Holstein cattle have been selectively bred for their high milk production. The seed of the cotton plant, Gossypium spp. is a good source of fibre, protein and carbohydrate. To improve milk production, whole cottonseed or cottonseed meal can be added to livestock feed. Cottonseed meal is cottonseed that has been processed by grinding. [Figure 2.1A] shows whole cotton seed and [Figure 2.1B] shows cottonseed meal. Free gossypol is a toxin found in cotton seeds. Some of this free gossypol binds to protein to form bound gossypol. This occurs during cottonseed meal production and in the rumen (forestomach) of the cow during microorganism fermentation. Free gossypol is easily absorbed. Bound gossypol is absorbed less easily and is not toxic.

1. (a)An investigation was carried out on the effect of different dry-matter diets on lactating (milk-producing) Holstein cows: • 30 healthy cows were fed on the same cottonseed-free diet for 14 days. • The cows were then divided equally into five groups, A to E, and fed on one of five experimental diets for 42 days. • The quantity of milk produced each day was recorded. • The cows had no visible signs of illness during the 42 days. For each diet, [Table 2.1] shows: • the cottonseed content and total gossypol (free and bound) content • the mean daily dry matter taken in during feeding (intake) • the mean daily lactation performance (milk yield). SE = standard error The differences in the dry matter intake for groups A to E were not statistically significant. Explain why the cows were all fed on the same cottonseed-free diet for 14 days.[1]
2. (b)(i)Using the information provided and the results shown in Table 2.1, a student made some statements about the effect of feeding different diets to lactating Holstein cows. The student stated that if the investigation is repeated, the groups of cows will produce milk in this order: (highest quantity) E → B → D → C → A (lowest quantity). With reference to Table 2.1, explain why the statement made by the student is not supported by the data.[3]
3. (b)(ii)Scientists are developing cotton plants that are genetically modified to produce seeds lacking gossypol. Table 2.1 suggests that there may be a link between total gossypol in the diet and milk yield. The student stated that repeating the investigation with no gossypol in diets B to E would result in a lower milk yield for each group. Explain whether or not Table 2.1 provides enough evidence to support the statement made by the student.[2]
4. (c)(i)Higher concentrations of gossypol absorbed into the circulation can cause an illness in cows known as gossypol toxicity. Low concentrations of gossypol can be detoxified in the liver. During the investigation, the gossypol intake for each cow was measured and analysed. At the end of the investigation (day 42), the blood plasma gossypol concentration of each cow was also measured. [Figure 2.2] shows the free gossypol intake for each cow, plotted against the concentration of gossypol in plasma on day 42. With reference to [Figure 2.2], explain whether there is a relationship between free gossypol intake and plasma gossypol concentration on day 42.[1]
5. (c)(ii)Gossypol produced by cotton plants can be found in two forms, the (–) isomer and the (+) isomer. The (–) isomer has higher biological activity within cells and is much more toxic than the (+) isomer. Both forms can bind protein to become bound gossypol. [Table 2.2] summarises the mean daily gossypol intakes for the five different experimental groups of lactating Holstein cows. The investigators knew that different diets presented different levels of risk of causing gossypol toxicity in cows. For example, the diet for group A cows did not present any risk of toxicity. With reference to Table 2.2, discuss the extent to which the different experimental diets for groups B, C, D and E present a risk of causing gossypol toxicity.[3]

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