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Farnan, Kirsty (2025) Effect of diet composition on the carbon footprint of dairy herd replacements and milk production. PhD thesis, University of Nottingham.

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Abstract

In the UK, dairy livestock contribute significantly to anthropogenic greenhouse gas (GHG)

emissions, with enteric methane being a major source, along with nitrous oxide is emitted as

ammonia from animal manure, fodder, and/or pastureland, and indirect emissions from nitrate

leaching. Feed resources used to formulate livestock diets play a crucial role in managing

methane, nitrous oxide, and ammonia emissions, while carbon dioxide emissions are

influenced by the demand for food grains, grazing land, and energy usage for farm operations,

feed transport, and deforestation. Dairy herd replacements are a substantial economic and

environmental cost of milk production, making herd replacement rate (RR), age at first calving

(AFC), and lifetime milk production (LMP) are important factors in optimizing feed use and

reducing the carbon footprint (CO2-eq.) of milk production. In the UK, dairy herd sizes and

individual cow milk yields have continued to increase, so Experiment 1 aimed to survey the

average lactation, RR, and AFC of 139 high-yielding herds across Scotland, Wales, and the

NW, SW, East, and West Midlands of England. The median annual milk yield was

approximately. 10,500 L/cow from 3.43-year-old cows, with a median 2.2 parities, 24.2 mo.

AFC (range 22.7 to 32.6 mo.). The median first lactation milk (FLM) yield was 70.4% of that

of adult cows, and the RR was 22.7% (range 19.0% to 29.8%). These findings suggest that

reducing CO2-eq./kg FPCM yield could be achieved by increasing FLM yield and reducing

variability in AFC and RR. However, few studies have assessed the CO2-eq./kg of dairy heifer

diets, which was assessed in Experiment 2, which evaluated the CO2-eq. of dairy heifers and

lactating diets using a global feed database and dairy heifer feed intake data. The CO2-eq./kg

of preweaning diets was higher, primarily due to the inclusion of milk replacer, compared to

growing dairy heifer diets. Feed CO2-eq./kg increased as dairy heifers grew older, reflecting

higher DMI and extended AFC. Early lactation diets had higher feed CO2-eq./kg than mid and

late-lactation diets, due to greater inclusion of concentrated ingredients. Improved calf health

and milk replacer feeding rates enhanced pre-weaning ADG, which could lower AFC and

increase MW at first calving, reducing premature culling and RR. Experiment 3 assessed the

effect of live yeast (LY) on pre-weaning ADG and FE in dairy heifers offered calf starter

concentrate with LY (n = 30) and no LY (n = 30). dairy heifers offered starter with LY had

greater pre-weaning ADG (P 0.049) up to 14 d postweaning (P 0.040), as well as higher

postweaning live weight (P 0.040), hip height (P 0.029) and width (P 0.007) and FE (P 0.006)

up to 84 d of age, and lower AFC (P 0.001) and feed CO2-eq at 12 mo. of age (P 0.022).

Additionally, FLM yield (P 0.019) and FPCM yield (P 0.049) were higher in the LY groups. In

growing dairy heifers, providing an adequate supply of digestible rumen undegradable protein

(DUP) and amino acids (AA) is critical in achieving sufficient ADG and mature weight at a

timely AFC. Experiment 4 investigated the effect of increasing dietary DUP supply with rumen

protected methionine and lysine in dairy heifer diets between 12 and 15 mo. of age. Seventy

17

six Holstein Friesian dairy heifers were offered either low (3.1%) or high (15.1%) DUP levels,

with RPML. Dairy heifers offered high DUP with AA had significantly greater ADG (P 0.036),

live weight at 15 months (P 0.012), FLM yield (P 0.038), FPCM yield (P 0.016), survival rate

(P 0.031), and fertility-related CO2-eq. (P=0.031), and feed CO2-eq./kg FPCM yield, though

body condition, AFC, and fertility did not differ between the groups. In conclusion, UK dairy

herds with high milk-yielding cows could reduce CO2-eq. of milk production by lowering herd

RR, reducing the variability in AFC, and increasing the FLM yield to reduce the risk of

premature culling, and increasing LMP, and lowering feed CO2-eq. related to dairy heifers.

Greater precision in dairy heifer nutrition, such as incorporating LY in calf diets and increasing

DUP with AA in growing heifer diets, can improve FLM yield, lower herd RR, and decrease

feed CO2-eq. and CO2-eq./ kg of FPCM yield.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Margerison, Jean Adejoro, Festus Connerton, Ian
Keywords:	Heifers; Carbon footprint; Growth; Replacement; Calving age; Yield
Subjects:	S Agriculture > SF Animal culture
Faculties/Schools:	UK Campuses > Faculty of Science > School of Biosciences
Item ID:	82485
Depositing User:	Farnan, Kirsty
Date Deposited:	12 Dec 2025 04:40
Last Modified:	12 Dec 2025 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/82485

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