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In a series of experiments, ewe milk production and associated plasma hormone and blood metabolite status were investigated. Separate comparisons were made between ewes suckling either single (S ewes) or twin (T ewes) lambs, ewes lambing in either January or April, ewes fed either 150 g (low) or 210 g (high) of crude protein/kg DM in the diet and between ewes of the East Friesland (EF) and Scottish Blackface (SBF) genotypes. In all comparisons milk yields, live weight, body condition score changes, fat, protein and ash contents and energy values of milk were determined weekly. Blood samples were collected on one day each week, at 20 minute intervals for 2 hours, prior to feeding. Samples were pooled within each week and each animal. Plasma glucose, non- esterified fatty acids (NEFA), 3-hydroxybutyrate (3-OHB), urea, albumin, protein, insulin, growth hormone (GH), cortisol, prolactin, triiodothyronine (T3) and thyroxine (T4) concentrations were determined. During weeks 2, 4 and 10 (and 14; genotype comparison only) of lactation blood samples were collection at 20 minute intervals for 8 hours and individually assayed for plasma insulin, GH, cortisol and prolactin concentrations. Ewes rearing twin lambs had higher milk yields than those rearing single lambs. This was associated with higher NEFA, 3-OHB, GH and cortisol concentrations and lower insulin concentrations in the plasma of twin-rearing ewes. There were no consistent differences in prolactin or thyroid hormone concentrations of the plasma of single and twin rearing ewes. The decline in milk production with advancing lactation was associated with an increase in the post prandial insulin:GH and insulintcortisol ratios and T4 levels and a decrease in prolactin levels. Feeding was followed by significantly higher insulin levels and slightly higher GH levels Ewes fed the high protein diet had higher milk yields and milk protein contents compared with ewes fed the low protein diet. This, however, was not apparently associated with a higher degree of adipose tissue mobilisation. There were no significant differences between the two protein treatments in any of the plasma hormones measured. Change in milk production with stage of lactation was associated with an increase in the insulin:GH and insulinrcortisol ratios and in circulating T4 levels, and a decrease in plasma prolactin levels. During early lactation, feeding was followed by a rise in insulin and GH levels but during late lactation only insulin levels increased. In contrast to previous observations there was no difference in milk yield or pattern of milk production between the ewes of the EF and SBF genotypes. SBF ewes produced milk of higher fat content compared with EF ewes. All ewes gained similar amounts of live weight throughout lactation. However, plasma albumin and protein levels were lower in EF compared with SBF ewes; plasma insulin, cortisol, prolactin and T3 levels were consistently lower in EF than in SBF ewes. The decline in milk production in late lactation was associated with an increase in the insulin:GH ratio and in T4 levels while prolactin levels decreased. Feeding was followed by increased insulin levels and cortisol levels (EF ewes only) and lower GH levels (during late lactation). The role of these hormones in the control of milk production is discussed and in particular the hormonal inter-relationships in relation to level and pattern of milk production and associated nutrient status throughout lactation.

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Sodium discharge from dairies in California has been identified as an important contributor to soil and water pollution. The Waste Discharge Requirements General Order For Existing Milk Cow Dairies (2007) aims to minimize the amount of fixed solids, including Na, that are discharged from dairies, aiming to maximise the useable lifespan of water resources. As sodium bicarbonate (SB) contains 270 g/kg Na, SB supplementation can substantially increase Na discharge from dairies. The aim of this study was to determine the effects of SB and a potentially alternative buffer that does not contribute to Na discharge and related negative impacts on soil and water quality, (i.e., calcium magnesium carbonate (CMC)), on the performance of high producing California dairy cows. This could help establish if CMC could substitute for SB while maintaining potential benefits of SB. It is well known that SB is a rumen buffer, but research indicates that its benefits are limited mainly to corn silage-based diets. Californian dairies use a wide range of forages, and tend not to base their diets solely or mainly on corn silage. Therefore, typical Californian lactation diets do not always conform to those reported in research publications involving SB. In addition, research parameters such as milk yield, DM intake and dietary ADF often differ substantially between reported studies and conditions present on commercial California dairies, and results are therefore not always practically applicable. Apart from its rumen buffering capacity, SB also has an influence on dietary cation anion difference (DCAD), and can therefore elicit a response via a change in blood acid base balance of cows. The experiment was a Latin square design with 3 treatments (i.e., control (C), SB and CMC), 3 pens of ~310 early lactation cows each, and 3 periods of 28 d. Sodium bicarbonate supplemented cows had elevated milk fat proportion, but a reduced milk yield, resulting in similar milk fat yield between SB supplemented and C cows. Based on a tendency for elevated faecal and in vitro rumen fluid pH, SB had a buffering effect on the gastrointestinal tract GIT, most likely in the rumen. However, it is likely that the difference in DCAD between the C and SB diets played a role in affecting milk yield and milk fat proportion, and a high intake of Na may have been the cause for a reduction in milk yield resulting in passive increase in milk fat proportion. There were no differences between C and CMC treatments, except for an elevated faecal pH of CMC cows. As CMC is not generally soluble at normal rumen pH, buffering likely occurred in the abomasum and small and large intestines. However, a lack of difference in productivity indicates that the buffering effect on the hindgut was not physiologically required. While there were no productive benefits of SB use, it likely substantially increased Na discharge, resulting in an increase in soil and water sodicity and the associated deterioration in soil and water quality. While CMC did not improve productivity or efficiency of cows, it also did not increase Na discharge from the dairy and therefore did not contribute to soil or water sodicity. It can be concluded that SB or CMC supplementation is not advisable for diets and conditions comparable to those present in this study, i.e., high producing dairy cows fed a diet with ‘normal’ aNDF levels and relatively low proportions of corn silage and starch (334.0, 104.0 and 160.3 g/kg DM, respectively, in our study). Copyright / Dissertation (MSc(Agric))--University of Pretoria, 2012. / Animal and Wildlife Sciences / unrestricted

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