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Published online 3 August 2006
Published in Agron J 98:1213-1220 (2006)
DOI: 10.2134/agronj2005.0264
© 2006 American Society of Agronomy
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Production Papers

Dairy Cattle Grazing Preference among Six Cultivars of Perennial Ryegrass

Harm J. Smita,*, Seerp Tammingab and Anjo Elgersmac

a Crop and Weed Ecol. Group, Dep. of Plant Sci., Wageningen Univ., P.O. Box 430, 6700 AK Wageningen, the Netherlands
b Anim. Nutrition Group, Dep. of Anim. Sci., Wageningen Univ., P.O. Box 338, 6700 AH Wageningen, the Netherlands
c Crop and Weed Ecol. Group, Dep. of Plant Sci., Wageningen Univ., P.O. Box 430, 6700 AK Wageningen, the Netherlands, and Dep. of Plant Prod., Faculty of Biosci. Eng., Univ. of Ghent, Coupure Links 653, 9000 Gent, Belgium

* Corresponding author (harm.smit{at}wur.nl)

Received for publication September 13, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Six endophyte-free diploid perennial ryegrass (Lolium perenne L.) cultivars (Abergold, Respect, Agri, Herbie, Barezane, and Barnhem) were examined in an experiment to investigate the selection behavior of grazing Holstein Friesian cows in July and September 2003 and May 2004 and to identify factors related to preference. Three groups of dairy cows (Bos taurus) were allowed to select among these six cultivars that were sown in replicates in a randomized block design with 12 plots (36.7 by 2.0 m). Each experiment lasted 4 d, and every day cows were offered a new field with 12 plots. Herbage intake was measured using the sward-cutting method, measuring herbage yield before and after grazing. The Chesson–Manly index was used to quantify cattle grazing preference. Dairy cows did select among the cultivars in every period. The pattern of selection was consistent over the three experiments, Abergold was selected most, and Agri and Respect were least selected (22 vs. 14%). In July 2003 and May 2004, cultivars differed in morphological characteristics, but this was not related with grazing preference. In all three experiments, distinct differences were found among cultivars in chemical composition [dry matter (DM), ash, neutral detergent fiber, and water soluble carbohydrates (WSC) concentration] and digestibility of organic matter, especially for WSC concentration, which, when averaged over experiments, ranged between 135 g kg–1 DM (Barezane) and 165 g kg–1 DM (Abergold). Dairy cows preferred the cultivars with low ash and fiber concentration and high WSC concentration and digestibility.

Abbreviations: ADL, acid detergent lignin • CP, crude protein • DM, dry matter • DOM, digestible organic matter • NDF, neutral detergent fiber • NIRS, near infrared reflectance spectroscopy • SSH, sward surface height • WSC, water soluble carbohydrates


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ALTHOUGH CATTLE are often considered to be indiscriminate grazers, still they are known to select if given the opportunity (Newman et al., 1995). Preference among various plant species is a well-recognized concept and has been frequently studied (Heady, 1964; Stephens and Krebs, 1986; Provenza et al., 1996). However, preference is also being given more attention by breeders of forages (Smith et al., 1997). Cattle have shown preference among cultivars of tall fescue (Festuca arundinacea Schreb.) (Shewmaker et al., 1997), annual ryegrass (Lolium multiflorum Lam.) (Aderibigbe et al., 1982), and perennial ryegrass (Lolium perenne L.) (O'Riordan et al., 1998). The mechanisms behind this preference are relatively unknown. Simon and Daniel (1983) concluded for sheep, and O'Riordan et al. (1998) for cattle, that tetraploid cultivars of perennial ryegrass were preferred to diploid cultivars. Diploid perennial ryegrass cultivars form the largest part of the seed market in the Netherlands and are widely used in practice (Bonthuis et al., 2003). Jones and Roberts (1991) reported that sheep selected among four diploid perennial ryegrass cultivars and concluded that cultivars with a high WSC concentration were preferred. Sward surface height (SSH) (Griffiths et al., 2003), bulk density (Laca et al., 1992), and sward morphology (Dumont et al., 1995) have been reported as key parameters for grazing behavior.

Most of the preference experiments were conducted with sheep (Simon and Daniel, 1983; Hazard et al., 1998) or nonlactating beef cattle (O'Riordan et al., 1998) in short-term experiments, and experimental animals are often fasted before experiments (Jones and Roberts, 1991). Detailed information on the preference of grazing dairy cows over longer periods (>1 d) is still lacking. The objective of this experiment was to investigate the selection behavior of grazing Holstein Friesian cows that could choose among six diploid perennial ryegrass cultivars and to identify factors related to preference.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Cultivars and Fields
Six endophyte-free diploid perennial ryegrass cultivars (Abergold, Respect, Agri, Herbie, Barezane, and Barnhem) were chosen from the Dutch cultivar list (Bonthuis et al., 2003). The first three cultivars are intermediate heading (25–28 May) whereas the latter three are late heading (7–9 June). Cultivars were known to differ in concentration of WSC and in vitro digestibility (Tas et al., 2005).

Three grazing experiments were conducted on the same paddock. Two experiments took place in 2003, in July and September, and the third experiment in May 2004. Experimental fields had been established in 1999 at experimental fields of Wageningen University, the Netherlands (51°58' N, 5°40' E; 7 m above sea level) and were cut for 3 yr during the grazing season and grazed by sheep during winter. Experimental details were reported earlier (Smit et al., 2005a). All experiments had a similar setup. Six endophyte-free cultivars were sown in duplicate in a randomized complete block design with four replicate blocks, resulting in 48 plots (110 by 2 m), which were separated by 0.2 m of bare soil. For this experiment, blocks were split in three parts with electric wires, resulting in 12 grazing areas of 36.7 by 26.4 m. Each grazing area consisted of 12 subplots (36.7 by 2 m) in which all six cultivars were present twice (Fig. 1 ). Three groups of cows were turned in at 1300 h and could graze one grazing area for a 24-h period. The next day, cows were moved to a new grazing area. Each experimental period lasted 4 d.


Figure 1
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  		Fig. 1. Setup of grazing experiments, with an example of the grazing area with the six cultivars offered during 1 d.

 
Management
Weather data during the three experiments are presented in Table 1. On 21 Mar. 2003, the paddock was fertilized with pig manure (9.3 g N kg–1, 4.0 g P2O5 kg–1, and 7.7 g K2O kg–1) at a rate of 25 m3 ha–1, and an additional 35 kg N ha–1 was applied. On 15 May 2003, a silage cut was made, and the fields were fertilized with 55 kg N ha–1. On 6 and 10 June 2003, the fields for Days 1 to 2 and 3 to 4 were harvested, respectively, and 75 kg N ha–1 was applied directly after harvesting. All cuts were made using a front mower with reciprocating blades set at a height of 6 cm. The first grazing experiment took place from 30 June to 4 July 2003. Thereafter, the residual herbage was removed by cutting and then collecting with a pickup wagon on 10 July 2003. On 19 Aug. 2003, the sward was harvested and fertilized with 60 kg N ha–1. The second grazing experiment took place from 15 to 19 Sept. 2003. During the winter of 2003 to 2004, the sward was grazed by sheep, which removed all residual herbage. On 13 Apr. 2004, the paddock was fertilized with 60 kg N ha–1. The third grazing experiment took place from 10 to 14 May 2004.


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  		Table 1. Average meteorological data during the three grazing experiments.

 
Animals
All experiments were approved by the Institutional Animal Care and Use Committee of Wageningen University and care was taken to minimize animal stress. Each experiment consisted of four consecutive days of grazing. The group size, some animal characteristics, and the amount of concentrate fed during the three experiments are presented in Table 2. All cows, which were all Holstein Friesian, had been adapted to grazing before the start of the experiments. Additional concentrate was supplied in portions of 1.5 kg during milking twice daily. The stocking rate use was definite to an herbage allowance of approximately 50 kg of DM per animal. With an average herbage intake of 20 kg of DM per animal, this would mean ample food available to select.


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  		Table 2. Group size, body weight (BW), milk production (MP), and concentrate fed during three preference experiments.

 
Field Measurements
Each morning, the SSH was measured with a falling plate (diameter 50 cm; Eijkelkamp Agrisearch Equipment, Giesbeek, the Netherlands). Five measurements of SSH were taken in each subplot and averaged. Then a pregrazing cut was taken with a mowing machine (Agria 3200, Agria-Werke, Möckmühl, Germany), at a cutting height of 4 cm, to measure initial herbage mass. Per subplot, an area of 3.5 m2 (2.80 by 1.25 m) was cut. The grass was collected in plastic bags, and fresh weight was recorded in the lab. Subsamples (200 g of fresh weight) were taken with a grass core and dried at 70°C for 24 h to determine the DM content.

After the cows were moved to the next grazing area, the SSH of the grazed plots was measured, and postgrazing cuts were taken with an Agria mower at a cutting height of 4 cm to measure residual herbage mass. Two strips of 3.5 m2 were cut per subplot. The two strips were situated immediately adjacent to the strip that had been cut before grazing. The cut grass was processed in the same way as the pregrazing samples.

Chemical and Morphological Analysis
The subsamples of the pregrazing herbage used to determine DM content were ground using a 1-mm sieve (Peppink, Deventer, the Netherlands) and analyzed for chemical composition [ash, neutral detergent fiber (NDF), acid detergent lignin (ADL), WSC, and crude protein (CP)] and digestibility [digestible organic matter (DOM)] with near infrared reflectance spectroscopy (NIRS). The NIRS predictions for chemical composition were validated with laboratory analyses (Smit et al., 2005c).

A plucked sample of the cut herbage of approximately 25 g of fresh material was collected and hand-separated into leaf blade, reproductive stem, pseudostem, and dead material.

Methodology to Express Preference
To measure preference in a grazing situation, various methods have been reported. All authors define the food that is eaten most as the most preferable. Many studies measured preference indoors, weighing the offered herbage and the residual and expressing preference as food consumed (offered – residual) (Aderibigbe et al., 1982; Provenza et al., 1996; Tolkamp et al., 1998). In a grazing situation, this is also possible by estimation of herbage yield before and after grazing, either visually (Shewmaker et al., 1997) or using the sward-cutting techniques (Meijs, 1981; Macoon et al., 2003). Before grazing, a small area is cut to measure the pregrazing yield, and after a period of grazing, this is repeated to determine the postgrazing yield. By subtraction, the consumption is calculated (O'Riordan et al., 1998; Jones and Roberts, 1991; Smit et al., 2005c).

As preference is determined in relation to the other options the animal has (van Dyne and Heady, 1965; Westoby, 1974; Chesson, 1983), it could best be expressed as a relative figure. Manly et al. (1972) and Chesson (1978, 1983) developed a methodology that takes food availability and depletion into account. They assumed that the probability of encountering a preferred cultivar decreases more rapidly than for a less preferred cultivar. They expressed preference according to the following formula:

Formula 1[1]
where an animal has a choice of m cultivars, consumedi is the amount of consumed herbage of cultivar i, and availablei is the amount of available herbage of cultivar i at the beginning of the foraging bout. The denominator term is the sum of all numerator terms of all cultivars (1,...m).

Statistical Analysis
The experimental unit in these experiments was the cultivar in each grazing area, so the two subplots per cultivar occurring in each grazing area were averaged. Each experiment resulted in a data set of 72 values. Sward surface height, pre- and postgrazing yield, consumption, chemical parameters, and preference were analyzed as a randomized block design with cultivar as treatment and the groups of cows in the three grazing areas and 4 d as blocks. The data were analyzed with the model:

Formula 2[2]
where µ is general mean, Ci is cultivar effect (j = 1,...6), Gj is grazing area effect (Gj = 1, 2, or 3), Dk is the day effect (1, 2, 3, or 4), Ci x Dk is interaction between cultivar and day, and eijk is residual term. Sward morphological parameters were analyzed as a randomized complete block design with six cultivars and 4 d as the samples from the three grazing areas within 1 d were pooled. An LSD test was preformed to differentiate among the cultivars.

To enable comparison of preference to morphological, sward structural, and chemical components, every component of each cultivar was expressed as an index value (100 = mean) relative to the mean during each experimental period. Correlation coefficients were calculated of the 18 obtained data points. Data were analyzed by the SPSS statistical package (SPSS for Windows, Rel. 10.0.5, SPSS, Chicago, IL).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Morphology
The morphological characterization of the swards is presented in Table 3. In July 2003 (Exp. 1), over 30% of the canopy consisted of reproductive material. There were significant differences (p < 0.001) among cultivars. Cultivars Barezane and Barnhem had the highest proportion of leaf blade, and cultivars Agri and Respect had the highest proportion of stem and the lowest proportion of pseudostem.


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  		Table 3. Sward morphology (percentage of leaf blade, stem, pseudostem, and dead material) of six perennial ryegrass cultivars during three grazing preference experiments (Exp. 1: July 2003; Exp. 2: September 2003; Exp. 3: May 2004).

 
In September 2003 (Exp. 2), no inflorescences were present in the sward. The cultivars did not differ in DM allocation. The amount of dead material was very high (22.5%) due to the hot and dry weather conditions during the summer of 2003.

In May 2004 (Exp. 3), the late-heading cultivars—Barezane, Barnhem, and Herbie—had the highest (p < 0.001) proportion of leaf blade and pseudostem, whereas the intermediate-heading cultivars—Abergold, Agri, and Respect—had the highest (p < 0.001) proportion of stems.

Chemical Composition
Table 4 shows the chemical composition of the six cultivars. The DM concentration of the herbage was high (>200 g kg–1 DM) in Exp. 1 and 2. In Exp. 3, the DM concentration was lower than in Exp. 1 and 2, and the variation among cultivars was larger. Cultivar Agri had in each experiment the lowest DM concentration. The range among cultivars in NDF concentration was small, with 39.3, 20.3, and 33.2 g kg–1 DM in Exp. 1, 2, and 3, and could be explained by the variation in stem proportion (Table 3). Cultivars with a high stem proportion also had a high NDF concentration and vice versa. In Exp. 1 and 3, the WSC concentration was much higher (165 and 166 g kg–1 DM) than in Exp. 2 (104 g kg–1 DM). The WSC concentration had the largest range of all examined traits among cultivars: 38.0, 37.1, and 30.2 g kg–1 DM in Exp. 1, 2, and 3, respectively, which was on average over 25% of the mean. The CP concentration varied among cultivars in Exp. 2 and 3. It had a stronger inverse relationship with the WSC (R = –0.864) than with the NDF concentration (R = –0.386). The in vitro DOM was 81.7, 76.8, and 82.3% in Exp. 1, 2, and 3, respectively. A significant (p < 0.001) difference among cultivars for DOM of 4% was found in each of the three preference experiments.


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  		Table 4. Dry matter (DM), ash, neutral detergent fiber (NDF), acid detergent lignin (ADL), water soluble carbohydrates (WSC), crude protein (CP) concentration (DM), and digestibility of organic matter (DOM) of six perennial ryegrass cultivars in three preference experiments (Exp. 1: July 2003; Exp. 2: September 2003; Exp. 3: May 2004).

 
Sward Surface Height
The SSH of the pre- and postgrazing swards and their difference is shown in Table 5. The difference between pre- and postgrazing SSH was always highest for cultivar Abergold and always lowest for cultivar Barezane. The SSH measurements were strongly correlated (R = 0.922; p < 0.001) with corresponding yield measurements (Table 6).


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  		Table 5. Pre- and postgrazing sward surface height (SSH) and the SSH difference of six perennial ryegrass cultivars grazed by dairy cows in three preference experiments (Exp. 1: July 2003; Exp. 2: September 2003; Exp. 3: May 2004).

 

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  		Table 6. Pre- and postgrazing yields, consumption of herbage [dry matter (DM) yield], daily herbage intake of this certain cultivar (kg DM animal–1), and Chesson–Manly preference index (CM index) of six perennial ryegrass cultivars grazed by dairy cows in three preference experiments (Exp. 1: July 2003; Exp. 2: September 2003, Exp. 3: May 2004).

 
Pre- and Postgrazing Yield, Intake, and Preference
Table 6 describes the pre- and postgrazing yields of the six cultivars during the three experiments. In all experiments, the pregrazing yield differed among cultivars. In Exp. 1 and 3, the intermediate-heading cultivars Agri and Respect were always among the highest-yielding cultivars, whereas the late-heading cultivars Barezane, Barnhem, and Herbie always had the lowest yields. In Exp. 2, however, the cultivar differences tended to be opposite.

In all experiments, postgrazing yields differed among cultivars. In general, the ranking for the postgrazing yield followed that of the pregrazing yield, especially in situations where there was large variation in pregrazing yields as in Exp. 1 and 3.

The level of consumption (kg ha–1) in Exp. 1 was much higher than in Exp. 2 and 3 because in Exp. 1, the group of dairy cows in one grazing area consisted of five cows, whereas in Exp. 2 and 3, it consisted only of three cows. The intake of herbage per cow did not differ significantly in the three experiments. The consumed herbage only differed among cultivars in Exp. 2 but tended to differ in Exp. 1. Cultivar Abergold was consumed most in all experiments although this was only significant in Exp. 2. In each of the experiments, differences (p < 0.05) in preference among cultivars were found. Cultivar Abergold was always among the best preferred cultivars, whereas cultivars Agri and Respect were always among the least preferred cultivars.

Relations with Preference
The relation between morphological, sward structural, and chemical components and preference is shown in Table 7. The significant effects are presented in Fig. 2 . Preference was significantly related (p < 0.05) to four parameters: negatively with ash and NDF and positively with WSC and DOM. The WSC concentration showed the highest correlation with preference (Fig. 2D). Water soluble carbohydrates were related to the other parameters that had an effect on preference: ash (R = –0.619, p < 0.01), NDF (R = –0.580, p < 0.01), and DOM (R = 0.638, p < 0.01). Morphology did not have an effect on preference in this experiment, but leaf blade, stem, and pseudostem proportions had large effects on the NDF concentration, one of the determinants of preference. Dry matter yield and SSH did not have an effect.


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  		Table 7. Correlation coefficients between preference and chemical components [dry matter (DM), ash, neutral detergent fiber (NDF), acid detergent lignin (ADL), water soluble carbohydrates (WSC), and crude protein concentration (CP)], digestibility [digestible organic matter DOM], morphological components (leaf blade, stem, pseudostem, and dead material), and sward structural components [dry matter yield (DMY), green leaf mass (GLM), and sward surface height (SSH)] examined in the three preference experiments.

 

Figure 2
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  		Fig. 2. The relation among (A) ash, (B) neutral detergent fiber (NDF), (C) digestible organic matter (DOM), and (D) water soluble carbohydrates (WSC) expressed as index figure to preference of six cultivars (cv) in July (black), September (open symbols), and May (gray symbols).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Selection, Preference, and Palatability
According to the strict definition given by Parsons et al. (1994), in this experiment, selection by dairy cattle among perennial ryegrass cultivars was observed, and no preference was measured. Nevertheless, the definition by Hodgson (1979) states: "Preference is a general term describing the discrimination exerted by animals between areas of the sward or the components of a sward canopy. Preference ranking is the ranking of a series of swards, herbage samples or morphological units, based if possible on the relative intakes determined in free-choice trials." In the experiments conducted here, animals had free choice to eat a certain cultivar. Therefore, the cultivar that was consumed most was considered as the most preferred cultivar. In this experiment, we tried to find factors (chemical and morphological) that were positively or negatively related to preference by dairy cows. Such associations would give grass breeders the opportunity to select for traits for preferable cultivars. The term palatability is avoided because it is too much related to taste, and it is unknown whether taste is the driving key behind preference.

Sward Surface Height and Morphology
In vegetative swards, cattle tend to select for taller swards (Bailey, 1995; Dumont et al., 1995; Griffiths et al., 2003). In this experiment, however, only in Exp. 2 was the sward completely vegetative, and then no differences among cultivars were found for SSH. The postgrazing SSH never fell below 7 cm, which is considered as limiting for cattle grazing (Gibb et al., 1997), showing that the herbage allowance was not limited. In Exp. 1 and 3, the sward was partly in a reproductive stage. No selection against cultivars with a high percentage of stems was found. Cattle can, however, graze selectively between stems and leaves (van Dyne and Heady, 1965) and generally tend to select for the vegetative patches when the sward height is 18 cm (Dumont et al., 1995). In this experiment, cultivars were in different stages of their reproductive phase. The intermediate-heading cultivars (Abergold, Agri, and Respect) had more inflorescences, whereas the late-heading cultivars had more pseudostem. This, however, did not have an effect on preference by dairy cows.

Chemical Composition
Chemical composition of forage is one of the major and best studied factors in preference behavior (Heady, 1964). In this experiment, negative effects on preference behavior were found for high ash and NDF concentration, and positive effects were found for high DOM and WSC concentration among cultivars of perennial ryegrass.

The ash concentration or minerals usually show a positive relation with preference (Westoby, 1974; Belovsky, 1978). In contrast, in this study, a negative relation was found, which might be explained by the fact that cultivar Abergold always had the highest WSC concentration and a low ash concentration as well. The ash fraction does not exist on a molecular level in the plant but can be related to sodium or potassium salts as triggers for preference (Belovsky, 1978; Arnold, 1981). However, no specific data were available for these fractions.

Digestibility was positively associated with preference, whereas the NDF concentration was negatively associated with preference. The two parameters were strongly (p < 0.01) interrelated, and there was negative relation between the NDF concentration and digestibility in each of the experiments. Heady (1964) reported a negative relation between cell wall constituents (lignin and crude fiber) and preference behavior of large herbivores, mainly in association with positive relations with sugars, proteins, and fats. Wright and Illius (1995) linked the aversion of herbivores to species with a higher NDF concentration to a higher energy expenditure for grazing and chewing.

A positive relation between WSC concentration and preference was found by others (Heady, 1964; Mayland et al., 2000b; Ciavarella et al., 2000), but, to our knowledge, never in diploid perennial ryegrass consumed by dairy cows. Ciavarella et al. (2000) directly related preference to WSC concentration. A pasture that mainly consisted of Harding grass (Phalaris aquatica L.) was partly shaded, causing a distinct decrease in WSC concentration. When sheep were given the choice between the shaded and the unshaded part, sheep consumed 2.6 times more of the unshaded (high WSC) part of the pasture. Researchers concluded that WSC played a large role in the diet selection of large herbivores and stated that it might be useful to select for high-WSC cultivars. This was supported by Mayland et al. (2000b), who described an experiment in which cows could select between eight tall fescue cultivars. They found that cattle chose an energy-dense diet and selected cultivars with high nonstructural carbohydrate levels. In a large examination of 24 perennial ryegrass cultivars in Ireland, clear differences were found among cultivars (O'Riordan et al., 1998). No chemical analyses were performed, but tetraploids were more preferred than diploids. Tetraploids have larger cell content than diploids and therefore a higher digestibility, CP, and WSC concentration (Smith et al., 2001). In the present experiment with diploid cultivars, in all experiments (July, September, and May) a significant (p < 0.001) positive effect of WSC on preference was found. In this experiment, all perennial ryegrass cultivars had a high N concentration (172–203 g kg–1 DM). This would then result in the preference for energy-dense grasses (high-WSC grasses). However, under lower N fertilization levels, the preference might change. Several studies (Heady, 1964; Tolkamp et al., 1998) have also found a positive effect of protein on preference of forages.

Cows can taste primary flavors sweet, salt, sour, and bitter (Chiy and Phillips, 1999) and seem to prefer sweet diets (Nombekela et al., 1994). However, WSC in itself cannot be tasted (Arnold, 1981) but is a collective noun of all mono-, di-, tri-, or polysaccharides, each of which will give a different sweetness response. Detailed information on saccharides in forages as a key to palatability is still lacking. Sucrose is found to be preferred in concentrate feeds (Chiy and Phillips, 1999; Nombekela et al., 1994). In perennial ryegrass, however, sucrose is only a small proportion of the total WSC fraction (15% of WSC)—fructans form the largest part (60% of WSC; McGrath, 1988). Mayland et al. (2000b) found a relation between total WSC and preference but could not find a significant relation with a single saccharide (Mayland et al., 2000a). Arnold (1981) commented that despite reservations about considering a gross indicator like WSC, it may still be a useful trait for breeding for preference in absence of better guidelines.

A higher preference for certain cultivars will not directly increase herbage intake of dairy cows if these cultivars are fed alone, and caution should be taken with interpretation of these results. In a grazing experiment (Smit et al., 2005b), where four of the six cultivars were fed alone to the same dairy cows, no differences (p > 0.05) between cultivars in DM intake were found. The trade-offs in the feeding behavior of animals may be different when they are given access to one feed rather than more feeds as a choice (Kyriazakis and Oldham, 1997). The question, "Why do dairy cows select for cultivars with high WSC?" remains unanswered and needs further examination.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Dairy cows selected among six diploid perennial ryegrass cultivars. Preference in this study was not related to morphological parameters but was positively related to the WSC and the level of digestibility and negatively related to ash and fiber (NDF) concentration. It is certainly possible for grass breeders and farmers to select for diploid perennial ryegrass cultivars preferred by dairy cows.


   ACKNOWLEDGMENTS
 
This experiment was carried out at experimental fields of Wageningen Plant Sciences Experimental Centre (PPW), using cows and animal housing facilities of the Ossekampen (Animal Sciences Group). This research was sponsored jointly by Barenburg Holland B.V. and the Ministry of Economic Affairs (EZ) in the Netherlands, project BTS 99170, Senter.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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