Evaluation of the n-alkane technique for estimating the individual intake of dairy cows consuming diets containing herbage and a partial mixed ration
Introduction
Researchers and farmers alike continue to be challenged by the need to accurately measure individual dry matter intake (DMI) of livestock in pasture-based production systems. In dairy cows, energy intake often limits milk production in high producing cows. Energy intake is defined as the product of feed DMI and the metabolizable energy (ME) concentration of that feed (Allen, 2000). Consequently, the estimation of DMI is important in assessing the efficiency of the conversion of DM into milk in dairy cows (Vazquez and Smith, 2000). Knowledge of pasture DMI is also essential for formulating supplementary rations that complement the nutrients received from pasture. Estimating DMI in grazing systems is problematic as herbage intake varies substantially at both a herd and an individual cow level (Smit et al., 2005; Wright et al., 2016; Wright, 2017). A number of techniques have been developed including the use of indigestible markers, sward difference measurements, reversed feeding standards, water intake methods, feeding behavior measurements and, more recently, the development of motion sensors (Langlands and Donald, 1978; Dillon, 1993; Mayes and Dove, 2000; Dove, 2010; Dutta et al., 2014). However, many of these techniques disrupt normal grazing behavior and cannot be used when cows are consuming heterogeneous herbage swards.
An indigestible marker method called the n-alkane technique has become a common method used in research for estimating individual intake in cattle. It is commonly used because of the presence of various n-alkanes in plant species, its ease of analysis, ability to determine diet selection in grazing animals and its accuracy when estimating intake (Mayes et al., 1986; Dove, 1993; Wright et al., 2019). Alkanes are saturated aliphatic hydrocarbons, present in various chain lengths (C21 to C37) in the cuticular wax component of plant species, and are mostly indigestible to ruminant species (Mayes et al., 1986). The n-alkane technique can be used to estimate DMI and diet composition from the concentrations of n-alkanes of ingested feed and feces excreted, and is unique in that it estimates digestibility in individual animals, allowing for a true estimation of individual DMI (Dove and Mayes, 1991, 1996; Dove, 2010). Alkanes found in plant species are predominantly odd-chain length and can be used, in combination, with orally dosed synthetic even-chain length alkane(s) to obtain estimates of individual intakes (Dove and Mayes, 1996). If using a single natural n-alkane and a synthetic n-alkane that is similar in carbon-chain length then a correction for incomplete fecal recovery is not required. However, when using the n-alkane technique to estimate diet composition using the differences in the concentrations of n-alkanes in various plant species, the various n-alkanes must be corrected for incomplete fecal recovery. Generally, n-alkane recovery rates increase curvilinearly with increasing n-alkane chain length and may be affected by the composition of the diet (Dove and Mayes, 2005; Dove et al., 2010).
It has been demonstrated that estimates of DMI using the n-alkane technique are most accurate when groups of animals are consuming herbage monocultures. Dove and Mayes (1996) reviewed nine scientific articles evaluating the n-alkane technique in both cattle and sheep and reported that the average discrepancies between known and estimated herbage DMI, at a group level, ranged from -2.60 % to 2.57 %. More recently, the n-alkane technique was validated in a herbage-only feeding system, and it was reported that the n-alkane technique accurately estimated herbage DMI and that its accuracy was not influenced by changes in herbage nutritive characteristics induced by changes pre-harvested herbage mass and season (Wright et al., 2019). Despite the success of the n-alkane technique in homogenous herbage swards, there is negligible research evaluating the use of this technique for the determination of DMI of grazing dairy cows offered more complex diets, where herbage is supplemented with large amounts of forage and grain/concentrates. To date the n-alkane technique has been used to estimate DMI of mixed grains and forage in sheep (Valiente et al., 2003). In dairy cows, the n-alkane technique has been used to estimate forage DMI in grazing systems, where individual supplement intakes were known or estimated using other techniques (Malossini et al., 1996; McEvoy et al., 2008; O’Neill et al., 2012). Bani et al. (2014) estimated individual DMI of dairy cows consuming total mixed rations using the n-alkane technique, however no fresh herbage was consumed in that study.
In temperate regions of the world, including Australia and New Zealand, dairying is primarily pasture-based due to its low to medium cost (Jacobs, 2014). Traditionally, pasture-based systems involve feeding cereal grain or concentrates in the parlor during milking however, an increasing number of dairy producers are increasing the amount of supplements offered to their cows, and as a result are transitioning to other supplementary feeding strategies (Auldist et al., 2019). One such strategy is feeding a partial mixed ration (PMR), where a mixed ration, is fed on a feed pad between periods of grazing. The PMR feeding system is used by approximately 12 % of dairy producers in Australia, with this number likely to increase (Wales and Kolver, 2017). Implementing a PMR feeding system can result in energy-corrected milk (ECM) production increases of between 2 to 5 kg/cow per day compared with offering equivalent amounts of energy as grain in the dairy and conserved forage in the paddock (Auldist et al., 2019). While the benefits of this feeding system have been demonstrated, the ability to measure individual cow DMI of fresh herbage and PMR has not been researched nor has the accuracy and precision of the n-alkane technique in this feeding system. As pasture-based dairying systems shift towards feeding more supplements, there is a requirement for a method to accurately estimate individual DMI of both fresh herbage and large amounts of supplements.
The objectives of this experiment were as follows: (1) to determine the accuracy and precision of the n-alkane technique for estimating individual cow DMI of fresh herbage and PMR; and (2) to determine the accuracy and precision of the technique at high and low amounts of fresh herbage and PMR.
Section snippets
Materials and methods
The experiment was conducted at the Department of Jobs, Precincts and Regions (DJPR) Ellinbank Centre, Victoria, Australia (latitude 38°14′S, longitude 145°56′E). All procedures were conducted in accordance with the Australian Code of Practice for the Care and Use of Animals for Scientific Purposes (National Health and Medical Research Council, 2013). Approval to proceed was obtained from the Agriculture Victoria Agricultural Research and Extension Animal Ethics Committee.
Accuracy and precision of the n-alkane technique
Lin’s concordance correlation coefficients contain measures of accuracy, with a bias correction factor, and precision was determined using Pearson correlation coefficient. The comparison of measured total DMI and estimated total DMI using the recovery rate corrections of Dillon (1993) resulted in a Lin’s concordance correlation coefficient of 0.96 (bias factor 0.99). When comparing measured and estimated total DMI with the average recovery rate correction, Lin’s concordance correlation
Discussion
The n-alkane technique accurately and precisely estimated total DMI in dairy cows fed fresh herbage in combination with a mixed ration. This is the first research that has reported the DMI of fresh herbage diets and mixed ration in dairy cows, where estimation has occurred without the application of artificial n-alkanes or beeswax mixtures onto the concentrate component of the diet. This finding, that the n-alkane technique can be accurately applied to a mixed ration feeding system in dairy
Conclusions
In conclusion, this research is the first to validate the use of the n-alkane technique for estimating intake of both herbage and PMR in dairy cows consuming mixed diets containing a combination of concentrate, conserved forage, and fresh herbage. While there were some discrepancies between estimated and measured intakes that were significantly different from zero, indicating systematic biases under some diets, these were on average ∼ 5% of individual diet component intakes, and are therefore
Conflict of interest
None.
CRediT authorship contribution statement
M.M. Wright: Conceptualization, Methodology, Investigation, Writing - original draft, Visualization, Project administration, Writing - review & editing. M.J. Auldist: Conceptualization, Methodology, Supervision, Writing - review & editing. E. Kennedy: Conceptualization, Methodology, Resources, Supervision, Writing - review & editing. F.R. Dunshea: Conceptualization, Methodology, Supervision, Writing - review & editing. N. Galvin: Resources. M.C. Hannah: Formal analysis, Validation. W.J. Wales:
Acknowledgements
The authors are grateful to G. Morris, D. Mapleson, A. McDonald, L. Dorling, T. Phillips, T. Hookey, S. Zeiro, D. Wilson, and Department of Jobs, Precincts and Regions (DJPR), Ellinbank farm staff for assistance with cow feeding and husbandry. The authors are also grateful for the n-alkane analysis by Teagasc Moorepark, Ireland. This research was funded by DJPR, Victoria, and Dairy Australia (Melbourne, Australia).
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