Abstract
The interactive effects of mechanical canopy management on the optimum cropping level and Ravaz index of Pinot gris in a warm growing region were determined. The study examined two pruning methods, three shoot thinning treatments, and two leaf removal treatments arranged factorially in a randomized complete block design with four replications. The pruning methods were applied by either hand pruning to 23 nodes/m or mechanically hedging and retaining a 100 mm spur height. The shoot thinning treatments were applied mechanically at modified E-L stage 17 to retain 23 (low), 33 (medium), or 45 (high, not thinned) shoots/m of row. The leaf removal treatments removed leaves on the east side of the canopy in a 450 mm zone in the fruiting zone, 20 days postbloom with a mechanical deleafer or no leaf removal was done. The interaction of mechanical hedging and medium shoot thinning treatments reduced the contribution of noncount shoots to total shoots. Percent canopy gaps and photosynthetically active radiation increased, while canopy leaf layers decreased with the application of leaf removal regardless of pruning or shoot thinning regime in both years. Yield increased and berry and cluster weight decreased in both years with the application of mechanical hedging and medium shoot thinning compared to hand-pruned vines without any detrimental effects on fruit composition. To achieve the economic crop level threshold of 8.15 kg/m, a Ravaz index of 10.2 to 12.0 kg/kg was needed. This Ravaz index corresponded to a pruning weight of 0.82 to 0.92 kg/m. The study identified a mechanical hedging and shoot thinning method where a 100 mm spur height was retained during the dormant season and 35 shoots/m of row was retained at E-L stage 17 to optimize crop load without adversely affecting pruning weight or fruit composition in a warm climate. The identified method provided 79% labor operations cost savings compared to hand pruning alone.
More than 50% of the 3.6 million tons of winegrapes grown in California each year are from the San Joaquin Valley (SJV) and 60% of the Pinot gris acreage is planted in the region. In the SJV, Pinot gris garners a price of $471.52 per ton (CDFA 2012) with an average yield of 16.8 tons/ha, while total operating costs are $12,637 per ha, of which 80% is attributed to manual labor operations (Kurtural et al. 2012). The increasing labor operation costs and unavailability of labor are threatening the long-term economic viability of SJV winegrape vineyards.
Many of the wine grapevines planted in the SJV are grown on a two- or three-wire single-curtain, non-shoot-positioned trellis (Gladstone and Dokoozlian 2003), commonly referred to as the California sprawl. This trellis type, while not capital intensive to install, is often used improperly, resulting in excessive fruit-zone shading under vigorous conditions (Dokoozlian and Kliewer 1995, Terry and Kurtural 2011). With narrow profit margins, the majority of growers do not apply principles of canopy management because of cost. To remain profitable they retain too many nodes, resulting in out-of-balance vines with less than desirable fruit quality at the farm gate.
Mechanization of canopy and crop load (Ravaz index; RI) management in vineyards was shown to reduce labor costs by 44 to 80%, maintain yield and quality at the farm gate, and reduce the overhead associated with human resources (Kurtural et al. 2012, Morris 2007, Poni et al. 2004). Literature indicates consistent production in vineyards is achieved by balanced cropping (Howell 2001, Morris 2007, Terry and Kurtural 2011). Balanced cropping aims to achieve equilibrium between vegetative and reproductive growth of the grapevine, and thus ensures consistent vineyard production. Canopy management can achieve balanced cropping in vineyards and provides a set of decision-making steps to improve the canopy microclimate. A combination of mechanical hedging and retaining 23 shoots/m of row with mechanical shoot thinning and regulated deficit irrigation in the SJV was proven successful in maintaining a pruning weight of 0.9 kg/m and improving berry skin phenolics while maintaining a yield of 7.9 kg/m with a RI of 9.9 kg/kg (Terry and Kurtural 2011). The economic threshold for winegrapes in the SJV at the planting density reported by Terry and Kurtural (2011) would correspond to 8.15 kg/m according to Peacock et al. (2005). Literature reports a RI (ratio of fruit yield to pruning weight) of 5 to 10 kg yield per kg of pruning weight to be optimal while maintaining a pruning weight per meter of row up to 1.0 kg/m (Kliewer and Dokoozlian 2005), but these values are not specific to the SJV. Thus, there is insufficient information on how to achieve vine balance in warm climates regarding the economic crop level of Pinot gris.
While there have been numerous reports on adapting mechanical pruning practices, shoot thinning, leaf removal, and regulated deficit irrigation on red winegrape cultivars, there is limited knowledge on how best to maintain yield and crop load of Pinot gris without adversely affecting fruit composition. The objective of this study was to identify interactive effects of mechanical canopy management on crop-load optimization while saving labor operation costs without adversely affecting pruning weight and fruit composition of Pinot gris in a warm climate.
Materials and Methods
Plant materials and site. This study was conducted in 2010 and 2011 at a commercial vineyard planted with Pinot gris/1103P (UC Davis clone 03) grapevines at 2.1 m × 3.4 m (vine × row) spacing in north-south oriented rows. The research site was located in Kern County, California (lat. 35°00’36”N, long. 118°31’46”W; 238 m a.s.l.) and was planted in 2004 on premier sandy-loam soil taxonomically classified as a coarse-loamy, mixed, superactive, calcareous thermic Xeric Torriorthent and described as a deep, well-drained soil formed in alluvium from granite rocks (www.nrcs.usda.gov/). The vines were trained to a single-plane, bilateral cordon at 1.4 m with two foliage support wires at 1.70 m (total height of canopy above vineyard floor), and a 20 cm T-top, otherwise known as the California sprawl. The vineyard was drip-irrigated with pressure compensating emitters spaced at 1.1 m, delivering 2.1 L/hr per vine. The vines were supplied with 16 kg/ha nitrogen at modified Eichhorn-Lorenz (E-L) scale (Coombe 1995) stage 12 in each season. The growing degree days (GDD) accumulation at the research site was 2566 and 2561 (10°C base) between 1 Apr and 30 Oct in 2010 and 2011, respectively. Within the same interval, the site received 404 mm and 178 mm of precipitation in 2010 and 2011, respectively. Pests were managed using an integrated pest management program. All other cultural practices were standard for the area and conducted according to the University of California Cooperative Extension guidelines.
The study area is a resource-limited environment and irrigation was managed as follows. The root zone was irrigated beginning in the third week of February based on a crop coefficient (Kc) of 0.2 and 80% of the daily evapotranspiration (ETo) obtained from the California Irrigation Management Information System (CIMIS) station in Arvin, CA. Starting at fruit set, Kc was measured every week as described elsewhere (Williams 2001). Between fruit set and harvest, irrigation was triggered when the midday leaf water potential was below -1.2 MPa, and the amount of irrigation application was calculated based on the weekly Kc calculation and cumulative ETo obtained from the CIMIS station. Postharvest, the vineyard was irrigated based on the product of a Kc of 0.3 and the weekly cumulative ETo demand.
The experiment was a two (dormant pruning type) × three (shoot thinning) × two (leaf removal) factorial with a randomized complete block design with four replicated blocks. Each experimental unit consisted of 386 vines within each block. There were 48 vines sampled per experimental unit based on a grid pattern of every fourth vine, or 21 m apart.
Canopy management treatments.Dormant pruning. Two dormant pruning treatments were applied: hand pruning and mechanical hedging. Hand-pruned vines were spurpruned to retain 23 nodes per meter of row (control). The mechanical hedging treatment consisted of pruning the previous year’s canes to a 100 mm spur height with a 600 mm Sprawl-Pruner (model 63700; Oxbo International, Kingsburg, CA) to a node density of 55 nodes per meter of row.
Shoot thinning. Three shoot thinning treatments were applied mechanically at modified E-L stage 17 (Coombe 1995) with a rotary-paddle shoot thinner equipped with a rotary brush (model 62731; Oxbo International). Treatments were applied to a target of 23 count shoots/m (low) (borne from count positions >5 mm distal to the base of the bearing surface), 33 count shoots/m (medium), or 55 count shoots/m (high, not thinned).
Leaf removal. Two leaf removal treatments were applied in the fruiting zone. Leaves were either removed 20 days postbloom in a 45 cm zone in the fruiting zone above the cordon or were not removed. The surface layer of leaves were removed with a vacuum-type mechanical leaf puller (model 62084; Oxbo International) that consisted of a rotating drum that drew in air and leaves that were sheared from the vine with a sickle bar. Leaves pointing to the interior of the canopy were not removed.
Shoot counts, canopy microclimate, and leaf area assessment. Number of shoots per meter of row was measured two weeks after budburst as described elsewhere (Terry and Kurtural 2011). Leaf layer number and percent canopy gaps were measured using the four-point quadrat method. Percent photosynthetically active radiation (PAR) was measured with a ceptometer (PAR80; Decagon Devices, Pullman, WA) using methods as described elsewhere (Gladstone and Dokoozlian 2003). Leaf area per shoot and total leaf area of the vine were determined at 50% veraison from 48 vines per experimental unit. Four shoots from the east and west sides of the canopy were sampled at random and stored at 4°C at 98% humidity until measured. The number of leaves per axis was counted, and leaf area was measured with a leaf area meter (LI-3000; LI-COR, Lincoln, NE). The total leaf area per vine was then determined as described elsewhere (Keller et al. 2008).
Yield, fruit composition, RI assessment, and labor operation costs. Fruit yield and cluster numbers for each treatment were measured as berries reached 22 Brix by handharvesting 48 vines from each experimental unit. Average cluster weight was calculated by dividing fruit yield per vine by the number of clusters harvested. Fruit composition was measured starting at 1200 GDD until harvest at four and three dates in 2010 and 2011, respectively, and at harvest. Only data from harvest are presented. On each date, a random 100-berry sample was collected from 48 vines from each experimental unit, placed in polyethylene bags, stored on ice and analyzed within 24 hr. Before analysis, the 100-berry sample was weighed and average berry weight was calculated. The samples were then crushed by hand and the juice was placed in 100 mL beakers. A 5 mL portion of each sample was used to determine the percent total soluble solids measured as Brix, juice pH, and titratable acidity (TA) as described elsewhere (Terry and Kurtural 2011). The Ravaz index (RI) was calculated by dividing the yield per vine by the dormant pruning weight per vine in the following winter and expressed as kg/ kg. Leaf area to fruit ratio was calculated by dividing the total leaf area of each vine by the yield per vine and expressed as m2/kg. Canopy management labor operations costs and the benefit for applied treatments were calculated as reported in Kurtural et al. (2012), based on previous methods (Bates and Morris 2009, Peacock et al. 2005).
Statistical analysis. All data were tested for normality using Shapiro–Wilk’s test, and no transformations were deemed necessary. Type III test of fixed effects was used to detect treatment variance appropriate for a three-way factorial at α = 0.05 using the MIXED procedure of SAS (ver. 9.2; SAS Institute, Cary, NC). The random variables used were replication and year. The effect of year was deemed significant at p > F 0.0001 after convergence criteria was met. Therefore data from each year is presented separately. Tukey’s honestly significant difference test (HSD) was used as the post-hoc adjustment method to detect treatment mean separation using LSMEANS options of the MIXED procedure in SAS at α = 0.05. Regression analysis was performed using simple linear regression and nonlinear regression using the REG and NLIN procedure in SAS where appropriate.
Results
Canopy architecture and microclimate. Pruning method and shoot thinning treatments interacted to affect count, noncount, and total shoots per meter row in both years (Table 1). Mechanically hedged treatments with shoot thinning were more effective in controlling the count, noncount, and total shoots per meter of row in both years. In 2010, shoot thinning and leaf removal treatments interacted to affect percent PAR transmittance through the fruiting zone (Table 2). In 2011, pruning method and leaf removal affected percent PA R transmittance. In both years of the study, leaf removal was the only treatment to decrease the canopy leaf layers. In 2010, shoot thinning and leaf removal treatments interacted to affect the percent canopy gaps. In 2011, only the leaf removal treatment affected percent canopy gaps, which increased by 55% compared to no leaf removal. In 2010, the shoot thinning and leaf removal treatments interacted to affect the total leaf area; medium shoot-thinned vines with leaf removal treatments had 21% less total leaf area compared to high shoot-thinned vines with no leaf removal, regardless of pruning method. In 2011, total leaf area for mechanically hedged vines was 74% greater compared to hand-pruned treatments; there was no effect of shoot thinning or leaf removal methods on total leaf area
Interactive effects of pruning method and shoot thinning on average number of count, noncount, and total shoots per meter of row on Pinot gris/1103P in 2010 and 2011.
Effects of pruning method, shoot thinning, and leaf removal on canopy microclimate post-shoot thinning and leaf removal of Pinot gris/1103P in 2010 and 2011.
Yield components. In 2010, pruning method and shoot thinning treatments interacted to affect berry weight (Table 3). A combination of mechanical hedging with high shoot thinning resulted in the smallest berry size. Mechanical hedging combined with low or medium shoot thinning resulted in the greatest berry size. However, in 2011, mechanical hedging decreased berry weight by 18% compared to hand pruning. There was no effect of leaf removal on berry size in either year.
Effects of pruning method, shoot thinning, and leaf removal on yield components of Pinot gris/1103P in 2010 and 2011.
Pruning method and shoot thinning interacted to affect the number of clusters harvested in 2010; the number of clusters per vine was the highest when mechanical hedging was combined with the high shoot thinning treatment. In 2011, mechanical hedging increased clusters per vine by 31% compared to hand pruning. High shoot thinning also increased the number of clusters by 23% compared to low and medium shoot thinning treatments in 2011.
There was no interaction of factors tested on cluster weight or yield in 2010. Compared to hand pruning, mechanical hedging reduced cluster weight by 14% and increased yield by 47%. High shoot thinning increased yield by 25% and 15% compared to low and medium shoot thinning, respectively.
In 2011, pruning method and shoot thinning interacted to affect cluster weight and yield. Mechanical hedging combined with high shoot thinning resulted in the smallest clusters. Hand-pruned vines with high shoot thinning had the greatest yield. A combination of mechanical hedging with low or medium shoot thinning reduced the yield by 8% and 5%, respectively, compared to the hand-pruned vines with high shoot thinning.
Fruit composition. The time to reach harvest target of 22 Brix was affected by pruning method and shoot thinning in 2010 (Table 4), but the same trend was not evident in 2011. In both years of the study, the TA of hand-pruned vines was higher than mechanically hedged vines. Increasing shoot per meter of row decreased TA of Pinot gris at harvest. Leaf removal treatments did not affect Brix or pH in either year of the study. Leaf removal increased TA in 2011, at harvest.
Effects of pruning method, shoot thinning, and leaf removal on fruit composition of Pinot gris grapevines at harvest on 17 August 2010 and 2011.
Yield efficiency. In 2010, pruning method and shoot thinning interacted to affect pruning weight and RI (Table 5). Hand pruning with medium shoot thinning had the highest pruning weight, while mechanical hedging with high shoot thinning had the lowest. In 2011, pruning weight was affected by pruning method, shoot thinning, and leaf removal. Mechanical hedging reduced pruning weight by 27% compared to hand pruning. Low and medium shoot-thinned vines had 16% and 9% less pruning weight than high shoot-thinned vines. Leaf removal decreased pruning weight by 13% compared to vines with no leaf removal.
Effects of pruning method, shoot thinning, and leaf removal on pruning weight, Ravaz index, and leaf area to fruit ratio of Pinot gris/1103P in 2010 and 2011.
The RI in 2010 was highest for mechanically hedged vines with high shoot thinning and lowest for hand-pruned vines with low shoot thinning. The RI of mechanically hedged vines with high shoot thinning was 77% greater than hand-pruned vines with low shoot thinning. Shoot thinning was effective in decreasing the RI of mechanically hedged vines, where 46% and 27% reduction was seen with low and medium shoot thinning compared to mechanically hedged vines with high shoot thinning. In 2011, the RI of hand-pruned vines with low and medium shoot thinning was 50% and 42% lower than mechanically hedged vines with medium shoot thinning.
In 2010, shoot density and leaf removal interacted to affect the leaf area to fruit ratio. The low shoot thinning and leaf removal treatment combination had lower leaf area to fruit ratio when compared high shoot thinning with no leaf removal. In 2011, pruning method and shoot thinning affected the leaf area to fruit ratio. Leaf area to fruit ratio of mechanically hedged vines was 49% greater compared to hand-pruned vines. Medium shoot-thinned vines had 37% less leaf area to fruit ratio than low or high shoot-thinned vines. There was no effect of leaf removal in leaf area to fruit ratio of Pinot gris in 2011.
Crop-load management and labor operation costs and benefits. The economic threshold of 8.15 kg/m corresponded to a RI of 10.3 kg/kg in 2010 (r2 = 0.8974, p < 0.0001) and 12.0 kg/kg in 2011 (r2 = 0.4012, p < 0.0001). The RI range identified (10.3 to 12.0 kg/kg) resulted in a pruning weight range of 0.82 kg/m to 0.92 kg/m in 2010 (r2 = 0.7480, p < 0.0001) and 2 0 11 ( r 2 = 0.7667, p < 0.0001). There was a nonlinear relationship between Brix and RI during the study, in which the range identified resulted in a Brix of 21.8 to 22.4 (r2 = 0.6274, p < 0.0002). There was a nonlinear relationship between shoot numbers and RI during the study. To achieve a RI of 10.3 to 12.0 kg/kg, after dormant pruning, 35 shoots/m need to be retained for Pinot gris (r2 = 0.6982, p < 0.0001). This shoot density was only achieved with mechanically hedging the previous year’s canes to a 100 mm spur height and then applying the medium shoot thinning treatment. The identified treatment cost $217.10/ha to apply and provided a 79% savings over hand pruning alone while generating positive cash flow (Table 6). Conversely, hand pruning alone did not meet yield threshold or generate a positive income under SJV conditions. Mechanical hedging alone generated the greatest gross income, lowest cost of application, and the greatest positive income per hectare.
Average labor operation cost for canopy management treatments and net benefit for canopy management of Pinot gris grapevines in 2010 and 2011.
Discussion
Canopy architecture and microclimate. In concert with previous reports, attempts to balance canopy parameters and yield with pruning severity or methods failed, since relying on node numbers per meter of row set by dormant pruning was a rough regulator of final crop level (Bernizzoni et al. 2011). Since the study area rarely receives any untoward weather events such as late spring frosts, the shoot numbers would not have been controlled otherwise in the hand-pruned or mechanically hedged treatments. Mechanical shoot thinning was applied rapidly and precisely to generate canopies of consistent and desired architecture as reported elsewhere (Morris 2007, Terry and Kurtural 2011). A similar leaf layer number was achieved in both seasons of the study across the shoot thinning treatments. This displayed a significant vegetative compensation occurrence with shoot thinning, in agreement with previous studies since shoot thinning might have led to a strong physiological impact in terms of supply-demand function at the canopy level (Bernizzoni et al. 2011).
The optimum amount of light penetration within the fruiting zone varies with growing region and cultivar (Dokoozlian and Kliewer 1995). Mechanical leaf removal is beneficial for growers in cool climates because they are able to apply this practice on a conditional basis depending on the cultivar growth response for a particular season to advance sugar accumulation (Bledsoe et al. 1988). Leaf removal was found to reduce canopy leaf layers from five to four in 2010 and from three to two in 2011, and these results are consistent with previous studies recommending three to four leaf layers in a warm climate (Main and Morris 2004, Terry and Kurtural 2011). In both years of the study, leaf removal increased percent canopy gaps as well as light penetration into the fruiting zone, as reported by Howell et al. (1994). This resulted in optimal bud development for the next season’s growth, as reported by Sanchez and Dokoozlian (2005). However, leaf removal was not effective in influencing total leaf area in either year of the study, as reported by Main and Morris (2004). The lack of a physiological response to late season leaf removal can again be attributed to vegetative compensation by the grapevine by the generation of a larger leaf area by the remaining leaves (Bernizzoni et al. 2011).
Yield components. Specific yield targets can be met using a combination of cultural methods presented in this study. Pruning method had the greatest effect on yield, and shoot thinning provided a higher level of crop control related to cluster number and yield. The cluster number results here were consistent with previous research demonstrating the proportional relationship between high node count and cluster number in mechanically hedged vines (Bates 2008, De Toda and Sancha 1999, Poni et al. 2004). The greater number of clusters retained per vine in mechanically hedged treatments compared to hand-pruned treatments was due to the increased number of nodes and shoots distributed on the mechanically managed canopies. Cluster number was further controlled with mechanical shoot thinning, as exemplified by the positive relationship between canopy density and clusters per vine (Terry and Kurtural 2011).
Previous research related to mechanical canopy management exemplified a direct relationship between shoot numbers and yield components (Bernizzoni et al. 2011, De Toda and Sancha 1999, Poni et al. 2004). Results from this study produced a similar relationship among pruning methods and shoot thinning, clusters per vine, and consequently yield. As the pruning severity and shoot density increased, the number of clusters per vine and yield were higher. The higher cluster number per vine demonstrates a corresponding increase in yield when comparing low to high shoot density for both hand-pruned and mechanically hedged treatments.
The fuctuation of yield from season to season in spurpruned treatments is a problem for many growers. The yield for hand-pruned vines in this study nearly doubled from 2010 to 2011. This increase in yield of hand-pruned vines was attributed to less canopy density in the previous year (Gladstone and Dokoozlian 2005) resulting from mechanical shoot thinning that led to increased bud fruitfulness the following season. Due to the lack of source limitation demonstrated by the consistently higher yielding mechanically hedged vines (Bates 2008, Terry and Kurtural 2011), the hand-pruned vines with minimal canopy management did not optimize the production potential of the vineyard. The ability to achieve similar yields in consecutive years with a mechanical canopy management approach is consistent with previous vineyard mechanization studies conducted in the SJV (Terry and Kurtural 2011).
The effect of canopy management on yield components such as berry size is a widely accepted concept, and previous studies have shown that altering berry size through cultural practices and irrigation is possible (Naor et al. 2002, Terry and Kurtural 2011). In both years of the study, the combination of mechanical hedging and high shoot density resulted in the lowest berry weight compared to other thinning treatments (Zabadal et al. 2002, Terry and Kurtural 2011). During the second year of the study, there was a reduction in berry weight as pruning severity and shoot density increased, a reduction attributed to increased shoot density and consequently crop level and load (Bates 2008, Reynolds et al. 1994, 1996, Zabadal et al. 2002). Cluster weight was consistently reduced in mechanically hedged vines compared to spur-pruned vines as reported by Terry and Kurtural (2011). These differences in berry and cluster weight due to canopy management demonstrate the capability to control cluster architecture with means other than irrigation and illustrate self-regulation in the grapevine depending on crop load (Naor et al. 2002). The greatest compensation effect observed was related to cluster weight. The reduction in cluster weight in both 2010 and 2011 of mechanically hedged compared to hand-pruned vines was direct compensation for the increased number of clusters retained on mechanically hedged vines. These results illustrate the vine’s ability to limit and manage the allocation of assimilates to developing clusters without exhausting all available resources necessary for other developing parts of the plant. Although this study was conducted throughout the course of two growing seasons, this compensation effect on cluster size due to increased cluster number was also demonstrated in an 11-year study, providing support that this trend is likely to continue (De Toda and Sancha 1999).
Fruit composition. Although the rates of ripening during veraison for Brix, juice pH, and TA did vary among treatment methods, the differences in these fruit composition values did not affect production standards at harvest and are consistent with similar studies in the SJV (Morris 2007, Terry and Kurtural 2011). Generally, vines with less shoots per meter of row, regardless of pruning method, reached the harvest target of 22 Brix value faster than vines with higher shoot density, as reported by previous studies (Bernizzoni et al. 2011, Morris 2007, Terry and Kurtural 2011, Zabadal et al. 2002). As reported by Howell et al. (1994), removal of leaves in the fruiting zone when berries were at pea-size stage had minimal impact on vine physiology or fruit composition, as such was our aim.
Crop-load management, yield effciency labor operation costs. The amount of pruning weight per meter of row attained in this trial was within previously defined optimal levels necessary to achieve vine balance between 0.5 and 1.0 kg/m (Kliewer and Dokoozlian 2005, Terry and Kurtural 2011). The optimum range to achieve the desired RI in this study resulted in 0.82 to 0.92 of pruning weight kg/m. Generally, the pruning weight per meter of row for hand-pruned treatments, regardless of shoot numbers per meter of row, was slightly above the desired range, while mechanically hedged treatments (greater shoot numbers per m of row) resulted in less pruning weight. This general decrease in pruning weight of mechanically hedged vines with increasing shoot numbers is consistent with similar studies on Grenache (De Toda and Sancha 1999) and Syrah (Terry and Kurtural 2011). In contrast to the response of Pinot gris to varying shoot density resulting with different pruning weights, Morris (2007) reported similar pruning weights with mechanically hedged and shoot thinned vines to replicate hand-pruned shoot densities, a possible of effect of vegetative compensation response by the grapevine to shoot thinning (Bernizzoni et al. 2011).
The current economic yield threshold at the planting density reported in our trial was 8.15 kg/m (Peacock et al. 2005). This yield resulted in a RI of at least 10.3 kg/kg in order to achieve the required crop level for the region and meet the 22 Brix fruit composition target. The RI range to achieve vine balance for a single-plane canopy in a warm climate was 5 to 10 kg/kg (Kliewer and Dokoozlian 2005, Terry and Kurtural 2011). Conversely, this study recommends a RI range of 10.3 to 12.0 kg/kg to maintain greater yields due to lower profit margins without any detrimental effects on fruit composition at the farm gate. The lack of RI variation between years in the mechanically managed canopies during this study was consistent with long-term vineyard mechanization studies (Bates 2008, De Toda and Sancha 1999, Morris 2007, Poni et al. 2004).
The RI range of 10.3 to 12.0 kg/kg correlated with mechanically hedged and medium shoot thinned treatments corresponding to 35 shoots/m. In contrast to previous studies, the optimal shoot density in this study is higher than previous recommendations of 15 shoots/m for cool-climate viticulture (Smart 1988) and higher than 23 shoots/m for red cultivars for warm climate (Terry and Kurtural 2011). Mechanical shoot thinning to 35 shoots/m provided an adequate crop level to meet production demands for the region, while maintaining a well-distributed canopy for subsequent seasons with a pruning weight of 0.82 to 0.92 kg/m. Leaf removal treatments had no consistent effect on RI or pruning weight.
The leaf area to fruit ratio achieved during the initial year of the study was higher than the 0.8 to 1.2 m2/kg recommended by Kliewer and Dokoozlian (2005). In the second year of the trial, hand-pruned vines saw a decline of leaf area to fruit ratio when number of shoots per m increased. In that year, only the hand-pruned treatment with low shoot thinning approached recommended leaf area to fruit ratio values. This outcome was likely a growth compensation response to shoot thinning as larger leaf blades were trying to fill the sparse canopy and gain photosynthesis (Bernizzoni et al. 2011). Conversely, during the second year of the trial, mechanically hedged vines resulted in an overabundance of leaf area with low and high shoot thinning. To achieve the leaf area fruit ratio recommended for warm climate viticulture (Kliewer and Dokoozlian 2005), a combination of mechanical hedging and medium shoot thinning was needed.
The mechanical canopy management approach identified in this study also provided significant labor operations savings and income benefit for the grower. The inclusion of mechanical shoot thinning adds a $61.50/ha cost for the grower. However, even with this additional management practice, the method offers a 79% savings over hand pruning alone and provides consistent production with positive cash flow. Although the mechanical hedging treatment without any shoot thinning or leaf removal generated the highest net income per hectare, it is not recommended. The reduction in pruning weight per meter of row with mechanical hedging alone was greater than the initial income benefit it provided, indicating a greater-than-needed devigoration of the grapevines for consistent production. Furthermore, mechanical hedging alone exceeded the recommended leaf area to fruit ratio for the SJV.
Conclusion
The results here show the effects of vineyard mechanization on canopy microclimate and its consequential effects on yield components and efficiency, while providing labor cost savings. In warm regions where the economic pressure for production per hectare is high and with a declining labor pool, the methods identified in this trial present an opportunity for growers to optimize crop load without adversely affecting fruit composition and pruning weight and to save on labor operation costs.
Mechanical shoot thinning provided a higher level of control of canopy microclimate, promoting vine balance and production. Based on these results, growers would be able to alter the yield to an economic yield threshold of 8.15 kg/m while achieving a Ravaz index of 10.3 to 12.0 kg/kg without adversely affecting pruning weight or fruit composition in a warm climate.
We conclude that mechanical hedging the previous year’s canes to a 100 mm spur height and shoot thinning to a density of 35 shoots/m resulted in balanced vines with consistent production. Leaf removal, while beneficial in reducing canopy leaf layers and improving percent gaps within the canopy, did not demonstrate any beneficial effects on yield components or fruit composition. Leaf removal would not be recommended based on these results and it also adds an additional cost for the vineyard owner. The precise role of mechanical canopy and crop-load manipulation will be better understood once their collective influence on the wine chemistry of Pinot gris is further examined.
Acknowledgments
The authors acknowledge the American Vineyard Foundation, the Bronco Wine Company Research Chair Trust funds, and Oxbo International Corp. for partial funding during the execution of this project.
The authors acknowledge the technical assistance of Geoffrey Dervishian, Lydia F. Wessner, Semih Tangolar, and Serpil Tangolar.
- Received March 2012.
- Revision received June 2012.
- Accepted July 2012.
- Published online December 1969
- ©2013 by the American Society for Enology and Viticulture
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