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Published online 5 June 2006
Published in Agron J 98:978-985 (2006)
DOI: 10.2134/agronj2005.0213
© 2006 American Society of Agronomy
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Production Papers

Morphological Characteristics of Perennial Ryegrass Leaves that Influence Short-Term Intake in Dairy Cows

P. Barrea,*, J.-C. Emilea, M. Betina, F. Suraulta, M. Ghesquièrea and L. Hazardb

a INRA-Unité de Génétique et d'Amélioration des Plantes Fourragères, 86600 Lusignan, France
b INRA-UMR1248, Agrosystèmes Cultivés et Herbagers, 31326 Castanet-Tolosan Cedex, France

* Corresponding author (philippe.barre{at}lusignan.inra.fr)

Received for publication July 19, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In sustainable agriculture, adaptation of swards to grazing is a major issue for breeding forage grass cultivars. Understanding the relationships between sward traits and animal production under grazing could help to identify criteria for selection. Intake is the major limiting factor for animal production under grazing. In this paper, short-term intake rate by grazing dairy cows (Bos taurus) is compared between eight diploid perennial ryegrass (Lolium perenne L.) populations. Four replicates per population were measured over four periods (April and May 2001, and April and October 2002) in two experiments per period, that is, intermediate- and late-maturity groups. For each period, sward structure was characterized by tiller density, leaf blade and sheath lengths, and green leaf and total dry matter (DM) yield, while nutritive value was characterized by in vitro dry matter digestibility (IVDMD), percent of DM, N, water-soluble carbohydrate (WSC), and neutral-detergent fiber (NDF) contents. For the late-maturity populations, the population with the shortest leaves had a lower fresh matter intake rate (10.5 kg h–1) than the three others (12, 12, and 11.6 kg h–1, respectively). For the intermediate-maturity populations, differences in intake rates among populations were significant (P = 0.06) for all periods and were highly significant (P < 0.0004) in April 2002. In this period, 49% of the variation in fresh matter intake rate could be explained by blade length. In conclusion, differences in short-term intake rate between populations of diploid perennial ryegrass are highlighted. In both maturity groups, blade length appears to be an important factor explaining these differences.

Abbreviations: ANOVA, analysis of variance • BDMW, blade dry matter weight • BLENGTH, blade length • BYIELD, blade yield • CV, coefficient of variation • DM, dry matter • DMI, dry matter intake rate • FM, fresh matter • FMI, fresh matter intake rate • IVDMD, in vitro dry matter digestibility • NDF, neutral detergent fiber • SDMW, sheath dry matter weight • SLENGTH, sheath length • TILLERDE, number of tillers • WSC, water-soluble carbohydrate


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
GRAZING, in the context of western agriculture intensification, was considered a less efficient way of feeding dairy cows than indoor feeding (Alard et al., 2002). However, the multi-functionality of grasslands combines economic performance, quality of animal products, and preservation of environment. Grazing is now promoted as a management practice to reduce animal feeding costs, to lower environmental impacts, and to raise the profile of animal husbandry. Adaptation to grazing has become a new issue for the breeding and testing of forage grass cultivars. To date, grass breeding programs have focused on DM yield under mechanical harvesting, rust resistance, and quality traits (Beerepoot and Agnew, 1997; Wilkins and Humphreys, 2003), which are likely to increase the forage feeding value for housed animals. However, Hazard et al. (1998) showed that the potential value of perennial ryegrass (Lolium perenne L.) cultivars under grazing cannot be drawn from feeding value assessments with housed animals. To be effective, forage grass improvement should be supported by animal grazing trials (Reed, 1994; Casler and Vogel, 1999).

Grazing is used in official testing procedures to estimate persistence and ground cover of cultivars but not to test animal performance parameters. Growing cultivars in mosaics (O'Riordan, 1997) allows their evaluation with regard to animal preference and regrowth potential (Rook et al., 1997).

Low intake has been identified as a major limiting factor in milk production from pasture (Mayne et al., 1987; Mayne and Gordon, 1995; McGilloway and Mayne, 1996). Long-term grazing experiments revealed significant perennial ryegrass cultivar effects on intake by sheep (Ovis aries) (Hazard et al., 1998; Orr et al., 2003) and dairy cows (Emile et al., 2000; Flores-Lesama et al., 2006). Animal trials were designed to identify intake-related plant attributes that could be used in grass breeding. Intake was found to be related to sward height and bulk density (Griffiths and Gordon, 2003), to green leaf mass per ha (Hazard et al., 1998; Penning et al., 1998), and to digestibility and N concentration (Orr et al., 2003). Under grazing, intake rate is limited by the difficulty for the grazing animal to gather the forage. Cows increase their bite mass by eating long-leaved plants using their tongue to pull leaves into the mouth before they are bitten off (Laca et al., 1992; McGilloway et al., 1999). In long-term experiments, the effect of ease of forage prehension on intake is confounded with post-ingestive effects due to both physical and chemical properties of the grass sward (Baumont et al., 2000).

Short-term grazing experiments can be used to test the ease of prehension of cultivars by estimating intake rate of grazing animals at the beginning of a grazing period. Moreover, the initial intake is known to be a key factor determining differences in long-term intake between forages (Moseley and Manendez, 1989). Barrett et al. (2003) modified the method developed by Penning and Hooper (1985) to compare short-term dairy cow intake rates between four contrasting ryegrass cultivars. Despite testing broad genetic variability, they did not find significant differences. Since intake rate by grazing cows is known to increase with leaf length (greater ease of prehension), we studied eight diploid ryegrass populations with contrasting leaf morphology using the same method as Barrett et al. (2003). The variation in short-term dairy cow intake rates grazing these ryegrass populations was investigated in relation to sward physical and chemical characteristics that could be used as selection and evaluation criteria.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Eight diploid perennial ryegrass populations were compared: four cultivars (intermediate cultivar 1: IC1, intermediate cultivar 2: IC2, late cultivar 1: LC1, and late cultivar 2: LC2) and four populations (BS+ for high blade/sheath ratio, BS– for low blade/sheath ratio, LL for long leaves, and SL for short leaves) obtained by divergent selections from a collection of French ecotypes (Charmet et al., 1990; Hazard et al., 1996). The IC1 has a "hay making type" (long leaves, large tillers, and low tiller density) and IC2, LC1, and LC2 have a "grazing type" (high tiller density).

These populations were managed in two sets: the intermediate-maturity group with BS+, BS, IC1, and IC2 and the late-maturity group with LL, SL, LC1, and LC2. Four replicated plots per population were sown in a randomized block design at the Institut National de Recherche Agronomique (INRA) in Lusignan, France (46°25'07 N, 0°07'06 E, altitude: 149 m), on 7 Oct. 1999. Plots measured 12 by 11.25 m (135 m2) and were sown in drills 18 cm apart at a rate of 25 kg ha–1.

The plots were managed by frequent cutting at 5 cm every 21 d to simulate grazing during the growth periods and after each measurement period. The plots were fertilized with 60 kg N ha–1 after each cut (about four times per year). An irrigation of 65 mm had to be supplied using a sprinkler on 30 Sept., 1 and 2 Oct. 2002 because of a dry period. Measurements started in the spring 2001 and were performed over four periods: April 2001 (P1), May 2001 (P2), April 2002 (P3), and October 2002 (P4). Each period, short-term (1 h) intake by dairy cows, structure and nutritive value of the swards were determined. On a single day, one of four replicates of the four populations was measured so that four consecutive experimental days were required to measure a complete replicated set of populations. The four intermediate-maturity populations were measured a week before the four late-maturity populations. Measurements were performed, in the morning (from 1000–1300 h), after regrowth of 22 to 25 d for P1, 24 to 27 d for P2, 24 to 27 d for P3, 27 to 30 d for P4 for the intermediate-maturity group, and, 28 to 31 d for P1, 31 to 34 d for P2, 26 to 29 d for P3 and 28 to 31 d for P4 for the late-maturity group. The weather conditions during measurement of intake are given in Table 1.


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  		Table 1. Daily weather data during the four experimental periods in 2001 and 2002.

 
Short-Term (1 h) Intake Rate by Dairy Cow Measurements
Fresh and DM intake rates (FMI and DMI, respectively, in kg cow–1 h–1) were estimated using the short-term weight gain method, adapted from Penning and Hooper (1985), as used by Barrett et al. (2003). Cows were weighed before and after grazing and harnessed with bags to collect urine and feces. The weight difference was corrected for metabolic weight losses. The latter were estimated for each individual cow once a week, by standing them on a concrete pad for 1 h after grazing and weighing them at the start and finish of this period. The Mettler multirange KG 3000G balance (Metler Toledo S.A., Viroflay, France) with a precision of 50 g was used.

Sixteen ‘Prim’ Holstein dairy cows were used for the experiment; they were lactating except during the last experimental period (P4). The 16 cows were divided into four groups to ensure equal milk yield and live weight between groups on the basis of pre-experimental data. At each experimental period, each cow was allocated to one of four groups balanced on the basis of parity, pre-experimental milk yield and live weight. Mean days in lactation, milk yield, and live weight were for P1: 183 d, 27.5 kg d–1, and 691 kg, respectively; for P2: 220 d, 25.7 kg d–1, and 678 kg, respectively; for P3: 172 d, 30 kg d–1, and 645 kg, respectively; and for P4 345 d, 0 kg d–1, and 723 kg, respectively. For each maturity group of ryegrass populations, each group of cows was allotted to a different population such that each group grazed each of the four populations. Cows were fasted for 3 h 45 min before the beginning of the experiment. Outside experimental time, cows were grazing in large paddock with perennial ryegrass. The cows were milked at the beginning of the period of fasting and at the end of the afternoon (1700 h). Cows started eating only perennial ryegrass at least 15 d before the experimental period.

Sward Structure Measurements
Before grazing each plot, all plant aerial parts were harvested for a 20-cm length of row, at five locations along a diagonal transect through each plot. For each sample, we counted the number of tillers longer than 5 cm (TILLERDE, number of tillers m–2) and for 10 tillers the blade and sheath length (BLENGTH and SLENGTH, respectively, in mm) of the youngest adult leaf were measured. The dry matter weight (samples were dried for 24 h at 78°C) of the leaf above and below the ligule of the youngest adult leaf (BDMW and SDMW, respectively, in mg per tiller) was established. Two variables were calculated: BYIELD = BDMW x TILLERDE that was the blade dry matter biomass per square meter (g m–2); YIELD = (BDMW + SDMW) x TILLERDE that was the total shoot dry matter biomass per square meter (g m–2).

Sward Nutritive Value Measurements
Just before grazing, between 0900 and 1000 h, fresh pooled forage samples hand-plucked from 16 to 20 locations were collected from each plot. Dry matter content (%DM) was determined (samples were dried for 24 h at 78°C). Dried samples were ground and analyzed for N (Anonymous, 1996), WSC (Lila, 1977), and NDF (Van Soest and Wine, 1967) concentration (in % of DM) and for digestibility by enzymatic solubility (IVDMD in % of DM; Aufrère, 1982).

Statistical Analysis
All the analyses were performed on plot means using SAS software (SAS Institute, 1988). For each maturity group, analyses of variance (ANOVA) for all traits were performed using the glm SAS procedure with three fixed factors: experimental period, population and replicate, and the interaction: experimental period x population. Comparison of means by Newmann–Keuls method (Miller, 1981) was obtained using the glm SAS procedure with the option: means factors/SNK. For the intermediate maturity group, differences of FMI between populations were found only for the period April 2002 (P3). To explain these differences by sward traits, a multiple linear regression (the reg procedure with the stepwise option) was performed, using the sward traits that were significantly different between populations (tiller density, length and weight of blade and sheath, and WSC content) to explain the FMI as the dependant variable.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Differences between Experimental Periods
The period when the experiment was conducted had a major effect on all measured traits for both maturity groups (Table 2). The experimental period x population interaction was not significant for any trait for both maturity groups but it was significant for the sheath DM weight within the intermediate maturity group and for the enzymatic solubility within the late maturity group.


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  		Table 2. Mean squares and significant levels of experimental period, population, replicate, and experimental period x population interaction on all traits for each maturity group.

 
Means per experimental period are compared in Table 2. During spring (P1, P2, and P3) the overall average of intake rate was 12.5 kg FM h–1 (2.3 kg DM h–1). These values are consistent with those of experiments using the same methodology (McGilloway et al., 1999; Barrett et al., 2001, 2003). These values fell in autumn 2002 (P4) to an average of 7.3 kg FM h–1 (1.3 kg DM h–1). Such a low intake rate could be due to both animal physiological status and sward characteristics: the cows were dry and the sward canopy comprised short plants, low tiller density, low yield, and low nutritive quality (Table 3). Very low levels of WSC were measured in autumn which could be the result of low levels of photosynthesis before the leaf harvest in the morning. Barrett et al. (2001) had also noted lower values of WSC at the end of September than at the end of July. Moreover, they showed an increase of WSC during the day with a particularly low value at 0700 h at the end of September.


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  		Table 3. Means per experimental period for all traits for each maturity group.

 
In the intermediate maturity group, FMI rates were not significantly different between spring experimental periods (P1, P2, and P3) despite large differences in sward characteristics. The intake of reproductive forage (P2) was similar to that of vegetative forage (P1 and P3) in spite of the former having greater sheath length and sheath DM weight associated with a high DM yield, a poor nutritive value, and a high DM content. This increased DM content led to the highest DMI rate when converting fresh into DMI rate (Table 3). Moreover, at the vegetative stage, the swards in 2001 (P1) had longer blades and a lower tiller density than the swards in 2002 (P3). Even with green leaf yield and total yield (BYIELD and YIELD, respectively) higher in P3 than in P1, FMIs were similar.

In the late maturity group, FMI rate was higher in April 2001 (P1) than in April 2002 (P3) despite similar yields, possibly due to differences in canopy structure. Tiller density was 20% lower in P1 than in P3 but blades and sheaths were 20% greater in P1 than in P3. As in the intermediate maturity group, yields were highest at the reproductive stage (P2) due to an increase in sheath length and DM. These differences coincided with a lower nutritive value and a higher DM content.

The poor quality of reproductive swards did not reduce the short-term intake rate compared to that of a high quality vegetative sward. This could have been due to a high DM content as DM forage intake by grazing ruminant is known to increase with DM content (Vérité and Journet, 1970; Butris and Phillips, 1987; John and Ulyatt, 1987). The method used to measure short-term intake rate was an estimation of the initial intake rate, which reflects the interaction between the motivation of the cows to eat and the ease of prehension of the sward (Baumont et al., 2000). Since the cows were fasted, the ease of prehension is likely to be the major determinant of initial intake rate. Quality traits of the eaten grass are more likely to induce post-ingestive effects that could regulate intake on a longer term basis. For example, it was found that daily intake was positively correlated with N concentration and digestibility of grass (Orr et al., 2003, 2004).

Differences between Populations
Intake Rate
Intake rate by dairy cows varied over to the ryegrass populations. The ANOVA reported in Table 2 shows a highly significant effect of population on both FMI and DMI rates by dairy cows within the late maturity group. Grazing the short-leaved population (SL) resulted in a significantly lower intake rate than grazing the other late-heading populations (Table 4). On average in the four experimental periods, a difference up to 1.5 kg of FMI in 1 h per cow was found. Within the intermediate maturity group, the effect of population was nearly significant (P = 0.06) on FMI rate but not significant on DMI rate. When the data were analyzed per period, highly significant population effects were found on both FMI (P = 0.0004) and DMI (P = 0.002) rates in April 2003 (P3). Intake rates on IC2 were significantly lower than those on the three other populations (Table 5).


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  		Table 4. Means per population for all traits for each maturity group.

 

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  		Table 5. Means per population and P values for all traits for the intermediate maturity group during April 2002 (P3).

 
The magnitude of differences between populations varies between periods of measurements. Table 6 shows the means and the coefficient of variation of FMI rates according to the experimental period and population. Early spring appeared to be the most favorable experimental period to run such a test with ryegrass populations. This period corresponds to maximal vegetative growth of perennial ryegrass. In northern Ireland, Barrett et al. (2003) failed to find any significant effect of grass genotype on intake rate by dairy cows in September and in June despite comparing different species and different ploidy levels. This was certainly due to a lack of repeatability with CV of 8.9% in September and 13.4% in June.


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  		Table 6. Fresh matter intake rate means per population and measurement period for each maturity group. Maximal differences of fresh matter intake rate between populations, coefficients of variation (CV) and P values are given.

 
Effects of Sward Characteristics on Intake Rate
It is clear from the results that there was a positive correlation between leaf blade length and short-term intake rate (Fig. 1 ). Highly significant differences among periods and among populations were found for leaf (blade and sheath) length. No significant population x experimental period interaction occurred for this trait. Within the late maturity group, the divergent selection for leaf length resulted in two contrasting populations, SL and LL, which were generally significantly different from the two cultivars (LC1 and LC2) for their sheath and lamina length (Table 4). The short leaf length of the SL population was associated with a significantly lower intake rate than that of the two cultivars. By contrast, longer leaf length for the LL population did not result in a higher intake rate when compared to that of the cultivars. Within the intermediate-maturity group, blade length explained 49% of the difference of FMI between populations observed in the P3 period (FMI = –4.33 + 0.1015 BLENGTH). The cultivar IC2 that showed the lowest intake rate during the period P3 (Table 4) had also the shortest leaf length (Tables 4 and 5).


Figure 1
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  		Fig. 1. Relationship between the blade length and the fresh matter intake among the eight perennial ryegrass populations on April 2002.

 
The significant differences in tiller density between populations, observed within both maturity groups had no effect on intake rate (Tables 4 and 5). For example, in period P3, although BS– and IC1 had the same leaf length and different tiller densities, they did not significantly differ in FMI rate.

Despite differences in leaf length and tiller density between populations, no significant difference was found for green leaf (BYIELD) and total (YIELD) biomass (Table 2). This absence of difference despite significant difference in sward structure indicates some kind of compensation between leaf growth and tiller density, that is, negative correlation between these two traits. This compensation is known as the self-thinning rule (Chapman and Lemaire, 1993). Breeding ryegrass for short and long leaves resulted in high and low tiller density. However, when comparing the populations, it appears that this relationship between leaf length and tiller density is flexible enough to allow concomitant genetic progress for both criteria: long leaves with high tiller density. For example, BS– seems to be a good compromise.

No genetic progress was made by selecting ryegrass for either high or low blade to sheath ratio (2.25 for BS+ and 2.24 for BS–; P = 0.96). The adjustment in the blade/sheath ratio is a major plastic response to environmental constraints such as defoliation height and frequency (Casey et al., 1999). Since genetic variation for this allometric relationship seems to be low, its effect on intake could be assessed by sward management rather than by intrinsic differences between perennial ryegrass populations.

Slight differences in nutritive value were found between ryegrass populations in both maturity groups but did not influence intake rate. Within the intermediate maturity group, IVDMD deferred significantly between populations (Table 2). Moreover, in period P3, BS+ had a significantly higher level of WSC than that of the other populations (Table 5). The BS+ had a lower FMI rate than IC1 which had similar sward characteristics except a lower WSC. When testing a ryegrass population selected for elevated concentration in WSC, Lee et al. (2001) found no effect of WSC level on intake by grazing ewes. Within the late maturity group, significant population differences were found for fiber content and IVDMD (Table 2). These differences were part of the short-leaf syndrome: selecting ryegrass for short leaves led to lower IVDMD and higher NDF content in the SL population than in the others.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Significant differences in initial forage intake by dairy cows were found between perennial ryegrass populations using a short-term live weight change method used by Barrett et al. (2003). Such a method is too costly, time and labor consuming, and too sensitive to climatic conditions to be used in a routine procedure of cultivar evaluation. Nevertheless, it can be used in research programs to identify sward traits influencing intake by grazing ruminants. The best period to perform the experiment appeared to be early spring. From a breeding prospective, the potential value of cultivars under grazing cannot be summarized by the measurement of short-term intake rate. However, the ease of forage prehension is a primary factor regulating herbage intake during short-term foraging strategy (Penning et al., 1998). Increasing the ease of forage prehension of perennial ryegrass would be valuable for its use under rotational grazing. Short-term intake measurements could help to identify plant traits related to the ease of prehension to be used as breeding criteria.

In this study, sward structure, not forage quality, made an impact on initial intake rate. Forage quality seems to be involved more in long-term intake by influencing the retention time of DM in the rumen rather than in short-term intake (Baumont et al., 2000). Variation in intake between experimental periods and between ryegrass populations were found to be mainly related to the variation in leaf length: populations of ryegrass exhibiting short leaves as a result of environmental conditions in autumn (P4) or as a result of the experimental selection for short leaves (SL) led to a low intake rate by the dairy cows. Leaf length as it determines extended tiller height and sward surface height could reduce the ease of prehension of the sward when too short. Bite mass decreases with sward height (Laca et al., 1992). Numerous studies have shown that biting rate and grazing time do not fully compensate for the change in bite mass that appears to have the greatest influence on intake rate (Forbes, 1988; Penning et al., 1998). The reduction in short-term intake induced by short-leaved swards is likely to impact intake rate on a long-term basis. Consequently, long leaves are desirable for forage cultivars of perennial ryegrass. Furthermore, leaf length is easily assessed and has a high heritability (Hazard et al., 1996). The blade/sheath ratio that could have influenced intake through the barrier effect of the sheath is not a good criterion to improve ryegrass ease of prehension since the experimental selection for low and high ratio was inefficient in creating contrasting progenies for that trait. There is a need for further investigation of the role of physical characteristics of the sward in influencing short-term intake rate under grazing condition. It is not clear from our results if DM content operates just as a corrective term when shifting from FMI to DMI, or if it affects the ease of prehension strength of the sward as does the resistance to fracture (Inoué et al., 1994).


   ACKNOWLEDGMENTS
 
We thank all the technical staff involved in this work, particularly the people who had to manage the cows. This work was supported by a grant from the Ministry of French Agriculture in the frame of the French forage breeders association (ACVF).


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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