The dairy industry is under increasing pressure to improve production efficiency and reduce greenhouse gas emissions, especially methane, in the face of an ever-growing global population and increasing demand for animal-based food products. Enteric emissions of methane is not only a major contributor to agricultural GHG emissions but it also represents a significant energy loss which could otherwise be utilised by dairy livestock for growth and development purposes. Rearing dairy heifer replacements is a key activity conducted within dairy systems and is required to replenish the milking herd, however, it is also the second largest cost on dairy production systems, and failure of heifers to reach target age at first calving further compounds the total cost. Studies have indicated that enhanced milk feeding has a long-term effect on age at first and first lactation milk production. Some studies have also reported potential benefits of providing forage to pre-weaned calves, yet the literature does stops short of examining the potential interactions between forage type and age of calves when it is introduced in the postnatal period. Furthermore, little information exists which investigates the potential residual effects of postnatal nutritional programmes on nutrient digestibility and energy metabolism at various stages during the post-weaned heifer rearing period. The studies reported in this thesis investigate the potential residual effects of postnatal nutrition on the performance of dairy heifer replacements during the pre-weaned and post-weaned stages of development.Study 1 evaluated the effects of providing various forage types to pre-weaned calves offered either enhanced or restricted milk replacer (MR) feeding programs on growth performance, feed intake, and age at first calving (AFC). The MR feeding regimens were: (1) enhanced (EN, 9L/d from d5-42; 6L/d from d43-56; 3L/d from d57-67; 2L/d from d67-70) and (2) restricted (RE, 4 L/d from d5-67, 2 L/d from d68-70). The forage treatments were: (1) chopped straw offered from 14 d of age (CS14), (2) chopped straw offered from 56 d of age (CS56), (3) grass silage offered from 56 d of age (GS56) and (4) no forage (NF) provision until weaning. The study therefore had eight treatment groups: 1) ENCS14, 2) EN-CS56, 3) EN-GS56, 4) EN-NF, 5) RE-CS14, 6) RE-CS56, 7) RE-GS56, and 8) RE-NF. To minimise the risk that calves would consume bedding as a forage substrate, all calves were bedded on woodchip. Growth performance was not influenced by forage treatment but was influenced by MR feeding regimen, whereby EN calves grew more rapidly and were subsequently heavier at weaning. Furthermore, daily concentrate intake between d 5 and 56 was greater in RE calves compared to EN calves, however, concentrate intakes were greater in EN calves compared to RE calves between d 56 and 70. Forage provision did not influence concentrate intake, however, calves in the EN.56 treatment group had greater total dry matter intake (DMI) than calves in the EN.NF treatment group. Furthermore, concentrate and total DMI were greater in RE.CS14 calves compared RE.CS56 calves. Milk feeding treatment did not influence live weight first service, age at first conception, or AFC. Study 2 investigated the effects of introducing various forage types to enhanced and restricted milk fed calves at different ages on nutrient digestibility, energy metabolism, and methane emissions, immediately pre-weaning. Study 2 had the same experimental design as study 1, whereby milk feeding regimens were: (1) enhanced (EN, 9L/d from d 5-42; 6L/d from d 43-56; 3L/d from d 57-67; 2L/d from d 67-90) and (2) restricted (RE, 4 L/d from d 5-67, 2 L/d from d 68-90). The calves were ~ 82 d of age during the measurement period, there milk feeding level was reduced to the same quantity for the duration of the study. The four forage treatments were: (1) chopped straw offered from 14 d of age (CS14), (2) chopped straw offered from 56 d of age (CS56), (3) grass silage offered from 56 d of age (GS56) and (4) no forage (NF) provision until weaning, giving a total of eight experimental treatment groups; 1) EN.CS14, 2) EN.CS56, 3) EN.GS56, 4) EN.NF, 5) RE.CS14, 6) RE.CS56, 7) RE.GS56, and 8) RE.NF. Calves were housed in individual digestibility crates for 5 days with feed intake and outputs of faeces and urine recorded on days 2-5 in order to determine total tract digestibility. Afterwards, calves were accommodated in open-circuit respiration chambers for 3 d, and gaseous exchange was measured during the final 48 hours for the calculation of methane (CH4) emissions and heat production (HP). A bedding substrate was not used; however, digestibility crates and respiration chambers were fitted with rubber mats for animal comfort. Milk feeding regimen did not affect concentrate DMI, however, calves which had no forage provision (NF) consumed less concentrate compared with calves from all other forage treatment groups. Furthermore, nitrogen intake (NI) and gross energy intake (GEI) were also lower in NF calves compared to calves which were supplemented with forage. Methane emissions were greater in GS56 calves than in CS56 and NF calves, and CH4 emissions in relation to metabolic live weight (LW 0.75) were greater in GS56 calves compared to CS56 calves and NF calves. Total tract digestibility was not influenced by previous milk feeding regimen or forage treatment.Study 3 evaluated the potential residual effects of providing various forage feeding options to enhanced and restricted milk fed heifers during the pre-weaning period, total tract digestibility, energy metabolism, and methane emissions in 8 mo old heifers. The two pre-weaned milk feeding regimens were: (1) enhanced (EN) (maximum daily offering of three meals of 3 L, 1350 g/d MR powder) and (2) restricted (RE) (maximum daily offering of two meals of 3 L, 900 g/d MR powder). The pre-weaned forage treatments were: (1) chopped straw offered from 14 days of age (CS14), (2) chopped straw offered from 56 days of age (CS56), (3) grass silage offered from 56 days of age (GS56) and (4) no forage (NF) provision until weaning, giving a total of eight experimental treatment groups;1) EN.CS14, 2) EN.CS56, 3) EN.GS56, 4) EN.NF, 5)RE.CS14, 6) RE.CS56, 7) RE.GS56, and 8) RE.NF. Heifers were transitioned to a standardised perennial ryegrass silage diet at ~ 8 mo of age. The digestibility measurements were conducted for 3 d in individual digestibility stalls, followed by 3 d within open-circuit calorimeter chambers to provide data measurements for a 6 d total balance. Gaseous exchange data was collected when heifers were accommodated in the calorimeter chambers to calculate CH4 and HP. Total DMI and GEI was greater in EN heifers compared to RE heifers, however, dry matter (DM) and organic matter (OM) digestibility was lower. Pre-weaned forage treatment did not affect feed intake, however, there was a residual effect of pre-weaned forage treatment detected in DM and OM digestibility measurements, whereby both digestibility measurements were lower in CS14 heifers than recorded in GS56 and NF heifers. Pre-weaned milk feeding quantity did not influence fibre digestibility in study 3. However, nitrogen (N) digestibility tended to be lower in EN heifers compared to RE heifers. Furthermore, daily CH4 emissions and CH4 energy outputs (CH4-E) were greater in EN heifers than in RE heifers. Pre-weaned milk feeding quantity also had a residual effect on energy metabolism, whereby EN heifers displayed lower DEI/GEI measurements compared to RE heifers. Study 4 investigated the potential residual effects of pre-weaned calf MR feeding regimen on energetic and total tract digestibility efficiencies of 16 mo old dairy heifer replacements. The effects were evaluated at two feeding levels using a PRG silage only diet at offered at maintenance energy compared to ad libitum access. Heifers had previously been subjected to EN or RE milk feeding programs as pre-weaned calves and the two MR feeding regimens were: (1) enhanced (EN) (maximum daily offering of three meals of 3 L, 1350 g/d MR powder) and (2) restricted (RE) (maximum daily offering of two meals of 3 L, 900 g/d MR powder). The study incorporated a changeover design whereby both EN and RE heifers were examined under experimental conditions whilst offered the PRG silage diet at both feeding levels during two separate experimental periods. In both periods, total tract digestibility measurements were conducted for 3 d with heifers housed in individual digestibility stalls, and then transferred into open-circuit calorimeter chambers to provide data for 6 d total balance. Gaseous exchange data was collected to calculate CH4 emissions and HP during the final 48 h when heifers were accommodated within calorimeter chambers. When offered both the diet at both maintenance energy and ad libitum access, EN heifers tended to exhibit greater daily DMI compared to RE heifers, however, total tract digestibility of DM, OM, fibre, and N were not different between the two pre-weaned milk feeding treatment groups. Faecal N excretion was greater in EN heifers compared to RE heifers when offered the diet and maintenance energy levels. Furthermore, urinary and faecal N were greater in EN heifers compared to RE heifers when animals were offered the diet at ad libitum access. Methane emissions were not different between EN and RE heifers when offered the PRG silage diet at either maintenance energy or ad libitum access. Both EN and RE heifers exhibited a negative energy balance when offered the PRG diet at maintenance energy level. Improving the performance of dairy heifer replacements is critical in the effort to increase dairy production while reducing the environmental costs associated with dairy production systems. The studies described in this PhD thesis aimed to investigate the potential residual effects of postnatal nutrition on the performance of dairy heifer replacements during the pre-weaned and post-weaned stages of development. A significant reason why dairy producers and nutritionists alike may be hesitant of both the adoption of enhanced milk feeding regimen and pre-weaned forage provision could be down the lack of undisputed verdicts regarding the long-term benefits, and perhaps even the perceived drawbacks of their implementation on dairy heifer performance, existing in the scientific literature. The results have shown that pre-weaned milk feeding regimen can have lasting benefits on dairy heifer growth performance, however, the residual effects shown in the digestibility studies reveal that compromises are made in some digestibility parameters as enhanced milk fed heifers get older. Nevertheless, the ability of enhanced milk feeding to reduce the age at first service offers a potential saving in production costs if age at first calving can also be reduced. The data presented in this thesis can help dairy producers and policy makers to make informed decisions regarding pre-weaned nutritional programmes for dairy heifer production systems. The results presented in this thesis regarding pre-weaned forage provision may also provide clarity to nutritionists and dairy producers who may be unclear as to what type of forage and what age they should introduce it to pre-weaned calves. Interest in feeding enhanced quantities of milk to pre-weaned calves is growing. The results in this thesis have also highlighted key areas for future scientific research. Future research on the influence of postnatal nutritional programmes on dairy heifer performance should include a larger data set of heifers to establish potential influences on RFI and first lactation performance of dairy heifer replacements. Similar to published literature in rodents, future calf rearing studies should also focus on genetic expression in order to more clearly understand if postnatal nutrition can cause metabolic imprinting in dairy heifer replacements. The development and validation of novel technologies for the examination of approximate methane emissions in dairy livestock may also provide a broader set of data for analysis in experimental conditions and may afford regulatory bodies the chance to measure methane on dairy livestock systems more easily.
| Date of Award | Jul 2022 |
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| Original language | English |
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| Awarding Institution | - Queen's University Belfast
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| Sponsors | Department of Agriculture, Environment and Rural Affairs |
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| Supervisor | Katerina Theodoridou (Supervisor), Steven Morrison (Supervisor) & Gillian Scoley (Supervisor) |
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