Grazing and supplementation strategies to improve pasture intake, milk production and reduce nitrogen pollution of grazing dairy cows in autumn

Abstract

During autumn, the low dry matter intake, low energy intake and imbalanced energy/protein ratio from pasture are the main problems in grazing dairy systems, which consequences are low milk production and high urine N excretion. Time of supplementation and changes in the grazing management such as pre-grazing herbage mass and time of pasture allocation have been evaluated as strategies to improve milk production and reduce urine N excretion, in response their effects on dry matter intake, energy intake, grazing behaviour and rumen function. However, the results have not been consistent among studies, suggesting that combination between these strategies could enhance their individual effects on dry matter intake, milk production and N partitioning. The aim of the present doctoral thesis was to evaluate the influence of time of pasture-supplement allocation and pre-grazing herbage mass on milk production, grazing behaviour, rumen function and nitrogen partitioning of lactating dairy cows during autumn. The first Chapter evaluated the effect of time of fresh herbage allocation and grass silage allocation on dry matter intake, grazing behaviour, milk production and nitrogen partitioning of grazing dairy cows during autumn. The dry matter intake, milk production, grazing behaviour and nitrogen partitioning were similar between treatments, however, morning grazing time was longer in cows receiving a new fresh pasture in the morning, while afternoon grazing time was longer in cows receiving a new fresh pasture in the afternoon. The second Chapter evaluated the effect of time of grass and maize silage supplementation on milk yield, grazing behaviour and N partitioning of lactating dairy cows during autumn. Dry matter intake and grazing time were similar among treatments, however, the time cows spent grazing between afternoon and morning milking was longer when maize silage was offered in the morning and afternoon compared with afternoon allocation. Milk production and urine N excretion were greater when maize silage was offered in the morning compared with was offered after morning and afternoon milking. The third and fourth Chapter evaluated the interaction effect of timing of herbage allocation and herbage mass on milk production, grazing behaviour and daily pattern of rumen ammonia production and nitrogen excretion of lactating dairy cows during autumn. Dry matter intake was similar between treatments; however, grazing time was lower when a high herbage mass was allocated in the afternoon. Milk production was greater when a high herbage mass was allocated in the morning or afternoon (H-AM and H-PM, respectively) compared with a low herbage mass allocated in the morning (L-AM). Concentration of urine N was lower in H-PM compared with H-AM and L-AM. It was observed a greater urine N concentration at 10:00 hours for H-AM. In addition, urine nitrogen concentration was greater for H-PM compared with H-AM at 16:00 hours. In conclusion, the interaction between time of pasture and supplement allocation and herbage mass modified the grazing time, milk production and nitrogen partitioning, although dry matter intake was not affected. However, the interaction between time of pasture allocation and herbage mass allowed a greater change in milk production and nitrogen excretion, in response its great effects on nutrient intake, grazing behaviour and rumen function.