Acoplamiento HPLC-MS

Predicted milk yield and MS production were largely within the range reported from studies investigating the milk production of dairy cows consuming similar diets and levels of intake (Woodward et al., 2010; Minneé et al., 2012; Muir et al., 2014). Milk-solids production increased when cows grazed non-legume swards with increasing rate of fertiliser N applied (an increase of 0.09 – 0.24 kg/cow/d, or 8 – 22%). While the predicted increase in MS from cows grazing plantain was positively related to N fertiliser applied to swards, the MS response from cows grazing chicory and ryegrass was marginal at fertiliser rates exceeding 200N. This reduced response at higher rates of N fertiliser, agrees with studies by Valk et al (2000) and Astigarraga et al. (2002). Extensive reviews by Kebreab et al. (2001) and Castillo et al. (2001a) concluded that the effect of increasing CP concentration of the diet on milk production is negligible when the diet contains a high proportion of RDP N and where N intake was sufficient to meet cow requirements for lactation and maintenance (NRC, 2001), as was observed in the current study. The model simulations in this study support the conclusion of those reviews, that there are no benefits of increasing N fertiliser application rate beyond 200N to ryegrass and chicory for MS production, but suggest that greater rates than 200N could be applied to plantain swards to improve MS production.

Although predicted MS production was strongly positively correlated with DMI and N intake (Figures 7.8 a-b), the efficiency with which the N is used declined by an average of 35%, and the proportion of dietary N partitioned to milk declined by 30% in non-legume diets under increasing N fertiliser

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applications. There was also a slight negative effect of N fertiliser to lucerne on MS production (Figure 7.7 b), indicating that applying fertiliser to lucerne is unwise. It would be valuable to investigate how N fertiliser application to mixed swards containing lucerne influences affects whole sward NUE.

7.5

Conclusions

Model predictions provided a likely outcome from the fertiliser treatments on both dairy cow production and digestive metabolism of N. These results suggest that adjusting to the application rate of N fertiliser to chicory, plantain and ryegrass provides a means to manipulate animal NUE and reduce N losses to the environment. In the context of this study, predictions of MS production and N excretion by the MINDY model suggests application of N fertiliser at 200 kg/ha could be the optimal, for compromise between MS production and total daily N excretion for chicory, plantain, and ryegrass swards. At this rate, predicted DMI and MS production from cows grazing chicory was maximised and no further gains were achieved through additional N fertiliser. Predictions for plantain and ryegrass showed higher (>200N) applications of N resulted in small increases in MS production, but the amount of N excreted increased exponentially. The simulations confirmed negative production and environmental consequences from applying N fertiliser to lucerne swards.

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8

General Discussion

8.1

Introduction

In New Zealand, grazed forages provide the majority of the diet of dairy cows. Forages provide New Zealand with a relatively low cost feed system for production of dairy products, and a competitive advantage in the global dairy market (Dillon et al., 2005). The efficiency of the system is maximised by high levels of consumption of pasture herbage. The dominant species grown is perennial ryegrass (Holmes et al., 2007) because this species can produce large amounts of herbage (up to 20 t/DM/ha/y), is tolerant of a wide range of environments and management practices, and is responsive to N fertiliser to promote plant growth (Kemp et al., 2000a). The adoption of N fertilisers to New Zealand farming in the 1990’s, and improvement in grazing management have increased herbage productivity, and contributed to the intensification of dairying in this country (through increased stock numbers and stocking rates). Over a similar period, however, N loading into land has increased (Scarsbrook and Melland, 2015) and the quality of New Zealand’s fresh water resources has declined (Verburg et al., 2010; Ballantine and Davies-Colley, 2014), and it is the N loss from agricultural systems that has been identified as a main contributor for environmental N pollution.

Herbage from forage species generally contains greater concentration of CP than is required by the animal, and in ryegrass, the addition of N fertiliser can exacerbate this (Tamminga, 1992a; Kolver et al., 1998; Whitehead, 2000). Excess dietary N is largely excreted, in the urine, and is a potential source of N pollution to groundwater (leaching) and nitrous oxide emissions to the atmosphere (Whitehead, 1995; Johnson et al., 2005). Forage species differ in their herbage CP concentration, and several reviews have shown a strong positive association between N intake and N excreted in the urine (Castillo et al., 2000; Kebreab et al., 2001; Parsons et al., 2011; Moorby, 2014). However, in some studies, measured differences in N excretion or urinary N concentration have not wholly been explained by N intake (Box et al., 2016; Minneé et al., 2017). The theory of synchronising nutrient supply, i.e. fermentable carbohydrates (i.e. non-structural carbohydrates, NSC) and N, suggests that maximal efficiency of N use for microbial protein synthesis (MPS) will occur when these nutrients are balanced (~ 5:1 ratio of

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NSC to N) (Tamminga, 1992a). Parsons et al. (2011) and Moorby (2014) also demonstrated that urine N excretion decreases as the ratio of NSC: N decreases. Generally, in forages, the amount of N is too high and NSC content is too low, and the imbalance results in the high urinary N excretion of cows consuming diets of forage species compared to those fed concentrate diets (i.e. as is common in the United States of American and Canada) (Rearte, 2005; Belanche et al., 2013). Beever and Cottrill (1994) suggested that the low utilisation of N and high N loss to urine observed in cows fed forage based diets is related to the pattern with which N and energy (from degradation of CHO) are made available to the rumen microflora. Meaning, that as well as the total concentration of N and NSC in herbage being unbalanced in quantity, the timing that N and degraded CHO is available to the microbes during digestion may also be unbalanced. Because much of the herbage N is contained within the plant cells, it is only accessible to microbial degradation once cell rupture (through mastication) has occurred. Therefore, the extent and timing of cell rupture will influence the delivery of N to the rumen, which in turn has potential to effect utilisation of N.

While there have been comprehensive studies investigating the ruminal degradation of N and DM from herbage of forage species (Waghorn et al., 1989; Burke et al., 2000; Chaves, 2003; Burke, 2004; Chaves et al., 2006), less is known about the extent of degradation and nutrient release during the initial phase of feed degradation, that is ingestion. A wide range (22 - 86%) of nutrient release from legume and grass species has been reported (Bryant, 1964; Boudon and Peyraud, 2001; Boudon et al., 2006; Acosta et al., 2007), but the characteristics of the herbage that influenced this were not determined. As a result, this study sought to expand upon previous work by developing methods to quantify and compare the nutrient release from a range of common forage species, and to determine the key characteristics driving variation. It was anticipated that by better understanding nutrient release from herbage during initial maceration we would improve our understanding of why some forage species result in reduced loss of N to urine when fed to cows, compared with that from other forage diets. Furthermore, determining whether N release and the ratio of N:NSC can be manipulated through management practices, such as varying N fertiliser applied to swards, could be used to inform fertiliser management practices to achieve balance between herbage production and loss of N to the environment.

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