|dc.description.abstract||Lucerne (Medicago sativa L.) is a major forage legume grown in approximately 45 million hectares worldwide. It is an excellent candidate for irrigation and can produce large quantities of quality herbage due to its high water use efficiency and deep root system. However, in New Zealand, the growth rate of lucerne is reduced in autumn/winter and early spring when temperature and solar radiation decrease. These periods of feed deficit usually provide opportunities for the use of supplementary feed to sustain high dry matter intake (DMI). Supplements are also offered as they may dilute the amount of nitrogen (N) eaten, thereby potentially reducing the quantity of N excreted in dairy cows grazing high N forages such as lucerne. Currently, little data exists on lucerne-supplement interactions in New Zealand dairy systems. This research study investigated the effect of feeding maize or grass silage supplements on dry matter intake (DMI), milk-solids (MS) yield, substitution rate (subR), grazing behaviour and N-output of dairy cows offered fresh lucerne in autumn. A field experiment was conducted at Lincoln University Research Dairy Farm (LURDF), Canterbury, New Zealand, between March and May, 2013. Three diets consisting of: 1. lucerne only; 2. lucerne plus maize silage, and 3. lucerne plus grass silage were allocated to 30 dairy cows in an incomplete randomised crossover design comprising of two periods of 15 days each. The silage supplementation, regardless of silage type, increased both total dry matter intake (DMI) and metabolisable energy intake - MEI (MJ ME/kg DM) compared to the lucerne only treatment, and reduced DM intake of lucerne herbage by up to 1.36 kg DM/cow/d (P = 0.04), resulting in a subR of 0.44 ± 0.03 kg DMI of lucerne/kg DMI silage and marginal milksolids response (MMR) of 0.02 ± 0.01 kg MS per kg DM silage intake. The type of silage had no effect on milk solids or milk and feed conversion efficiency (P > 0.05).
Generally, cows had low utilisation of lucerne herbage per hectare (44 ± 2%) and high utilisation of lucerne per cow (88.5 ± 3.8%), which had no effect on the rate of liveweight (LWT) gain and body condition (BC). The silage supplements reduced grazing time by up to 7 minutes (P < 0.001) during the morning grazing session of 4 hours, which could have been the effect of the supplements on physical fill of the rumen. The nitrogen use efficiency (NUE) averaged at 18.8 ± 0.5% for all cows in the three dietary treatments; however, feeding maize or grass silage did not improve the NUE. The supplements reduced the excretion of N via urine by 19 % and increased faecal N output by 8.7% and 4.0% for cows fed grass and maize silage respectively. The concentration of milk urea nitrogen (MUN) was highest in cows grazing lucerne only (15.85 ± 0.39 mmol/L; P = 0.044) compared to the supplemented cows (15.12 ± 0.39 mmol/L). The dietary treatments had no effect on urinary concentrations of creatinine, purine derivatives (PD) i.e. allantoin and uric acid, calculated PD index and the total microbial N supply. Conclusively, findings of this study show that feeding maize or grass silage to mid-late lactation dairy cows grazing lucerne in autumn significantly increased total DMI and MEI; however cow responses to the supplements in terms of milk-solids yield, subR, MMR, FCE, LWT gain and BCs were generally low. This therefore implies that supplementary maize or grass silage feeds can be used in autumn to sustain high DM and ME intakes, and enable substitution of the sown lucerne crop to increase herbage cover on the farm during periods of reduced growth in autumn/winter and early spring. The results also demonstrated the use of either maize or grass silage as a mitigation strategy to reduce nitrogen excretion through urine in dairy cows grazing autumn lucerne. Thus, based on these results, it would be appropriate to recommend the restriction of lucerne allowance when supplements are fed. This would improve the utilisation of lucerne per hectare, reduce the N-intake (particularly from lucerne herbage), and maintain milksolids production, milk yield and liveweight gain in mid lactation.||en