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For condensed tannins in white clover to be of use in agriculture, the clover needs to produce enough feed for animals, and to persist long enough to be cost effective. HT clover had good winter and autumn growth, but grew less herbage than Huia in spring and summer, which is a disadvantage for seasonal dairying. Good summer growth of white clover is important for maintaining milk production when pasture quality is declining due to ryegrass flowering and accumulation of dead matter (Woodfield et al. 2001). Leaf size and number per stolon were similar between treatments, so the higher seasonal dry matter yields of Huia over HT were probably due to its higher growing point density.

HT clover has not undergone any selection for agronomic performance and hence may lack adaptation to intensive grazing. Siral white clover, which was used as a parent of HT because of its unusually prolonged flowering, was developed in Australia from an

Algerian ecotype. Woodfield & Caradus (1 994) compared Siral to Huia under sheep

grazing and found lower growing point densities, a lower proportion of clover in the pasture and lower clover DM production for Siral than for Huia. In New Zealand, Mediterranean clovers such as Siral are often winter active and become summer dormant during periods of high temperatures and drought (Caradus 1 994). These effects were evident in this experiment when comparing HT to Huia.

Stolons are the primary sinks for carbohydrate storage in white clover (Hart 1 987) and therefore support clover persistence in temperate environments. Stolon yields were greater for Huia than HT for the first two measurements, 1 5 and 1 7 months after sowmg. This difference, and the consistently lower growing point density in HT suggests HT plants are likely to be less persistent than those of Huia. This reduced persistence is evident in the loss of treatment effects in 2003 and the high levels of contamination in HT in January 2004.

3.5.3 Clover morphology

Plant b ranching order and loss of tap root

The transition of plants from tap rooted to clonal is associated with a decline in productivity and persistence, as plants are smaller and rely on small nodal roots for water and nutrient supply. The faster loss of tap roots in Huia than HT may relate to the former having larger and more complex plants at the first measurement (Brock &

Tilbrook 2000).

This loss of tap roots was faster than previously reported by Brock & Tilbrook (2000) and Brock et al. (2000) in sheep grazed perennial ryegrass/white clover pastures. This may be due to greater damage from treading in cow versus sheep grazed pastures, more rapid plant development in monoculture or greater attack by pathogens (Thomas 2003).

The average distribution among branching orders of plants in this experiment (27% first, 47% second, 2 1 % third, 4% fourth, 1 % fifth and sixth) was similar to that reported

for Waikato dairy pastures (21 % first, 47% second, 25% third, 6% fourth, 1 % fifth and sixth) by Harris (1 994), but with slightly more first order and less third order plants. Once all plants had assumed a clonal growth form (September 2002), more than 20% of the population was as first order plants, except for Huia in Nov 02. In contrast, Harris

( 1 994) reported more than 20% of the population as first order only from August to December and in February.

First order plants are normally most abundant in spring (Brock et al. 1 988; Harris 1 994;

Pinxterhuis 2000), but were highest in January 2003 in this experiment. As these

pastures were sown as mono culture, grazing frequency differed to that in mixed ryegrass/white clover pastures. The lack of grazing in winter and early spring in this experiment may have reduced spring fragmentation, thereby reducing the proportion of first order plants at this time compared with other experiments. White clover stolons also have the potential to initiate branches during mild winters, when competition from

other species is minimal (Patterson et al . 1 995).

Differences between treatments in flowering and in some cases, stolon length, were evident from January 2002 to January 2003, but disappeared after a dry spell in February 2003. Mortality of HT plants may have been low in the first summer due to the majority of plants still being tap rooted. The higher proportion of first order plants in HT than Huia in spring 2002 may have increased plant mortality over the following summer.

Stolon length, branching and flowering

Differences between treatments in growing point densities were not directly related to the number of branches per plant, per node or per stolon. HT had more intact plants per turf than Huia in November 200 1 , so despite having less stolons per plant, it had similar growing point densities to Huia. The trend for higher densities in Huia for the remainder of the experiment could not be clearly explained by plant morphology.

The higher flower densities in HT than Huia were due to a greater number of flowers per stolon and per node. Increased flowering can reduce stolon branching (Thomas 1 980), as each node has the potential to produce either a flower or a branch. This relationship was not evident in this experiment. The proportion of nodes supporting

either a flower or a stolon branch was always less than 20% for both clovers. However, the flux of birth and death of branches and flowers was not measured, and the percentage of nodes that did branch or flower could therefore have been considerably higher. Newton et al. ( 1 992) noted that 40% of axillary buds may be non-viable. Loss of viability may have had a greater effect on branching than the loss of sites due to flowering, as less than 6% of nodes contained flowers at any one time in both Huia and HT.

Turves used to study white clover morphology were collected in areas that did not have excessive weeds or bare ground, whereas growing point density measurements included all areas. As HT pastures tended to have more weeds in autumn and winter, and more bare ground in summer, growing point densities were lower. Huia had similar numbers of stolons per plant to HT on most occasions, but tended to have more intact plants per turf. It appears that differences between treatments in growing point density were due mainly to better survival of Huia plants. Flowering has long been associated with reduced clover persistence (Gibson 1 957), and the shorter internode length of Huia produced denser stolon branching, providing less room for weeds to grow. Differences between the clovers in root characteristics or resistance to invertebrate pests or viruses may have also affected persistence (Woodfield & Caradus 1996) but these were not studied.