Growth and urinary nitrogen excretion in soil of heifers with diverse genetic merit grazing low quality forage

Dairy cattle that graze perennial ryegrass and white clover pasture often receive nitrogen (N) in excess to their requirement (Pacheco and Waghorn 2008). This excess N can be lost to the environment, particularly when water infiltration and runoff levels are high such as in autumn and spring (Di and Cameron 2002). Cows with greater genetic merit have increased feed efficiency and reduced N excretion (Cheng et al. 2014). Limited studies have been conducted to understand the differences between animals with high and low genetic potential in a dryland grazing system with low-quality forage typical of south eastern Australia. This study was conducted on a dryland ryegrass pasture at Dookie college in northern Victoria to evaluate the growth and urinary N loading of heifers with diverse genetic merit. Forty-eight Holstein Friesian heifers with known genotype of Balanced Performance Index (BPI) were grouped into high (HBPI; n = 24) and low (LBPI; n =24) genetic groups. Each genetic group was further divided into six replication grazing groups of four heifers per replicate. Apparent dry matter intake (DMI) was calculated based on the difference between pre- and post-grazing herbage mass, herbage regrowth rate and the area grazed for 29 days. All heifers were weighed after a fasting for twelve hours overnight at the start and the end of the study. Blood and urine samples were collected from each heifer on measurement days 16 and 25 for analysis of plasma urea N (PUN). Urinary N excretion was estimated from PUN and live weight.

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33rd Australian Association of Animal Sciences Conference, 2021, Fremantle, Perth

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