Agronomy and Crop Improvement
Utilisation and management of white clover (Trifolium repens) for dairy cows in pastures
Introduction The utilisation of pastures for grazing largely depends on careful monitoring of conditions and their effect on crops and livestock, and the flexibility in management to optimise production. The main constraints on dairy pasture productivity are (i) the unpredictable varying seasonal conditions (rainfall, temperature fluctuations) (Campbell and Bryants, 1978), (ii) vegetation sensitivity to abiotic stress, competition (Sanderson et al., 2003), diseases, pests, trampling (Drewry et al., 2008), (iii) soil composition (Greenwood and McKenzie, 2001) and (iv) correct application of fertilisers (Andrews et al., 2007). The introduction of white clover (WC, Trifolium repens, Fabaceae) into pasture-based dairy production systems as fresh feed or silage, has the potential to (1) increase milk yield and (2) decrease nitrogen (N) fertiliser application (Lane et al., 2000; Lane et al., 1997; Caradus et al., 1996; Chestnutt and Lowe, 1970). Milk yield per dairy cow between 2001-2002 and 2011-2012 has increased by 18% on average but the number of dairy cow numbers declined by 19% (DairyCo, 2012) in the UK. Overall, the total milk production on dairy farms has decreased by 4% (Andrews et al., 2007). Therefore, improving dairy pastures in a cost-effective and sustainable way is crucial to sustain milk production in the UK. For example, in New Zealand, the financial contribution of white clover to the economy reaches the total of $3.1 billion (Caradus et al., 1996) through nitrogen fixation, forage yield, seed production and honey production. The major biotic constraints of WC is to maintain an optimal clover content within a pasture (Harris et al., 1997) while it has numerous competitors such as grasses, weeds, pests, diseases in the field while sustain its growth under trampling pressures by the cows. This essay reviews the major benefits of WC introduction in dairy pastures, requirements for its establishment, and effects of different management practices and briefly discusses appropriate crop protection and breeding strategies. Main benefits of WC compared to grass monoculture dairy pastures One of the major benefits of WC is its ability to fix nitrogen with Rhizobium trifolii. It is estimated that 200-300 kg N per annum can be fixed by WC inclusion which varies on the WC content within the pasture and it can save ÂŁ125-160 ha-1 annum-1 on a field (Andrews et al., 2007). Numerous studies have investigated the effect of N fertiliser and have shown that 400 kg N/ha application negatively effects clover growth and morphology (Harris et al., 1996), while 150 kg N/ha application has shown to increase clover content, but 250 kg N/ha and 200 kg N/ha had no significant effect under low (3.2 cows/ha) and high (4.5 cows/ha) stocking rates (Clark and Harris, 1996; Egan et al., 2018). McDonagh et al. (2017) found that WC on a mixed pasture with 100 kg N/ha under 21 grazing days yielded 1009 kg dry matter (DM)/ha more than on 250 kg N/ha pure perennial grass pasture with 30 grazing days throughout the whole year. Therefore, WC reduces the cost of N fertilisation by its N fixing ability and produces higher yields than ryegrass pastures with high amounts of fertilisers.