Effects of growth stage on chemical composition, organic matter digestibility, gross energy and fatty acid content of safflower (Carthamus tinctorius L.)

P G Peiretti

Institute of Sciences of Food Production, National Research Council, Grugliasco, Turin, Italy
piergiorgio.peiretti@ispa.cnr.it

Abstract

Safflower (Carthamus tinctorius L.), which is grown as a high-quality forage crop for dairy cattle and dairy sheep in Mediterranean conditions, has been studied to determine the chemical composition, in vitro organic matter digestibility (IVOMD), gross energy (GE) and fatty acid (FA) profile of the plant during growth.

Herbage samples were collected five times at progressive morphological stages from the late vegetative to the early flower stage. The dry matter, organic matter, fibrous fractions and GE content increased with increasing growth stage, while the ash, crude protein and IVOMD decreased during growth.  Linoleic and α-linolenic acid were the two main FAs in the safflower plant during the growth cycle and they ranged from 158 to 188 g/kg  and 521 to 586 g/kg  of the total FA, respectively.

The quality and IVOMD of safflower, like other forages, depends on the development of the plant and decreases with increased maturity. A forage with a good nutritive value can be obtained by harvesting safflower at the early flowering stage. 

Key words: Carthamus tinctorius, crude protein, gross energy, lipid, morphological stage

Introduction

Safflower (Carthamus tinctorius L.) is an annual oil-seed crop that originated in the eastern Mediterranean area (Knowles 1976). It has earned a reputation as a drought-resistant plant, and for this reason it has been cultivated successfully in semiarid regions (Quiroga et al 2001; Yau 2007).

Safflower provides four principle products: oil, meal, seed, and fodder (Knowles 1989). The oil is used by both food producers and in industry. The meal (about 24% protein), which is relatively low in energy and high in fibre when the hulls are included. The  decorticated meal (about 40% protein), with most of hulls removed and a reduced fibre content, can be used as a protein supplement for low protein forages in livestock diets or in poultry backgrounding diets. Safflower seed can be used as a protein and energy supplement for beef cattle (Bottger et al 2002; Scholljegerdes et al 2004; Bolte et al 2002) and sheep (Kott et al 2003). Prepartum supplementation with safflower seeds high in either linoleate or oleate has increased the subsequent conception rates in primiparous beef cows (Lammoglia et al 1997).

Safflower is a high-quality forage crop that can be grown in arid and semiarid regions with limited water resources (Bar-Tal et al 2008; Leshem et al 2000) and can also be grown successfully on poorly fertile soil and in areas with relatively low temperatures (Koutroubas  and Papadoska 2005). Safflower has the potential of lengthening the duration of lush green pastures under arid Mediterranean conditions and cattle and sheep can graze succulent safflower regrowth and stubble fields after the harvest. Safflower pastures are adequate for growing ruminants which have moderate requirements for pasture quality (Landau et al 2005). Grazed safflower has been shown to sustain satisfactory growth rates in Australian steers (French et al 1988). Sheep have been able to utilise safflower forage and thorough chewing of spines likely prevents mouth ulceration. Ewes selectively consume the most nutrient-dense parts of the forage (Stanford et al 2001). A safflower monoculture at the pre-blooming stage can be used safely as the only feed for grazing sheep (Landau et al 2004) and the intake of safflower green fodder, cut to a 30 cm height, has exceeded maintenance requirements for energy and protein in sheep (Vonghia et al 1992). Spineless safflower cultivars, which could be used as fodder with dry-matter (DM) yields of up to 22 t/ha, and a high DM digestibility for safflower hay fed to heifers, have been introduced (Leshem et al 2001). Research conducted in Montana and Alberta indicates that safflower crops damaged by frost may be cut for hay. The thorny nature of the plant naturally causes concern, but in feeding trials conducted with ewes in Alberta, no aversion to the forage was noted and intakes were similar to a conventional alfalfa hay-based diet. It has also been reported to improve fertility in Canadian ewes, in comparison to alfalfa-grass hay (Stanford et al 2001). Safflower hay, given ad libitum, has successfully been used as the only food for late-pregnant dairy cows (Landau et al 2004).

Safflower cropped at the budding stage can be ensiled successfully (Weinberg et al 2002; Weinberg et al 2006) and it can substitute cereal silage in the diet of high-yielding dairy cows, without affecting dairy performance (Landau et al 2004).

As safflower has only recently been re-discovered as a fodder crop, there are no data available on its fatty acid (FA) profile and there is little information available regarding its nutritive value during the growth cycle or the optimum time for harvesting. The purpose of the present work was to study the effects of growth stage on the chemical composition, in vitro organic matter digestibility (IVOMD), gross energy (GE) and FA content of safflower.

Materials and methods 

Plant material and environmental conditions

The study was conducted in the Western Po Valley near Cuneo, Italy (latitude 44°N, longitude 7°E). The stands were seeded on 20 May 2006, and no irrigations or fertilisers were applied after sowing. Herbage samples were collected with edging shears (0.1 m cutting width) at five progressive morphological stages from the late vegetative to the early flowering stage, on subplots of 2 m² randomly located in 3 x 10 m² plots with three replicates cut to a 1 to 2 cm stubble height. The sampling time ranged from June to July 2006. Sampling was not performed on rainy days and was carried out in the morning, only after the disappearance of dew.

Chemical analysis

Whole plant samples were immediately dried in a forced-draft oven to constant weight at 65°C to determine the dry matter (DM) content and were then air equilibrated, ground in a Cyclotec mill (Tecator, Herndon, VA, USA) to pass a 1 mm screen, and stored for later analyses. The proximate composition of the dried samples was determined according to the AOAC method (AOAC 1990). The samples were analysed to determine the total N content, ash by ignition to 550°C, and ether extract (EE) using the Soxhlet method. Neutral detergent fibre (NDF) and acid detergent fibre (ADF) were determined as described by Van Soest et al (1991). The GE was determined using an adiabatic calorimeter bomb (IKA C7000, Staufen, Germany), while the IVOMD was determined according to the two-stage rumen fluid technique (Tilley and Terry 1963).

Fatty acid analyses

Fresh samples of the whole plants were immediately frozen, subsequently freeze-dried and ground to pass a 1 mm screen. Lipid extraction was performed on freeze-dried samples according to Hara and Radin (1978), while the transesterification of the FAs was carried out according to Christie (1982), with the modifications described by Chouinard et al (1999). The FA methyl esters were then determined by gas chromatography according to Peiretti and Meineri (2008).

Statistical analysis

The variability in FA and the herbage chemical composition of the samples  harvested at five stages of maturity were analysed by one-way analysis of variance (ANOVA) using the Statistical Package for Social Science (v 11.5, SPSS Inc., Chicago, Illinois, USA) to test the effect of the growth stage. When the values of F were significant (i.e., P<0.05), the Duncan range test (Duncan 1955) was used to detect differences among means.

Results and discussion 

Crop quality

The herbage quality and GE at the five different stages of development are reported in Table 1.

The DM content was very low throughout all the studied stages and increased with advancing morphological stage from 83 g/kg fresh matter (FM), for the late vegetative stage, to 157 g/kg FM for the early flowering stage. In Italy, similar trends have also been observed in safflower cultivated for hay and ensiling purposes, where the DM content increased from 123, at the bud initiation stage, to 520 g/kg FM at the seed filling stage (Corleto et al 2005). In Sardinia (Italy), Landau et al (2005) found that safflower DM content increased from 110 to 220 g/kg FM during the first spring growth cycle, while in Israel, safflower DM content remained steady at less than 400 g/kg FM throughout the growth cycle. 

The low DM content of safflower before the flowering stage, found in the present research, prolongs the wilting period and this could have a negative effect on the drying speed during wilting for ensiling purposes. Wilting the safflower forage for one or more days is very effective at the early stage of harvesting, as this practice greatly increases DM, and, at the same time, reduces the intensive fermentation and proteolysis processes of the silage. If harvesting is performed at the flowering stage, wilting is not necessary (Corleto et al 2008).

The CP and ash contents decreased, while OM, NDF, and ADF contents increased with time as the plant advanced to flowering. A similar trend has been observed in safflower harvested from bud initiation to the seed filling stage in Southern Italy by Corleto et al (2005), who showed that the decrease in CP content and the increase in fibrous components were related to a decrease in the leaf portion of the biomass, which has a higher CP concentration, and that an increased proportion of stems is related to the advance in maturity. As maturity stage increased, there was an increase in the DM content, whereas the safflower forage quality declined with time; this indicates that plants cut at an early stage of growth yield less forage, but have a higher concentration of CP and lower cell wall constituents, which result in a high nutritive value of the herbage. As forage advances in maturity, the quality change results in a decrease in the nutritive value of the herbage (Corleto et al 2005). As far as the CP content is concerned, in the present research, it decreased from 272 to 124 g/kg DM during the growth cycle. However, the average daily decline in CP content with increasing stage was higher than in the South Italian study (5.5 g/kg DM/day vs. 2.2 g/kg DM/day) (Corleto et al 2005). In Israel, the CP safflower content declined from 140 to approximately 110 g/kg DM, while in Sardinia the CP content in the DM of ingested safflower decreased from 190 to approximately 130 g/kg DM (Landau et al 2005). The safflower hay used by Landau et al (2004) was harvested at the late budding stage. It therefore had a higher CP content than the full-bloom hay used by Stanford et al (2001) (134 and 97 g/kg DM, respectively). Rahamatalla et al (1998) found that the CP content of the safflower seed increased rapidly with time and there were considerable variations in CP in the studied cultivars. These variations may have been due to the differences in the level of crude fibre in the seed which decreases the CP content of the seed and meal (Peiretti and Gai 2009); the decrease in CP content during the latter stage of development could also be due to the seed utilized protein for growth and development, as indicated by a rapid increase in seed volume with time (Rahamatalla et al 1998).

In the present research NDF and ADF increased from 313 to 491 g/kg DM and from 172 to 415 g/kg DM, respectively. The increase in fibrous fractions with increased stage of maturity is due to the progressive translocation of the soluble cell contents from the leaves and stems to the seeds. Landau et al (2005)  found that NDF ranged from 410 and 490 g/kg DM in safflower cultivated in Israel, while grazing safflower in Sardinia, the NDF content of safflower DM ingested by sheep was in a range of 270 and 370 g/kg DM. During a research carried out in Southern Italy, to evaluate the evolution of biomass and safflower quality from bud initiation to seed filling stage, the NDF and ADF content varied from 295 to 585 g/kg DM and from 193 to 403 g/kg DM, respectively (Corleto et al 2005).

The EE content at the late vegetative and full branching stages was higher than in the other stages. These variations are probably due to to the progressive translocation of the lipid cell contents from the leaves and stems to the seeds. In the safflower seed, the oil content reached its maximum and then slightly decreased. This may be due to the stabilization in oil content after completion of seed maturation which occurs 30 days after flowering (Rahamatalla et al 1998).

The GE content increased slowly with increasing growth stage and was highest in the last analyzed stage; this difference was primarily due to variations in the ash content, which was at its lowest in the early flowering stage. Similar trends have been observed in other oilseed plants, such as chia (Peiretti and Gai 2009), false flax (Peiretti and Meineri 2006) and evening primrose (Peiretti et al 2004).

In vitro organic matter digestibility

As the safflower advanced in maturity, the quality change resulted in a decrease in nutritive value of the herbage and a decrease in the IVOMD of 805 to 588 g/kg OM with a mean decrease of 3.3 g/kg OM/day (Figure 1).

Forage crops are usually harvested to obtain an optimal compromise between yield and nutritional value. Leshem et al (2000) studied the effects of three sowing dates (November, December and the beginning of February) on DM yield and quality of safflower for forage which was harvested simultaneously at the end of April. The DM yields were 22, 15 and 8 t/ha, the DM digestibilities were 489, 521 and 655 and the CP concentrations were 100, 114 and 146 g/kg DM, respectively. Thus, younger plants resulted in a lower DM yield, but a higher nutritional value.

In southern Italy, safflower utilized as fodder crop has shown a similar daily DM intake and digestibility values to those of a vetch–oat mixture when it was offered to rams and its GE content was around 18.0 MJ/kg DM (Vonghia et al 1992). Landau et al (2004) measured the digestibility of a safflower hay product for pregnant dry cows when it was fed as the only feed. The in vivo digestibility of the DM from the safflower hay was 723 g/kg, while the in vitro DM digestibilities (IVDMD) of the ingested hay, leaves, stems, and orts were 646, 729, 546, and 505 g/kg, respectively. In Israel, the in vitro DM digestibility of the safflower plant remained remarkably steady between 650 and 660 g/kg; while grazing safflower, when the sheep consumed only leaves and buds and chose not to eat the coarse stems, the in vitro DM digestibility decreased from 790 to 720 g/kg during the spring growth cycle (Landau et al 2005).

Fatty acid profile

The differences in the FA profiles over the five morphological stages are reported in Table 2.

α-Linolenic acid (ALA, 18:3 n-3) was the dominant FA in the safflower during the growth cycle and ranged from 521 to 586 g/kg of the total FA. Linoleic (LA, 18:2 n-6) and palmitic (PA, 16:0) acids were the next most abundant FAs and they ranged  from 158 to 188 g/kg of the total FA and from 90 to 111 g/kg of the total FA, respectively. The concentrations of ALA decreased by 11% while the concentrations of PA increased by 19% between stem extension and early flowering. The remaining FAs were present in all the safflower stages with a variable trend.

The FA profile in the plant during growth differs from the oil in the seed. Standard safflower oil contains about 6–8% PA, 2–3% stearic acid (SA, 18:0), 16–20% oleic acid (OA, 18:1 n-9), and 71–75% LA (Velasco and Fernandez-Martinez 2001), but safflower is one of the best examples of plant that shows variability in the FA composition of seed oil (Knowles 1989). Gecgel et al (2007) showed that the oil content and the four major FAs in the safflower seeds were affected by sowing and harvest dates and that the moisture content declined 15 days from the flowering period to maturity, while the oil content increased. In the safflower seed, the saturated fraction mainly included PA and SA, and although the levels of PA regularly decreased during seed development, the levels of SA showed fluctuations (Gecgel et al 2007). Hamrouni et al (2004) demonstrated that rapid decreases in PA were accompanied by slight decreases in SA at the end of ripeness. Ladd and Knowles (1970) reported that their safflower varieties, with high levels of SA, contained lower levels in PA.

As far as the unsaturated fraction is concerned, an inverse relationship emerged between the development of the OA and LA in the seeds. The OA content increased in the high oleic variety, and the LA content increased in the high linoleic variety during seed development. Furthermore, changes in the composition of the FAs, especially the OA and LA, regarding the sowing date, were largely attributed to seasonal weather differences, particularly moisture and temperature, during the growing season (Ladd and Knowles 1970; Nagaraj and Reddy  1997).  Browse and Slack (1983) have reported the existence of an inhibitory effect of high temperature on desaturases in safflower seeds. The involved mechanism appears to be the direct effect of temperature on the activity of desaturase enzymes which convert OA to LA, and on the solubility of oxygen, which seems to play a regulatory function in that activity (Harris and James 1969). The enzymes that convert OA to LA are inactivated at a high temperature. Under low temperatures, the LA content increases, while under high temperatures the OA content increases.

During the growth cycle, ALA was the dominant FA in the whole safflower plant, while, during the process of seed formation and ripening, LA predominated in every lipid class of the seed and the ALA content was present in less than 3 g/kg of the total FA of the seed (Gecgel et al 2007), decreased with increasing maturation and was absent in fully mature safflower seeds (Nagaraj and Reddy 1997; Sims et al 1961). Hill and Knowles (1968) have shown that ALA is not found after 10 days of flowering in any studied variety, while McKillican and Sims (1963) have reported that low levels of this FA are contained in their safflower varieties as late as 30 days after flowering.

Conclusions 

• As occurs in other forages, the chemical composition and IVOMD of safflower is closely connected to the stage of maturity of the plant.

  
  

• Safflower is a readily available, natural and economic source of polyunsatured fatty acids (PUFA) and provides good nutritive value forage, when harvested at the early flowering stage.

  
  

•  Our data have confirmed that safflower has the potential to be a valuable fodder and could also be proposed as a possible source of PUFA for productive ruminants.
   

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Received 8 October 2009; Accepted 26 October 2009; Published 3 December 2009

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