Dynamics of soil ingestion by growing bulls during grazing on a high sward height in the French West Indies

The aim of this study was to evaluate the kinetic of daily soil ingestion by growing bulls at tether-grazing when they received a very high sward and a large grazing area in which they stay during 11 days (no stake moving).

Use of herbage resource

The average sward height was 17.6 ± 0.3 cm (mean ± s.e.m.) along the 11 days of measurements from D2 to D12. Nevertheless, pregrazing sward height (at D2) was quite high with 49.2 ± 0.9 cm and significantly higher from those on D3 to D12 (14.4 ± 0.1 cm; P < 0.001) with a 68% decrease along the first 24 h (15.6 ± 0.6 cm at D3; Fig. 2). On the last day (D12) on the experimental plot, the sward height was 13.9 ± 0.5 cm corresponding to a reduction of about 72% in 10 days (Fig. 2). This postgrazing sward height near to 14 cm seemed to be relatively high compared to the range of 3.3–8.2 cm reported in previous works^5,6 but the important differences in postgrazing vegetation structure (long lying stems vs. short erected ones) limiting the comparison of postgrazing sward heights with these studies. Indeed, animals grazed on the experimental plot until resource depletion and at D12 the remaining vegetation cover was only constituted of lying fibrous stems forming an ‘insulating carpet’ above the ground (Fig. 1c). Sward heights decreased rapidly, mainly due to trampling by animals and the movements of the chain, and reflected more a rapid change in the structure of the vegetation (grass lying on the ground) than in the forage supply. Grass allowance decreased across time, but more progressively than sward heights, and the animals were moved to a new plot on D13 when they had ingested all the leafy parts (daily observation of the plot) to avoid any limitation to meet their nutritional requirements.

Daily grass ingestions, individually estimated during 9 days from D5 to D13, ranged from 1.33 to 2.62 kg organic matter (OM)/100 kg BW and 1.42 to 2.79 kg DM/100 kg BW (Fig. 3a). Average daily ingestions measured from D5 to D9 (1.99 kg OM/100 kg BW and 2.11 kg DM/100 kg BW) were significantly higher than those measured from D11 to D13 (1.61 kg OM/100 kg BW and 1.72 kg DM/100 kg BW; P < 0.0001). On average, animals ingested 1.84 ± 0.06 kg OM/100 kg BW and 1.95 ± 0.07 kg grass DM/100 kg BW (mean ± s.e.m.). The total faecal output was on average 1.51 ± 0.03 kg OM/100 kg BW and 1.76 ± 0.07 kg DM/100 kg BW (mean ± s.e.m.). Expressed in OM, it was significantly higher on D7, D9, D13 than on D11 (1.57 vs 1.39; P < 0.01); expressed in DM, there was a tendency to observe the same differences (1.87 vs 1.49; P = 0.078). OM digestibility of ingested grass was on average 68.9% but significantly decreased across time. It was significantly higher on D5–D9 (71.1%) than on D11 (67.8%) and D13 (63.7%); the difference between D11 and D13 being also significant (P < 0.0001; 94% OM in the grass). The evolution of grass ingestions across time was related to the progressive decrease in both the amount of grass available and its digestibility. The observation at the end of the experiment that the remaining grass was only a ‘carpet’ of stems lying on the ground suggests that animals consumed more and more of the more fibrous parts of the grass which results into a decrease in OM digestibility.

Soil ingestion

Ti contents in soil (STi), faeces and washed grass were 3821, 157 and 2.6 µg/g DM, respectively. The daily faecal amount of titanium (FATi) and the daily titanium intake due to grass intake (TiIG) (obtained using titanium content in washed grass) were on average 102.0 ± 8.1 and 5.0 ± 0.2 mg Ti/100 kg BW, respectively (mean ± s.e.m.). Daily soil ingestion did not significantly increase along the 9 days (i.e. D5–D13) when the six bulls grazed on the same surface and was on average 26.9 ± 2.6 g DM/100 kg BW (mean ± s.e.m.; P = 0.224; Fig. 3b) corresponding to 1.4 ± 0.1% of their total daily DM ingestion. There was a tendency for daily soil ingestion, expressed in percentage of total ingested DM, to increase over time (P = 0.112) with daily averages ranging from 1.1 (D5) to 1.4 (D13), the highest value being 1.5 at D11 (Fig. 3c). This tendency can be explained by a lower grass ingestion on the last days of measurements (D11–D13). Nevertheless, there was a high degree of interindividual variability with median, minimal and maximal daily soil ingestions of 27.1, 7.9 and 58.4 g DM/100 kg BW, and 1.3, 0.4 and 2.7% of total ingested DM, respectively (Fig. 3b and 3c). Previous studies conducted in tether-grazing growing bulls already reported a large range of variability in soil ingestions^5,6 (Table 1). Given that in these experiments the animals presented similar age and BW, this interindividual variability can be attributed to differences in feeding behaviour between individuals.

The pregrazing sward height was substantially higher than these two previous studies (Table 1), which is consistent with the idea suggested by Jurjanz et al.⁵ and Collas et al.⁶ that the higher the grass, the lower the soil ingestion. In dairy cows strip grazing in spring on temperate pastures, the highest soil ingestion (0.63 vs 0.17 kg DM/day) had been associated to the lowest post-grazing sward height (3.6 vs 5.3 cm) and to the lowest daily pasture allowance (20 vs 35 kg DM/cow above ground level⁸). In another trial with strip grazing in autumn and different daily pasture allowances (40 vs 65 kg DM/cow above ground level), these authors reported soil ingestions of 0.85 and 0.64 kg DM/day for post-grazing sward heights of 4.1 and 4.7 cm, respectively. Kirby and Stuth⁹ showed soil ingestions from 0.28 to 0.84 kg DM/day by growing steers in May and June in the arid Central Texas. In Idaho, soil ingestions of 0.73 and 0.99 kg DM/day were estimated by Mayland et al.¹⁰ for cattle at grazing in June and August, respectively. While studies in cattle generally reported soil ingestions up to 10% of the daily ingested DM, studies in sheep can show higher rates. For example, studying soil ingestion by sheep from January to July, Abrahams and Steigmajer¹¹ reported in March a median soil ingestion of 17.6% of the daily ingested DM (with individual values from 4.7 to 45.1%). These authors observed the lowest rates in July with a median soil ingestion of 1.5%. These studies suggest that not only the animal species, but also several confusing factors as season and weather conditions, type of soil, and grazing practices may influence soil ingestion by ruminants.

While literature reported soil ingestion by cattle reaching 10% of the daily ingested DM^5,8, the consequences on animal health, or on the diet digestibility, of such soil ingestions remain poorly documented to date. In chickens, high soil ingestion has been shown to decrease diet digestibility¹². Others have mentioned the role of soil ingestion in pathogen exposure¹³, and its interest for a mineral supply or to buffer the pH of the digestive system^14,15, particularly in wildlife. However, soil ingested by herbivores would contribute to their exposure to environmental pollutants (especially organic pollutants as chlordecone that have a high affinity with soil OM⁴) and to the transfer to animal tissues. Contamination of food products of animal origin would limit the sustainability of economic activities related to livestock for farms located in contaminated areas.

Grass soiling

Regardless of sampling location in the grazing circle, titanium content of unwashed grass did not present temporal variation throughout the experiment with an average of 7.2 µg Ti/g DM (P > 0.05). Contrarily, the titanium content of unwashed grass differed significantly between the three different sampling zones in each circle: samples from the peripheral area (5.0 µg Ti/g DM) were significantly lower than those in the intermediate (8.8 µg Ti/g DM; P = 0.007) and central zones (8.5 µg Ti/g DM; P = 0.013; Fig. 4). There were no significant differences between the titanium contents of grass sampled in the central and in the intermediate zone. The lesser extent of sward soiling in the peripheral area of the grazing circle suggests that animals would have explored the grass resource from the centre to the border of the circle. Thus, the peripheral area would be less soiled due to less trampling. In addition, the chain may not be in contact with the ground in the peripheral area depending on the head movements of the animal. Hinton et al.¹⁶ observed an increase in soil loading on grass with increasing intensity of grazing by sheep by comparing several stocking rates. The results of the present study suggest that the animals would have ingested the most appetent and not very soiled grass first and, in this way, would have limited their soil ingestion throughout their stay on the same grazing circle.

The differential of titanium contents between washed and unwashed grass evidences that soil particles may adhere to leaves and stems, so that soiled grass ingestion also contributes to soil ingestion. Titanium contents of unwashed grass ranged from 5.0 (periphery of the grazing circle) to 8.8 (intermediate zone of the grazing circle) µg Ti/g DM. Subtracting the average titanium content of the washed grass (2.6 µg Ti/g DM) gives information on the sward soiling induced by grazing with values from 2.4 to 6.2 µg Ti/g DM depending on the zone of the grazing circle. Considering a daily grass ingestion of 1.95 kg DM/100 kg BW and an average titanium content of 3821 µg Ti/g dry soil, the sward soiling estimated during this experiment could induce a daily titanium ingestion from 4.7 to 12.1 mg Ti, and a daily soil ingestion from 1.23 to 3.17 g DM/100 kg BW depending on which zone of the circle is grazed. In this experiment, indirect soil ingestion due to soiled grass represented 4.6–11.8% of the total soil ingestion (considering an average daily soil ingestion of 26.9 g DM/100 kg BW). The representativeness of the unwashed grass samples analysed for their titanium contents compared to the samples of ingested grass can be discussed and the previous estimation of indirect soil ingestion did not allow to quantify precisely the contribution of soiled sward to the total soil intake. Nevertheless, the study of sward soiling gives elements for a better understanding about where the ingested soil comes from. It suggests that indirect soil ingestion, via soiled grass, exists and would contribute to the total soil ingestion, and by consequence, to the exposure to pollutants if the grazing area would be contaminated.

Soil particles may adhere on grass with transport via suspension, or soil disturbance by animal trampling or rainfall splashes¹⁷, and be retained for several days (above 53 µm) to over a week (below 53 µm) according to the size of soil particles¹⁸. Sheppard¹⁹ reported that soil particles which resiliently adhere to plant leaves are generally clay-sized (i.e. < 2 μm). Several authors also used titanium to estimate soil load on grass²⁰, and Sheppard et al.²¹ mentioned the interest of rare earth elements. The presence of elements not readily absorbed on plants highlighted the adhesion of soil particles on grass. The titanium content of washed grass was due to soil particles which resisted despite grass washing.

The sward soiling increases when animals grazed on water-saturated surface compared to dry soil surface, as shown by Collas et al.⁶ and measured also by the titanium content of unwashed grass. In the present study, daily rainfall was recorded and was not significantly linked to titanium content in unwashed grass (P > 0.05). The recorded daily rainfall showed higher values for the four last days (D9–D12) on the experimental plot whereas sward soiling did not change during the whole experiment. This study did not allow to evidence a link between daily rainfall and sward soiling but soil surface moisture was neither controlled nor measured, contrary to the study of Collas et al.⁶ for which the two levels of soil surface moisture were well contrasted because of an experimental irrigation of 20 mm per night.

Source: Ecology - nature.com