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Ayers, Ariel

Animal and Veterinary Sciences

2020

M.S.

Organic dairy producers rely on pasture-based diets during the grazing season to provide their lactating herd with the nutrients to support lactation. This can present challenges for producers in the Northeast US, as pasture nutrients can vary with season and weather, ultimately impacting milk productivity and animal health. There is currently limited literature outlining feeding guidelines for producers to meet dairy cattle nutrient requirements on pasture-based systems. This thesis outlines an investigation of the nutritional strategies that are utilized on organic dairy farms in Vermont, as well as a case study utilizing a modified supplement to identify the implications of this modified supplement profile on animal health and productivity during the grazing season. The first objective (Chapter 2) was to survey nutritional management strategies commonly used on organic operations in the Northeast, and evaluate the impact on animal health and productivity. Sixteen Vermont organic dairy farms were evaluated during the grazing season of 2017 and data collected included management, animal, and pasture data. Farms were assessed at three timepoints throughout the grazing season, including once in the spring, summer, and fall. Farms were later divided into two groups based on their measured milk production for subsequent comparison: 1) high milk group (HMG, n=8) or 2) low milk group (LMG, n=8). Non-forage supplementation for the HMG ranged from 4.78-7.03 kg DM per animal and the LMG ranged from grass-only to 7.93 kg DM per animal. Protein and fat yield were greater in the HMG compared to the LMG. Protein yield decreased from spring to summer, and remained similar from summer to fall. The rumen-undegradable protein (RUP) concentration in pasture followed a similar trend as the protein yield observed in both HMG and LMG. The second objective (Chapter 3) was to utilize a modified supplement on commercial organic dairy farms during the grazing season to determine whether dietary protein limitations were the primary factor limiting milk production on organic dairies in Vermont during the grazing season. Six Vermont organic dairy farms participated in a 6-week trial consisting of a 2-week baseline period and 4-week experimental period, with management, animal, and pasture data collected throughout the 6-week trial. Farms were paired by their 2017 summer milk urea nitrogen (MUN) profile, and farms within each pair were assigned to 1) continuation of their regular supplements (n=3, control group, CON), or 2) supplement targeted to 16% crude protein (CP, as % of dry matter) formulated using an organic barley and roasted soybean mix (n=3, treatment group, TRT). The modified supplement CP (% DM) averaged 14.8% for CON and 19.4% for TRT during the experimental period. Milk production was higher during the experimental period for TRT vs. CON. Milk composition was different between groups, with fat and protein percent being higher in the CON group compared to the TRT for the 6 weeks. During the experimental period TRT had higher MUN concentrations compared to CON. This research outlines nutritional strategies used on organic dairy farms in Vermont and identifies some potential nutritional drivers impacting milk production and composition. These results also indicate that altering the CP content of dietary supplements fed to grazing organic dairy cattle during the summer period in the US Northeast could be a useful mechanism to maintain milk production. Further research could be done to investigate the economic variables for these feeding strategies.