Effect of winter crop systems on rumen degradation and grazing behaviour of dairy cows

Abstract

Winter forage crop-fed dairy cattle excrete the largest proportion of their annual nitrogen (N) leachate from the dairy system, during the winter period. Adjusting the supply of N to carbohydrate in the diet is suggested to enhance the use of dietary N by the reticulorumen microbial population, reducing the amount of N lost in urinary excrement. 150 pregnant, non-lactating, spring calving Friesian Jersey cross dairy cattle were randomly blocked to three wintering systems: HK, early sown kale, high allowance with barley straw (14, & 3 kg DM cow⁻¹ day⁻¹), MK, late sown kale, low allowance with green chop oat silage (11, & 5 kg DM cow⁻¹ day⁻¹), and FB, fodder beet with ryegrass balage (8, & 6 kg DM cow⁻¹ day⁻¹). Using the in sacco method, three ruminal cannulated cows blocked to each wintering system had their respective crop (leaf and stem together) incubated for 0, 3, 6, 9, 12, 24, and 48 hours intra-ruminally. On three separate occasions 5 random cows from each winter system had their grazing behaviour observed for 6 hours, including grazing activity, stance, and fecal and urination events. In the in sacco study, crops had similar organic matter (OM) degradability fractions with total rumen degradability reaching 91.5%. Similarly N degradability was also similar for the three crops at 98%, where N disappearance rate was more rapid for FB compared with kale diets. The apparent disappearance rate of N to OM was lowest for FB to that of both kale diets (4.6 to 27.3 verse 13.5 to 48.4 g N kg OM⁻¹), suggesting N use by reticulorumen microbes may be more enhanced in the FB diet. In the behaviour study, FB cows spent less time grazing crop than kale feed cows (156.7 verse 254.7 minutes). By 6 hours from allocation due to the high digestibly of all crops intake satisfactory behaviour possible indicated the end of the grazing bout, with maximum crop utilisation achieved (98, & 88 %, FB and kale diets) by this time. FB diet could be expected to have the most influence on reducing urinary N loss in winter grazing systems, with microbial N use. Rapid crop intake allows for wintered cattle to be feed in situ while managed with a standoff pad to capture and spread nutrient output, potentially migrating environmental risk.