Quantifying the individual intake of grazing dairy cows offered mixed rations

© 2017 Dr. Marliene Margaret WrightDairy systems in temperate regions of the world, including Australia and New Zealand are primarily pasture-based. However, due to variability in pasture dry matter (DM) production throughout the year, pasture is often supplemented with conserved forage, cereal grain and/or pelleted concentrates. As a result, feeding systems have emerged that enable large amounts of supplements to be more optimally fed to dairy cows; one such feeding system is termed the partial mixed ration (PMR) feeding system. This feeding system is defined as a feeding system where a mixed ration is fed in between periods of grazing. However, such feeding systems may be exposed to large between-cow variation in dry matter intake (DMI) as a result of the presentation of feed to groups of dairy cows. Over the last 10 years there have been several published research articles investigating milk production responses to PMR feeding. Supplementary feeding is an area of dairy nutrition that has seen a lack of consistency in production responses. This may be a result of large between-cow variation in feed intake. However, research in the area of between-cow variability is lacking and, as a result variation between cows in feed intake is an area of dairy science that is not fully understood. Even more limited is research regarding between-cow variation in pasture-based feeding systems, due largely to the difficulties in measuring individual intake in pasture-based systems. This thesis had the aim of determining whether between-cow variability existed in the PMR feeding system and identify some system-level factors that might be controlled to minimise between-cow variability in feed intake. To determine if this variation existed, feeding behaviour measurements were used as a proxy for feed intake because of the difficulties in measuring individual intake in pasture-based feeding systems. Feeding behaviour was measured using automated feeding behaviour devices on 48 cows, in each of spring and autumn when cows were in the PMR feeding system and offered one of two supplement amounts in combination with one of two pasture amounts. The variation in feeding behaviour between cows within a group was influenced by the amount of supplement but not the amount of pasture offered. The variation among cows in pasture eating time approximately doubled when the amount of supplement offered increased. This indicated that in order to reduce the variability between cows in feed intake, supplement feeding management strategies need to be considered. Increasing pasture allowance had no effect on pasture eating time, although pasture intake increased as a result of increased grazing intensity compared with the low pasture allowance. Increasing the amount of supplement offered in the PMR feeding system reduced pasture eating time by 51 min/cow per day, a reduction of 8.5 min/cow per day for each kilogram of supplement consumed. The accurate determination of individual cow intake in pasture-based systems has been a theme of dairy nutrition research for at least 60 years. A method for determining individual intake was investigated to enable determination of variation in intake between cows. The n-alkane technique was selected for the investigation of individual intake because of its proven accuracy in pasture-only feeding systems. In order to evaluate the n-alkane technique for intake determination, it was applied to a herbage-only feeding system prior to its application with a combination of mixed ration and pasture diets. Eight Holstein-Friesian dairy cows were used in this experiment and were offered perennial ryegrass ad libitum, at a low and high herbage mass during two different seasons, to determine the effect of altering herbage mass on the accuracy of the n-alkane technique. Cows were housed in individual metabolism stalls to ensure accurate determination of both measured DMI and total faecal output. Herbage DMI was estimated using the n-alkane technique by dosing cows twice daily with C32 n-alkane for each day of each period. Season and pre-harvest herbage mass did not affect the accuracy of the n-alkane-estimated DMI, although season and herbage mass effects may have been masked by variation. However, there were differences in the faecal recovery rates of a number of n-alkanes between summer and autumn, suggesting that the effect of season needs further investigation prior to the application of reference recovery rates when investigating diet fractions. The n-alkane technique was then applied to diets where mixed rations were fed in combination with fresh herbage to evaluate this technique for the estimation of intake. Diet fractions were evaluated in cows fed a mixed ration with fresh herbage. Sixteen rumen-fistulated Holstein-Friesian dairy cows were housed in individual metabolism stalls and were offered a low or high amount of herbage in combination with a low or high amount of supplement. The n-alkane technique accurately estimated intake when correcting for incomplete faecal recovery, using recovery rates published by Dillon (1993) and recovery rates calculated in this experiment. For some n-alkane estimates of intake, several mean discrepancies with measured intakes were significantly different from zero. This indicates systematic bias with some treatment combinations, however these were less than 5%. This research has important implications for the future use of the n-alkane technique, especially in mixed ration feeding systems. The n-alkane technique was then applied to a field-based PMR feeding experiment in spring (early-lactation) to estimate individual DMI. Measurements of feeding behaviour were also recorded using automated feeding behaviour recorders. These techniques were used in a spring experiment with 32 Holstein–Friesian dairy cows when offered two amounts of two supplement treatments; a PMR containing canola meal (PMR+C) and without canola meal (PMR–C). All cows were offered a low pasture allowance of 10 kg DM/cow per day to ground level. Replacing some wheat in a ration with canola meal increased pasture and total eating time. However, DMI did not differ between PMR–C and PMR+C cows, as estimated using the n-alkane technique. This experiment also demonstrated that altering the amount of supplement did not affect the increase in eating time that occurred as a result of the inclusion of canola meal. Increasing the amount of supplement reduced pasture intake as a result of a reduction in grazing time and grazing intensity. An experiment with similar treatments was then conducted in autumn (late-lactation), with the same measurements for DMI and feeding behaviour with 32 Holstein-Friesian dairy cows. The data from this autumn experiment was combined with the spring data (mentioned in the previous paragraph) to enable the investigation of the variability between cows in feed intake, feeding behaviour, and milk production, in different seasons. This research found that there was a treatment type by supplement amount interaction on between-cow variability in terms of ration DMI. There was an interaction between season, supplement type and the amount of supplement for pasture DMI. This is the first research to report considerable variability between individual cows in terms of feed intake, in mixed ration feeding systems. In conclusion, this thesis offers insights into the variability between cows in intake, feeding behaviours and milk production. It provides details on feeding management strategies that may impact between-cow variability. The measurement of between-cow variability was investigated following the validation of the n-alkane technique for estimating intake following an investigation of the applicability of the n-alkane technique for measuring DMI in herbage-only and mixed ration diets. This investigation has also provided some valuable insights into the future use of the n-alkane technique for intake estimation in both pasture-only and pasture-based supplementary feeding systems