Validación de hipótesis

The digestive disorder ruminal tympany (bloat) is associated with many forage legumes, including red clover and lucerne, which has a much stronger effect on cattle than sheep (Frame, 2005; Kemp et al., 1999). In leafy productive legumes there are high levels of plant proteins that cause frothy gas content in the rumen. The occurrence of bloat occurs when the animal’s production of gas exceeds its ability to release the gas (Majak, McAllister, McCartney, Stanford, & Cheng, 2003). There are various preventative techniques to avoid the digestive disorder such as supplementary minerals and other additives. Management techniques are used including monitoring the grazing herd, as well as using mixed swards containing grass and a given legume species, which is commonly practiced throughout New Zealand to avoid this problem (Frame, 2005).

2.6.2. Persistence

Longevity of red clover stands could be considered the greatest limitation especially as persistence of a pasture is the basis of profitability (Ford & Barrett, 2011). This is why great importance should be placed on the management of a stand to avoid unfavourable conditions. Generally persistence can be strong for 2 to 3 years in a pasture stand followed by a steep decline (Frame, 2005; Kemp et al., 1999). As a result compensatory growth occurs by remaining plants in a stand as population numbers decrease which can emphasise plant loss when it is discovered (Frame et al., 1998). Persistence is further limited in places of low rainfall, especially less than 500-800 mm (Hyslop, 1999). Breeding red clover to gain more vigorous germplasm has been the

most common goal of breeders since red clovers popularity grew as a forage legume (Abberton & Marshall, 2005; Wang, Hampton, & Hill, 1994).

Crown integrity and resistance to external damage among other morphological traits are apparently the largest contributors to susceptibility to pathogens (Abberton & Marshall, 2005). Frequent grazing decreases future persistence of red clover, as carbohydrate reserves are depleted in the tap root (Brock et al., 2003; Kemp et al., 1999), especially under heavy grazing in winter which will further decrease stand population (Kemp et al., 1999). This can be attributed to the degradation of tap root and reliance on the more shallow adventitious root systems (Spedding & Diekmahns, 1972). Grazing when soil has high moisture content can cause treading damage opening opportunity for disease and general plant destruction (Hay, 1985). Tap roots of red clover root as deeply as lucerne or sweet clover and are commonly prominent for 18-24 months; subsequently the rooting systems is largely made up by adventitious roots (Sheaffer et al., 2003; Spedding & Diekmahns, 1972). Grazing is examined further in the section (Red Clover Growth and Development, 2.4). The survival of the tap root in red clover is a fundamental determinant of the perpetual life of the plant (Abberton & Marshall, 2005; Kemp et al., 1999). Experiments conducted by Skipp & Christensen (1990) illustrated the destruction of functional tap root by root rot or other pest disturbance caused early death to majority of the affected plants. The deterioration of the tap root or the crown is a primary way that population numbers are depleted (Skipp & Christensen, 1990).

2.6.3. Oestrogens

After some confusion in the 1960s over which isoflavone was causing reproductive problems in ewes, ‘formononetin’ was proven to cause oestrogenic effects related to the conception rates after passing through the rumen (Anwar, 1994; Kemp et al., 1999). A trial undertaken by Shackell, Wylie, and Kelly (1993) indicated that long term exposure to ‘ Grasslands Pawera’ which possessed high levels o f phytoestrogens caused significant reduction in ewe reproductive performance. Today, clover induced infertility is a well-documented topic in terms of reproductive issues with ewes (Kelly, Shackell, & Allison, 1980; McDonald, Anwar, & Keogh, 1994; Turnbull, Braden, & George, 1966). Grazing before mating with high levels of formononetin in a pasture is now avoided with ewes (Kemp et al., 1999). When affected by oestrogenic red

clover, ewes can display two variations of infertility which are temporary or permanent infertility (Anwar, 1994). There are also negative reproductive effects that can be seen in cattle but there is no scientific confirmation that a permanent infertility has appeared from continuously grazing of phytoestrogens, and therefore it is suspected to have weaker effects on cattle and deer (Kemp et al., 1999). However, there are now low formononetin level cultivars which are available on the market (Stewart & Charlton, 2003). These low formononetin cultivars include the two red clover cultivars ‘ Grasslands Relish’ and ‘ Grasslands Sensation’ which are included in this study (Ford & Barrett, 2011).

2.6.4. Pest and Diseases

During establishment there is a low capacity to compete with weeds especially in terms of pure stands. As canopy cover develops red clover stands gain better competiveness (Frame, 2005). The primary axis for red clover, the crown and roots, are the primary way in which plant persistence can be jeopardised (Skipp & Christensen, 1990). Predominant crown disease (Selerotinia trifoliorum) and root diseases

(Fusarium andRhizoctonia) are responsible for the rotting of the primary axis (Hyslop,

1999; Skipp, Christensen, & Biao, 1986). Wounds that occur to the plant from grazing or otherwise open up the plant and particularly the crown for fungal and bacterial infections will usually cause death. Variation amongst cultivars also changes susceptibility, for example, late flowering cultivars have often had higher persistence in mixed swards as well as ploidy levels have displayed positive correlated effects for pest and disease resistance and therefore persistence (Hay & Ryan, 1989). There are many insects that feed or attack red clover including nematodes, pea aphid, slugs, grass grub, porina and clover leaf weevil (Hyslop, 1999).