No abstract

Las parasitosis gastrointestinales (PGI) causadas por nematodos constituyen una limitante sanitaria y económica de la producción ovina en sistemas pastoriles. Su control ha dependido del tratamiento con productos antihelmínticos, y esto ha resultado en el desarrollo de resistencia de los parásitos a los productos utilizados. Las consecuencias de las PGI sobre la producción ovina varían de acuerdo a la severidad del efecto del parásito considerado, la carga parasitaria, la categoría ovina considerada, y el estado nutricional y fisiológico, y llegan a decenas de millones de dólares. El método más difundido para la identificación de ovinos resistentes es el conteo de huevos de parásitos por gramo de materia fecal (HPG). La precisión utilizada más comúnmente para la medición de esta variable es de 100 HPG, pero no hay estudios que indiquen si debería ser mayor o si podría ser menor. Utilizando propiedades de la distribución uniforme, estimamos con qué precisión debería registrarse para distintos valores de desvío estándar de HPG y de la magnitud del error de medición en relación a este parámetro. Concluimos que la precisión con que usualmente registran esta característica los laboratorios (100 HPG) es más que suficiente. En programas de mejora genética, en caso de existir capacidad ociosa de medición, sería más beneficioso aumentar el número de animales medidos que la precisión con que se mide HPG.

Sheep production is expanding among small farmers in Southern Uruguay. Currently, Corriedale and Milchschaf are being used, but not Highlander. However, there is lack of experimental information regarding the relative performance of these breeds. We conducted an experiment where these three sheep breeds were run together at the Southern Regional Centre, located in the region in question. Wool, body, reproductive and lamb growth traits were recorded from 2015 to 2019. Results were used as a basis for the calculation of gross margins for each breed, which we calculated for a range of production and economic scenarios. For scenarios based on the results of the present study Highlander was the best performing breed. If it was assumed to be wool-less, it performed even better. In some scenarios Corriedale performed best, especially when the higher wool price it used to enjoy was assumed. However, fetching such a price in the foreseeable future is unlikely. We concluded that in the absence of wool-less sheep that perform in a manner similar to Highlander or Milchschaf in terms of reproduction and lamb growth, Highlander is currently the best option for small farmers in Southern Uruguay. Until now, Milchschaf has been the recommended breed for the region and production system in question. The recommendation should be reviewed, Highlander should be recommended instead, and the performance of wool-less breeds should be investigated. Results could be applicable to other temperate regions in Latin America where similar production systems exist or may be developed.

Gastrointestinal nematodes (GIN) constitute a problem in many sheep production systems, including those in Uruguay, causing reduced productivity and increased expenses. The main strategy to control GIN has consisted of the use of anthelmintics.However, GINs have developed resistance to anthelmintics, reducing their effectiveness. Genetic resistance to GINs has been found in flocks of different breeds. To date, there have been no reports about GINs breaking down genetic resistance in sheep.

Faecal egg count (FEC) is used in sheep to decide when to drench. It is also used in genetic evaluations, in which case FEC should be recorded when the parasite burden is large enough so that variation among animals can be expressed. Another requisite is that the fraction of animals with zero FEC should be below 0.1. In order to decide the appropriate moment for drenching, or FEC recording in the whole group in the case of genetic evaluations, FEC is monitored in a random sample of animals from the group. Two questions arise: (i) what is the appropriate sample size to determine if the average FEC has reached a threshold? and (ii) what is the sample size needed to determine that the fraction of animals with a FEC of zero is 0.1 or less? In this paper we calculate the number of animals to be sampled for a range of FEC means and standard deviations, as well as for different fractions of animals with a zero FEC. Regarding FEC, we found that sample size was greater than the recommended 10 to 20 animals. With respect to fraction of animals with a FEC of zero, sample size was even greater than for FEC. Given the insufficient sample size currently used, we recommend a revision of the topic and a statistically based reformulation of sampling guidelines.