Effects of nutrition on milk production and reproduction of dairy cows : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Science at Massey University, Manawatū, New Zealand

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2022
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Massey University
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Sri Lanka is developing its dairy industry in a concerted effort to become self-sufficient in dairy products. Dairy farming in Sri Lanka occurs largely in small (≤10 cows) or medium (11-100 cows) scale farms, characteristically using diets that are based on tropical forages and various concentrate supplements. Despite this reliance on forages, little is known about their nutritive value or about the most appropriate ways of managing such forages in dairy rations. Consequently, per cow yields are generally low, and poor fertility is a significant limitation to the viability of the dairy industry. The primary objective of this thesis was to examine the management of forages in Sri Lankan dairying systems and the consequences of that management upon dairy cow productivity and fertility. As there was a lack of systematic information regarding the nutritive value of forage-based diets, generating such data was the first focus of the study. These data were used to calculate the extent to which supplied diets met the energy and protein requirements of dairy cows. Direct observation of cows and blood-based measures of metabolic status were used to verify cows’ nutritional status. As there was also a lack of systematic information on the fertility of Sri Lankan dairy herds, this was also evaluated. The first study (Chapter 3) investigated the feeds and rations used in medium-scale dairy production systems of Sri Lanka. Metabolisable energy (ME) and crude protein (CP) content were assessed in the two most commonly used forages (Guinea grass ecotype A (Panicum maximum), hereafter referred to as Guinea grass, and Hybrid Napier CO-3 (Pennisetum purpureum* P. americanum), hereafter referred as CO-3 grass) and in the secondary forages Gliricidia (Gliricidia sepium) and maize stover (Zea mays L.). Mean ME and CP content of Guinea and CO-3 grasses were 7.8 and 9.3 MJ/kg dry matter (DM) and 8.0 and 8.8% DM, respectively. Total dietary intakes of ME and CP were calculated using these values. Daily ME intake across the entire late-dry to mid-lactation period was consistently 7% lower than calculated requirements, whilst CP intake (13.5% DM) was below requirements in early lactating (16 18% DM required) but not dry cows (10 12% DM required). Metabolic profiling (serum albumin, urea, β hydroxybutyrate and non-esterified fatty acid concentrations) over the same period confirmed the presence of widespread energy and protein deficiencies (Chapter 4). However, body condition scores (BCS) were consistent over the lactation at 4.3 4.5 (1-10 scale). Insulin-like growth factor-1 (IGF-1) concentrations (Chapter 6) were higher during Days 1-14 of lactation than during Days 43 57 (60.8 ± 2.6 ng/mL vs. 72.3 ± 3.6 ng/mL), and also in cows with higher post-calving BCS, but were not clearly related to calculated energy status. The major conclusion from Chapters 3 and 4 was that nutritional deficiencies in Sri Lankan dairy cows could largely be attributed to poor control of stage of maturity of harvested forages. Reproductive performance of cows (Chapter 5) was assessed using farm records (including farmers’ observations of oestrus) and serum progesterone assays over Days 43-120. Progesterone profiles showed that 61.6% of cows resumed ovarian cycles by Day 120; but only 42% of these were observed in oestrus. Further, of the cows that progesterone assay had not shown resumed cyclicity by Day 120, 20.9% had been considered by farmers to have displayed oestrus. Pregnancy rates were 12.7% by Day 120 and 59% by Day 400, average intervals from calving to conception were 196.1 ± 8.9 days, and from calving to first artificial insemination (AI) were 116.0 ± 2.5 days, with 3.05 ± 0.2 inseminations required per conception. The high proportion of anoestrous cows and low conception rates were largely attributed to inadequate nutrition during the transition and early lactation periods, compounded by inaccurate oestrus detection. Parity (1st) and breed (Holstein-Friesian, HF) of cows were both risk factors for delayed resumption of oestrous cycles. Concentrations of IGF-1 were largely unrelated to reproductive outcomes, except those values on Days 43-57 were negatively related to the interval between calving and first AI. Chapter 7 investigated the nutritional composition of ryegrass-white clover pastures in New Zealand that were grazed with different rotation lengths, on the hypothesis that longer rotations lengths would result in more mature swards, with lower quality herbage. Farms were selected with longer (winter ~35 days, spring ~25-30 days) or shorter (winter ~30 days, spring ~20-25 days) grazing rotations. Both ME and CP were reduced by higher pre-grazing DM and rotation length, whilst post-grazing DM and residual height were positively correlated (r2 ≥0.8) with pre-grazing DM. Thus, the quality of the forage from long grazing rotations was poorer, and, therefore, the amount of DM consumed by grazing cows may be affected. Many cow-related factors significantly affected the reproductive performance (Chapter 8) of dairy cows in New Zealand. Cows with >85% of HF genes had lower pregnancy rates in the first three (PR21) and six (PR42) weeks from the start of breeding compared with those with ≤85% HF genes. Cows of parity 1-4 had higher PR21 and/or PR42 than those of parity ≥5. Pregnancy rate to first service (PREG1) was lower (odds ratio (OR): 0.7) in cows with higher BCS change ( 0.5 to 1 units: 1 10 scale) between calving and the BCS nadir. Cows that calved in July had higher PR21, and PR42 (OR:1.96, and 2.51, respectively) than those that calved in September, while cows that calved in August had higher submission rates in the first three (SR21) and six (SR42) weeks from the start of breeding (OR: 2.39, 3.21, respectively) than those that calved in September. Higher fat production and total milk solids production were associated with significant negative energy balance (NEB) (≥ 30 MJME/day), but NEB was less (or was positive) in cows with lower milk yields. Therefore, it appears that breed, BCS, parity, energy balance, calving month and milk composition are all useful indicators of future reproductive efficiency. Taken together, these studies for the first time, characterise the quality of dairy forages in Sri Lanka and the adverse effects of inadequate nutrition upon cow productivity. The studies in New Zealand confirmed that forage quality and intake were adversely affected by over-maturity of pasture, driven primarily by rotation length, and these findings supported conclusions made regarding forages in Sri Lanka. Characterisation of reproductive performance of Sri Lanka dairy cows showed that it was at a level that undermines the viability and productivity of the dairy industry. Reproductive performance is not only impaired because of the adverse effects of nutrition, but also due to the inadequacy of farmers’ reproductive management. Whilst these results are ostensibly depressing, they point to some relatively easy-to-implement interventions (e.g. improving forage management, better ration formulation, better education of farmers) to boost the future of the Sri Lankan dairy industry.
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Dairy cattle, Nutrition, Sri Lanka, Milk yield, Reproduction
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