TIPOS DE METODOLOGÍAS PARA LA DETERMINACIÓN DE

3.4.1

Morphology of perennial ryegrass

This study provides information on key morphological characteristics of perennial ryegrass affected by the presence of white clover and cultivar. The presence of white clover exerted little effect on the morphological characteristics of perennial ryegrass, such as the proportion of the lamina,

pseudostem, reproductive stem and dead material. This may reflect a low overall proportion of white clover (< 10% DM above ground level) and thus its little opportunity to affect perennial ryegrass morphology. Further, it is noted that quantitative trait loci are responsible for morphological traits in perennial ryegrass, including plant height, tiller size, leaf length and leaf width, with little

environmental impact (Yamada et al. 2004). Therefore, perennial ryegrass’s morphology remains stable no matter growing with or without white clover.

As for the cultivar effect, a key result was that lamina proportions were greater in tetraploid than diploid cultivars at the vegetative stage, which agrees with the previous study (Orr et al. 2004). At the reproductive stage, the emergence of inflorescences reduced the proportions of both lamina and pseudostem and thus diminished their differences among cultivars. Similarly, Laidlaw (2004) found that cultivars of different heading dates expressed no significant differences in leaf lamina proportion after heading. In this study, under rotational grazing, intermediate-heading perennial ryegrass cultivars, Kamo and Commando, did not produce more reproductive stems than late-heading

cultivars, but their reproductive stems had greater ADF and NDF concentrations, suggesting that they were more mature.

3.4.2

Nutritive value of perennial ryegrass morphological components

There were interactions between morphological component and cultivar for nutritive value, indicating that the nutritive value differences among cultivars were different in each morphological component. To be specific, greater differences in the CP concentration among cultivars were found in lamina than pseudostem and reproductive stem, while the variation in the WSC concentration was greater in pseudostem than lamina. Despite the interactions, lamina consistently had a greater CP and a lower WSC concentration than pseudostem, which agreed with previous studies (Waite & Boyd 1953; Chaves et al. 2006b). These findings showed that a greater variation in the concentration of a certain nutrient among cultivars would be found in the plant part that had a greater concentration of this nutrient.

sheaths than the leaf blades (Wilman et al. 1996). WSC are highly digestible (Heeren et al. 2014) and accumulates in the leaf sheaths (Fulkerson & Donaghy 2001), which could also contribute to a greater digestibility of the pseudostem. In practice, the dead material attached to the sheaths, making the digestibility low in the lower sward layer. In terms of feeding value, which is a

combination of herbage nutritive value and voluntary intake (Chapman et al. 2014), lamina is still greater than pseudostem, because pseudostems are hard to access (Chapman et al. 2012) and require more breaking force to be ingested (Bryant et al. 2008). Therefore, it is still necessary to increase lamina proportion and depress the amount of dead material through pasture management to achieve a higher herbage feeding value (Casey et al. 1999).

3.4.3

Effect of white clover on nutritive value of perennial ryegrass

Including white clover into perennial ryegrass pastures led to a greater CP concentration and lower ADF and NDF concentrations of overall herbage (Chen et al. 2016). This was due to a high CP concentration and a low fibre concentration of white clover than perennial ryegrass (Evans et al. 1996). Additionally, in this study, the presence of white clover increased the CP concentration in the accompanying perennial ryegrass at two vegetative stages. This could be a consequence of the N transfer from legumes to grass via decay of plant tissues, root exudation of ammonium and amino acid and mycorrhizal hyphae (Paynel et al. 2008). Interactions between white clover and

morphological components indicated that lamina responded more sensitively to the presence of white clover than pseudostem.

At the reproductive stage, the presence of white clover resulted in lower herbage WSC

concentrations in Base, Alto, Kamo and Prospect, but greater herbage WSC concentrations in Bealey, AberMagic, Commando and One50. Fibre concentrations (ADF and NDF) decreased in Bealey, AberMagic, Commando, Kamo and One50, with greater digestibility when white clover was included. In contrast, other cultivars were the opposite. These interactions indicated that growing with white clover had different effects on nutritive value for different perennial ryegrass cultivars at the reproductive stage. The reason for these interactions is not clear, but it may be related to the interactions between perennial ryegrass cultivars’ reproductive growth and external N input (Bahmani et al. 2001b). However, the interaction only occurred at the reproductive stage and the magnitude was small.

3.4.4

Effect of cultivar on nutritive value of perennial ryegrass

In this study, the high-sugar cultivar, AberMagic, had a higher nutritive value, as determined by the greatest herbage WSC concentration and the greatest DOMD with low ADF and NDF concentrations. Tetraploid cultivars, Base and Bealey, followed, while the intermediate-heading cultivars, Commando

and Kamo, had a relatively low herbage nutritive value. These findings confirmed that high-sugar cultivars (Miller et al. 2001; Lee et al. 2002; Moorby et al. 2006), tetraploid cultivars (Wims et al. 2013; Solomon et al. 2014), late-heading cultivars (Gowen et al. 2003) had a higher nutritive value and the potential to increase milk production, compared with their counterparts.

Herbage nutritive value varied in different seasons and at different regrowth stages, and this was partly due to the changes in the ratio of green leaf, stem and dead material, which are different in chemical composition and digestibility (Stewart & Hayes 2011). However, the results suggested that the differences in morphology (such as lamina proportion) is not the main reason why perennial ryegrass cultivars differed in nutritive value. There are three findings supporting this point. Firstly, nutritive value varied among cultivars within the same morphological component (Table 3, 4 and 5). For example, the high-sugar cultivar (i.e. AberMagic) had a greater WSC concentration than all the other cultivars in all morphological components, including the lamina, pseudostem and reproductive stem. Secondly, it was found that lamina had a greater CP concentration and a lower WSC

concentration than pseudostem. However, cultivars with greater proportions of lamina did not necessarily have greater overall herbage CP concentrations, while a greater proportion of

pseudostem did not lead to a greater overall herbage WSC concentration. For example, tetraploid cultivars had greater lamina proportions, but their CP concentrations were not always greater than diploid cultivars. For another, Commando and Kamo had the greatest proportions of pseudostem at the pre-heading vegetative stage, but their WSC concentrations were less than AberMagic. Finally, when morphological proportions were similar among cultivars at the reproductive stage, the chemical composition of cultivars was still different regardless. Therefore nutritive value variation among cultivars, in part, reflected the genetic variation in metabolism of nutrient synthesis and turnover (Wilkins et al. 2000), but not the morphology.