Introducción a la espectrometría de masas

The NSC: CP ratios increased when fertiliser N was applied at rates exceeding 200N in chicory and ryegrass, and there was a slight increase in the ratio in plantain herbage fertilised at 500N compared with unfertilised plantain (Table 7.4).

Table 7.4 Non-structural carbohydrate: crude protein ratio of herbage from four species fertilised at increasing rates of nitrogen.

N fertiliser rate (kg N/ha/y)

0 100 200 350 500

Chicory 0.6 0.6 0.7 0.9 1.0 Lucerne 2.8 3.1 3.1 3.3 2.9 Plantain 0.7 0.7 0.7 0.7 0.8 P. ryegrass 1.1 1.2 1.2 1.6 1.6

The model predicted mean rumen ammonia concentration to increase by 15, 7, 4 and 10 mmol/L in chicory, lucerne, plantain and ryegrass herbage respectively when N fertiliser application increased from 0 – 500N (Figure 7.3 a). The largest increases (~ 162%) were predicted in chicory, and diets where fertiliser application exceeded 200N. Predicted plasma urea N concentrations followed the same pattern as rumen ammonia concentration (Figure 7.3 b).

The predicted partitioning of dietary N by cows fed forage fertilised at increased rates of N is summarised in Figures 7.4 a-d. Nitrogen partitioned to milk was predicted to decline by an average of 23% when chicory, plantain, and ryegrass fertilised with increasing rates of N were fed. Nitrogen partitioned to faeces also tended to decline with increasing fertiliser N when chicory or ryegrass were grazed. While N partitioned to urine increased by 35% for chicory diets, and by 14 and 29% for plantain and ryegrass diets respectively when fertiliser applied increased from 0 – 500 kg N/ha/y. In contrast, the partitioning of N in cows grazing lucerne was largely unaffected by N fertiliser application rate.

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Figure 7.3 Predicted daily mean rumen ammonia (a) and plasma urea nitrogen (b) concentrations of dairy cows fed diets of chicory (○), lucerne (▲), plantain (+) and perennial ryegrass (●)

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Figure 7.4 Predicted partitioning of nitrogen (N) to urine (■), faeces (■), and milk (□) in dairy cows fed chicory (a), lucerne (b), plantain (c), and ryegrass (d) fertilised with increasing rates of fertiliser N (0 – 500 kg N/ha/y).

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When MINDY grazed plantain and ryegrass diets, predicted urinary N concentration and total daily N excretion increased when fertiliser N application rates exceeded 200N. Whereas in chicory diets, N excretion increased progressively with N application rate, but urine N concentration peaked when fed lucerne diets receiving 200N (Figure. 7.5 a-d). Predicted N excretion in urine from MINDY when fed lucerne did not appear to be altered by N fertiliser rate.

The model predicted a positive linear association between N intake and rumen ammonia concentration and urine N excretion (R2 _{> 0.85; Figure 7.6 a, b). Simulations within species suggest a quadratic}

association between N intake and rumen ammonia concentration when plantain or ryegrass diets were fed, with the point of infection occurring when N intake exceeded 0.4 kg/cow/day (Figure 7.6 a). Strong linear responses were observed between N intake and daily urinary N excretion for all species (R2 _{> 0.9;}

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Figure 7.5 The predicted effect of increasing N fertiliser rate applied to swards of chicory(a), lucerne (b), plantain (c), and perennial ryegrass (d) on the concentration of urea in urine (open symbols) and total daily urinary N excretion (closed symbols).

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Figure 7.6 Simulated relationship between N intake and rumen ammonia (a) concentration, and daily urinary N excretion (b) of dairy cows fed chicory (○), lucerne (▲), plantain (+) and

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Predicted milk (L/cow/d) increased with increasing N fertiliser application, for cows grazing chicory (12.3 – 15.3 L/cow/d), plantain (12.3 – 14.8 L/cow/d), and ryegrass (12.4 – 13.4 L/cow/d) (Figure 7.7 a). Similarly, milk-solids (MS) yield (kg/cow/d) also increased with increasing N rate, by 22, 17 and 8% from diets of chicory, plantain, and ryegrass, respectively (Figure 7.7 b). Unlike plantain and ryegrass where the milk production response was linear, chicory demonstrated a diminishing response when fertiliser application rate exceeded 200N (Figure 7.7). Predicted Milk and MS yield declined marginally (1%) when N fertiliser was applied to lucerne swards. The predicted efficiency of milk solids production (g MS/g N eaten), declined with increasing N fertiliser application rate by an average of 35% in diets of chicory, lucerne, and plantain (Figure 7.7 c).

Investigation of the relationships between intake and MS yield across forages showed that N intake was strongly positively correlated with MS yield (Figure 7.8 b), and a moderate correlation between MS and DMI (Figure 7.8 a). Dry matter intake was also moderately positively associated with CP concentration of herbage (R2 _{= 0.71; y = 0.0185x + 0.8481)}

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Figure 7.7 The predicted effect of increasing N fertiliser rate applied to swards of chicory (○), lucerne (▲), plantain (+) and perennial ryegrass (●) on milk yield (a), milk solids yield (b) and

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Figure 7.8 The predicted relationship between milk production and dry matter (a) and nitrogen intake (b) of dairy cows grazing swards of chicory (○), lucerne (▲), plantain (+) and perennial

ryegrass (●).

7.4

Discussion

The multiple herbage-based variables used as inputs into the model (i.e. sward physical characteristics and herbage chemistry) make it difficult to elucidate the precise drivers for influencing DMI in this study. However, the simulations undertaken did make it possible to explore the practical aspects of cows ingestive behaviour, N metabolism and production, when grazing forages grown under different N fertiliser rates.

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Overall, the model predicted DMI and MS production within the range observed in studies from cows grazing similar diets under similar N fertiliser regimes, at the same stage of lactation (Woodward et al., 2010; Minneé et al., 2012; Muir et al., 2014). It appears the model, however may be under-predicting bolus weight, particularly for ryegrass as measurements detailed in Chapter 6 and other studies involving cows fed ryegrass or lucerne, reported much greater weights (range of 88 – 173 g FW/bolus) than predicted in this simulation (Gill et al., 1966; Boudon and Peyraud, 2001; Boudon et al., 2006; Acosta et al., 2007). Though it must be recognised that the study in Chapter 6, and most others noted, were conducted in a tie-stall, feeding cut fresh herbage, where according to the study of Boudon et al. (2006) boli from grazing cows would be expected to be smaller than cattle fed indoors.