A rumen, animal and farm systems evaluation of fodder beet when used to supplement ryegrass during lactation : A thesis by manuscript submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

The purpose of this thesis was to identify the functional changes at the rumen, individual animal and whole-farm scale when FB is used to supplement a ryegrass-based sward during lactation. A review of the literature (Chapter 2) explored the potential of FB to improve the feed base of the farm system and advance low infrastructure grazing systems which are common to New Zealand. However, the review also suggested greater biological, tactical and financial risk may be associated with growing and feeding FB on the milking platform, and the potential net advantage/disadvantage and had not been well defined in grazing dairy systems. Of primary concern was the incidence of acute and sub-acute ruminal acidosis (SARA) which has been underestimated across the herd in confinement and pastoral dairy systems worldwide. The incidence of SARA in cows transitioned to FB using industry-approved methods, and alternative FB feeding strategies to reduce SARA was highlighted as an area requiring further evaluation. In the first experiment (Chapter 3), the effect of feeding FB during early lactation on milk production and milk fatty acid composition of grazing dairy cows was explored. Sixty Friesian × Jersey cows were blocked into six groups of 10 cows, and groups randomly allocated to three replicates fed either 18 kg DM/day of ryegrass herbage (H), or 14.4 kg DM/day of ryegrass herbage + 4 kg DM of harvested FB bulbs (FBB). There was no advantage to milk production when 30% of a ryegrass-based herbage diet was substituted for FB bulb although, this also indicated that FB might provide an adequate substitute for herbage during periods of feed deficit. Supplementation of herbage with FB increased (P < 0.001) de novo synthesis of saturated fatty acids (particularly; lauric, myristic and palmitic acids) and reduced substrate availability of unsaturated fatty acids for ruminal biohydrogenation which reduced (P < 0.001) the content of long-chain and unsaturated products in milk. While the sward's chemical composition differed between treatments, the fatty acid content of herbage was not different (P > 0.1). The altered biohydrogenation end-products in milk and the high soluble-carbohydrate content of FB compared with herbage indicated altered rumen microbial communities and rumen function. The second experiment (Chapters 4 & 5) was split into two chapters to evaluate two objectives. The first objective was to assess the industry recommended method for transitioning lactating dairy cows (+ 0.5 kg DM FB/day) to moderate (40% DMI) amounts of FB on changes in rumen fermentation, pH and risk of SARA. In a cross-over design, eight rumen cannulated cows during early lactation were fed one of two diets either; herbage only (HO) or 60:40 ryegrass herbage + FB bulb (FBH). Response variables were analysed as a 3x2 factorial arrangement of FB adaptation stage (Stage 1: transition day 1-12, Stage 2: adaptation day 13-17, Stage 3: post-adaptation day 18-20) and dietary treatment. Two animals experienced severe SARA (pH < 5.6 for >180 min/d), one during each period, they were closely monitored but were able to self-correct rumen pH without intervention. Across each treatment, the FBH diet increased estimated DMI (measured by calibrating sward height with sward mass), but milk production was similar to the HO diet. Ruminal pH of cows fed FBH declined below HO between 0100 h and 1200 h each day even during stage 3 of adaptation, which may have reduced the microbial degradation of structural carbohydrates and limited the milk response to FB. The large content of water-soluble carbohydrate content of FB prevented rumen pH from stabilising within 20 days of adaptation and elevated the risk of SARA in specific individuals. An extended period of transitioning and low FB allocation may be needed to prevent the risk of SARA grazing dairy cows supplemented with FB. The objective of Chapter 5 was to evaluate the effect of supplementing spring ryegrass with moderate amounts (40% of total DMI) of FB on digestive and ingestive processing. We hypothesised that the decline of ruminal pH caused by supplementing ryegrass with FB would reduce the rumen function and microbial degradation of ryegrass. Following day 20 of adaptation (Chapter 4), the eight ruminal cannulated cows' rumen contents were removed at 0000 h, weighed and returned to the rumen and cows were fasted for ~10 h overnight before rumen contents were again removed and weighed. Samples determined particle comminution, pools of fermentation-end products and fractional neutral detergent fibre degradation between each bailing session. Minced samples of ryegrass and fodder beet were incubated separately, in sacco over 20 h on day 20 of each period (between 0400 – 0000 h), to evaluate DM disappearance. Each cow's total jaw movement was recorded on day 16 and 18 of adaptation to FB to identify changes in behaviour (grazing, ruminating and idling) and oral processing (mastication and prehension). While calibration of pasture mass form height reported in Chapter 4 indicated the FBH treatment consumed greater DMI than HO, estimation of DMI from energy output in maintenance, milk production and loss of body condition indicate DMI between treatments was similar (P > 0.10). In addition, the rumen pool of DM, ADF and NDF measured at the first (0000 h) rumen bailing, also reflected DMI when calculated from animal output and maintenance. Cows fed FBH spent 86 min/day longer ruminating and chewing intensity while ruminating increased 38% compared with those fed HO, while grazing time declined 20 min/kg DM of FB eaten. While the fractional degradation of neutral detergent fibre was similar between treatments, the FBH diet reduced the total VFA pool compared with HO following fasting (3.67 versus 4.03 mol), due to reduced ruminal concentrations of acetate and propionate. Despite greater rumination and chewing intensity, the rumen pool of large particles (> 2 mm) following fasting, declined 28% in cows fed FBH compared with those fed HO. In sacco DM disappearance of ryegrass following 20 h of incubation also declined 19% (P < 0.01) in the FBH treatment. The decline of VFA pool, reduced particle comminution and DM disappearance of ryegrass in sacco support the hypothesis that supplementing grazing dairy cows with moderate (40% of DMI) amounts of FB reduces the microbial activity of the rumen and limits the milk response to FB. The results suggest minimal advantage and high risk to rumen function and animal welfare of individual cows supplemented with FB. The third experiment (Chapter 6) evaluated the effect of a combined substrate containing ryegrass and increasing proportions of FB bulb (0, 15, 30 and 50 % of DM) on cumulative gas production and fermentation-end products in vitro. The objective of this study was to evaluate the dose-dependent response to supplementing ryegrass with FB bulb on the formation of fermentation end-products and gas production in 100 ml glass syringes. The total gas accumulated increased with the proportion of FB incubated (P < 0.05). The concentration of butyrate and propionate increased, while the concentration of acetate declined (P < 0.01), following 24-h of incubation. Production of carbon dioxide (CO2) formed from buffering VFA and methane (CH4) formed from fermentation, were calculated using stoichiometry. The percentage of CH4 declined yet, the total accumulation of CO2 and CH4 increased with the amount of FB included in the substrate. The effect of treatment on gas production diminished (P > 0.10) when the greater OM content of FB was accounted for, which indicate that while FB may reduce fractional CH4 emissions, total methane emission may increase compared with ryegrass, due to the greater fermentable organic matter content of FB bulb. Chapter 7 aimed to characterise changes of rumen pH, milk production and total discomfort from FB and define practical feeding strategies of a mixed herbage and FB diet. The deterministic, dynamic, and mechanistic model, MINDY, was used to compare a factorial arrangement of FB allowance, herbage allowance (HA), and allocation time. The FB allocations were 0, 2, 4 or 7 kg DM/cow per day (0, 2, 4 and 7FB, respectively) and HA were 18, 24 or 48 kg DM/cow per day above ground. All combinations were offered either in the morning or afternoon or split across two equal meals. Milk production from 2FB diets was similar to control but declined 4, and 16% when FB increased to 4 and 7 kg DM. MINDY predicted that 7FB would result in SARA and that rumen conditions were sub-optimal even at moderate FB allocations (pH < 5.6 for 160 and 90 min/d, 7 and 4FB respectively). Pareto Front analysis identified that splitting the 2FB diet into two equal meals fed each day alongside daily HA of 48 kg DM/cow provided the best compromise between high milk production and low total discomfort. However, due to low milk response and high risk of acidosis, we conclude that FB is a poor supplement for lactating dairy cows. In Chapter 8 a multi-component, whole-farm modelling approach was used to predict milk solids (MS) production and the economic farm surplus (EFS: operating surplus – adjustments) over two seasons (2016-2018) for an irrigated farm in Canterbury (South Island) and a non-irrigated farm in the Waikato (North Island), of New Zealand. The financial risk of the dairy business was measured using stochastic modelling in which the ratio between mean return on assets (ROA) minus an assumed 5% risk-free ROA, and the standard deviation of ROA was calculated from 300 combinations of climate, milk and feed price, land appreciation and interest rate. Four scenarios of autumn and spring supplementation of pasture were considered at each geographical location; imported maize silage (Base), maize silage crop grown on the platform (MSC), FB crop is grown on the platform (FBC) and FBAC a FB crop with an outbreak of acute (1% stock fatality) and SARA (5% decline of feed intake). Crop yield of FB increased with irrigation (21 versus 23 t DM/ha; irrigated and dryland, respectively) and was greater than maize silage (19 versus 21 t DM/ha; irrigated and dryland respectively). The DM yield of maize silage increased with the dryland system due to the warmer climate in the Waikato region of New Zealand (NZ). The MSC scenario improved EFS 5.8 % compared with Base when introduced to either the irrigated or the dryland system. The predicted response to MSC reflected a combination of greater milk production, lower feed expenses and shorter crop rotation compared with either Base, FBC or FBAC. While FBC increased EFS by 4.8% compared with Base under irrigation, EFS was similar to Base under dryland conditions ($2,711 and $2,759/ha, respectively). The limited advantage of growing FB under dryland conditions reflect reduced herbage supply due to the extended crop duration of FB compared with maize silage (14 versus 11 months between grazing of herbage). Model predictions suggest FBAC would reduce EFS by 6.5% (irrigated) and 7.1% (dryland) compared with Base, due to reduced milk production and livestock sales. In the absence of any adverse health risks, farm performance from supplementing FB crop was comparable to maize silage under irrigated conditions. However, in dryland conditions, and when the potential economic cost of acute and sub-acute ruminal acidosis is considered, there is little advantage from growing FB on the milking platform. While there is some support that minor allocation of FB with herbage will improve animal production, the novel methods of feeding and grazing FB in NZ increase animal welfare risk of individual animals within the herd, preventing the elimination of SARA risk when feeding FB to support lactation. Besides a few recent studies, previous research of FB feeding systems in NZ has focused on the herd as an experimental unit. However, the dynamics of feeding FB to individuals within the herd are variable, and the risk of SARA caused by supplementing ryegrass-pastures within commercial dairy systems of NZ may be underestimated. Further research should focus on factors responsible for individual risk to SARA such as competition, grazing and feeding behaviour, epithelial function, and morphology and rumen fermentation. Attention is needed when feeding FB to large herds in minimal infrastructure systems which prevent individualised feeding of FB as the variation of FB and herbage intake between individuals and days alter the allocation of FB to the remaining individuals within the herd. The results from this thesis suggest feeding small amounts of FB may help improve milk production and reduce feed deficits; however, the risk of SARA increases with FB allocation. Profit comparisons indicate limited financial incentive to growing FB on the milking platform to supplement ryegrass during early and late-lactation compared with lower-risk alternatives such as maize silage. In conclusion, from a rumen, individual animal and farm systems perspective, there is no advantage to supplementing grazing dairy cows with fodder beet to support lactation.