appears that a saving of more than 25% water is possible using drip irrigation. Inevitably the smaller wetted soil volume allows less tolerane in terms of duration between irrigations, as was demonstrated during the first two years of the trial when drip irrigation was not effetively timed by means of tensiometers. speially on sandy soil drip irrigation will require a higher level of managerial skills and ontrol measures than a full surfae wetting system. Although the adequay of water applied ould only be judged by means of tensiometers and shoot growth, the ontention is that the respetive mean gross quantitites of about 569 mm and 411 mm of water atually applied with miro-sprinklers and drippers, an serve as useful guides in irrigation planning in the Hex River Valley. Aording to tensiometer readings, it was seldom neessary to ommene with irrigation before November, a pratie not followed by most produers, who tend to start irrigating too early. This probably partly explains their laims of water needs of about 1000 mm or more per season. LITRATUR CITD MYBURGH. 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