L A EVALUACIÓN DEL APRENDIZAJE : “E NTRE DOS LÓGICAS ”

It is often assumed that dagginess is a result of a high worm burden. Consequently drenching is often timed based on dag score of the mob rather than worm count. However, this theory does not always convert to practice. High dag scores have been associated with both low faecal egg counts (FEC) (Bisset et al., 1997; Morris et al., 1997) and high FEC (McEwan et al., 1992b). The McEwan et al. (1992b) study was performed on lines selected on production traits (number of lambs born, 100 day weight and hogget fleece weight). Between lines: FEC was genetically correlated with dagginess (0.61) and faecal consistency scores (0.54). There was also a positive genetic correlation between dag and faecal consistency scores (0.86). However the relationship between dags and FEC was negative for half-sib genetic correlations (-0.55), indicating low FEC equals high dag score as conferred by Bisset et al. (1997) and Morris et al. (1997). The half-sib heritability estimate for dag score (0.06 ± 0.06) was low compared to other estimates published. Heritability for faecal consistency was moderate 0.27 ± 0.09 and for total FEC was 0.14 ± 0.07. Dags were positively correlated phenotypically (0.46) and genetically (1.37) with faecal consistency. Therefore, in this study, there was an unclear association between dags and FEC.

A study on Australian Merinos calculated heritabilities across 8 recording times from weaning to approximately 400 days old (Pollott et al., 2004). Dag score ranged from 0.07 to 0.32, overall was 0.11 with moderate trait repeatability (0.25). Faecal consistency score had a heritability range from 0.03 to 0.22, overall 0.12 with a repeatability of 0.12. FEC heritabilities ranged from 0.19 to 0.60, overall 0.28, trait repeatability was 0.22. Dag score and faecal consistency score had phenotypic and genetic correlations of 0.33 ± 0.01 and 0.63 ± 0.14, respectively. There were no other significant correlations.

Bisset et al. (1996) went further and noted that the low FEC lines had a higher immunological response to internal parasites, with increases in global leucocytes/mucosal mast cells, connective tissue type mast cells, and eosinophils in the intestinal mucosa. This study did not provide a correlation with dag score; however, it did use animals from the same Wallaceville selection lines used by Bisset et al. (1997) and Morris et al. (1997). In a different study, sheep with severe dags due to the Australian ‘winter flush’ tended to have low FEC counts, high faecal moisture content, and a hypersensitive inflammatory response, characterised by changes in lymphocyte

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populations, increased eosinophilia in the gut and small intestine, and mucosal damage (Larsen et al., 1999).

The assumption that the correlation between dags and FEC is due to high worm burden has not been proven. The above literature has shown there is potential for low FECs to increase dag formation. The response of the sheep’s immune system to worm burden, independent to the level of burden and FEC, maybe the factor involved in dag formation.

1.4.2.1.2 Pasture

It has been noted that pasture quality, composition and quantity can affect the incidence of dag formation in sheep. Notably when sheep have a ‘flush’ of grass (i.e. allowed to eat ab libtum after a period of little to eat) that there is a marked increase in

the proportion of the flock with high dag scores. Several factors about pasture including condensed tannins, fungal endophytes, and pasture composition have been studied in relation to dag formation.

Condensed tannins have been shown to improve animal productivity through reducing; dags, internal parasites and flystrike incidence in sheep (Leathwick and Atkinson, 1995). This has been attributed to the ability of condensed tannins’ to bind leaf proteins, creating complexes only released in the abomasum. This increases the amount of amino acids absorbed in the small intestine and thus helps improve animal productivity (Ramirez-Restrepo et al., 2004). A 1994 study by Leathwick and Atkinson (1995) observed lambs grazing endophyte ryegrass had greater dag score, higher average dry weight of dags and higher percentage of lambs struck than those grazing tannin containing Lotus corniculatus. In a subsequent study there was an average of 36,

29, 26, 11, and 7% of lambs flystruck when grazing L. corniculatus for a period of 0, 4,

7, 10, or 14 continuous days each fortnight respectively (Leathwick and Atkinson, 1998). Mean weight of dags decreased as each grazing treatment period increased; there was a significant difference between grazing for 0 to 4 and for 7 to 14 days. There was also a significant difference between mean dag weight of flystruck and non-flystruck lambs (185 vs 105g respectively, P<0.01) (Leathwick and Atkinson, 1998). Ramirez- Restrepo, et al. (2004) also showed reduced dag scores for animals grazing on L. corniculatus versus perennial ryegrass/white clover pastures. Condensed tannins also

have positive effects on live weight (Leathwick and Atkinson, 1995, 1998; Min et al., 1998; Ramirez-Restrepo et al., 2004). Conflicting results have also been found for the

49 effect of condensed tannins on wool growth (Leathwick and Atkinson, 1995, 1998; Min et al., 1998; Ramirez-Restrepo et al., 2004).

In New Zealand, the majority of pastures contain perennial ryegrass (Fletcher et al., 1999). There is a mutualistic relationship between asymptomatic fungal endophytes and the ryegrass plant (van Heeswijck and McDonald, 1992), with the presence of endophyte providing the ryegrass with resistance to Argentine Stem Weevil (Schaeffer, 2006). Adverse effects, however, are experienced by herbivores grazing endophyte containing grasses. These include reduced growth rates, ryegrass staggers, diarrhoea, fescue toxicosis, and heat stress (Fletcher et al., 1999; van Heeswijck and McDonald, 1992). High faecal moisture in those animals grazing high endophyte grasses appears to contribute towards diarrhoea and consequently dag formation (Pownall et al., 1993). Pownall et al. (1993) observed that faecal moisture content was 8.6% higher in those animals grazing high endophyte pasture compared to those on nil endophyte, with faecal moisture content only accounting for 15% of the variation in dag score. Fletcher et al. (1999) also observed that animals grazed on endophytpe containing ryegrass developed higher dag scores than those on endophyte free ryegrass. During the summer/autumn grazing trial, animals on high endophyte pasture also had a higher proportion of animals flystruck, 15-30% vs 2% (Fletcher et al., 1999). Endophyte toxins can also affect the body temperature of animals. Flystrike incidence in ryegrass staggers susceptible and resistant flocks was similar (8%), however, the majority of strikes (54%) in the susceptible line occurred on the body (Young et al., 2004). The endophyte toxins can increase body temperature; and it has been shown that increased sweating in the wool increases flystrike risk, due to increased humidity (Davies, 1948).

Spring pastures often contain low levels of neutral detergent fibre (NDF), below the 26% required for optimum rumen function (Davidson et al., 2006). This can lead to softer faeces, and scouring seen during the spring flush. Davidson et al. (2006) supplemented sheep grazing spring pastures with additional fibre. Animals supplemented with fibre had higher worm burdens, higher faecal dry matter percentage, and lower dag scores than control sheep. There was no correlation between worm burden and dags. Only a small proportion of the sheep in this study were examined for flystrike, however, within this group the flystruck sheep had greater median dag scores compared to unstruck sheep (median dag score 4 vs 3). Reid and Cottle (1999), measured mineral and dry matter content of faeces from daggy and non-daggy sheep

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from 2 flocks, a high and low FEC selection line and a grazing trial on endophyte or no endophyte pasture. In both experiments, daggy sheep had higher potassium and lower magnesium content. There may be a difference in mineral absorption in animals that tend to accumulate dags, but this needs to be investigated further.

In summary, controlling the composition and species of pasture can help to reduce dags. Pasture that is high in condensed tannins, low in endophytes and with levels of NDF above 26% will provide the highest potential to reduce dags. The potential for certain environments to grow such pasture is limited. There is also conflicting information on the effect condensed tannins have on other major production traits such as live weight and wool growth. These need to be further investigated before pasture replacement can be recommended.