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Dairy cows in Thailand are Holstein Friesian crossbreds (87.5% HF) and it is generally accepted that current milk yields of these cows are below their potential. The main reason for the low milk production of these dairy cows in Thailand is related to the prevailing environmental conditions. The tropical climate is an important constraint for milk production because of two main reasons. First, it is well known that the digestibility of forages is negatively affected by tropical conditions. Unfavorable growth conditions can result in low contents of protein and minerals in the plant and high amounts of so called structural carbohydrates. Secondly, heat stress affects animal performance by affecting feed intake. The objectives of this thesis were to provide a basis for improvement of milk production in small farm holders in Thailand through supplementation of protein and salt (NaCl). Four experimental objectives were identified for this research, and it was anticipated that the outcome of the research can be applied so as to improve dairy production in Thailand and potentially other countries where dairy cows are maintained under tropical conditions. In Chapter 2, a preliminary investigation is described into the relationship between ration composition and milk production of dairy cows in Thailand. The positive correlation between milk yield and crude protein (CP) intake was high during early lactation (r = 0.65). Cows consumed 5 and 8% greater CP than requirement during early- and mid-lactation, respectively. Interestingly, significant positive correlations were found between Na intake and milk yield. This observations is somewhat difficult to explain as Na intake was highly correlated with concentrate intake (r = 0.66). In other words, the increased milk yield may been related to a higher intake of Na but this was confounded with the amount of concentrate intake. In conclusion, protein, non-fibre carbohydrates and Na intake may have limited milk production by dairy cows on the Thai farms participating in the current study. The study in Chapter 3 investigated the Na requirement of lactating dairy cows under tropical conditions by measuring Na in saliva, milk and faeces. The Na intake did not change the concentrations of Na and K in milk, faeces and serum, but did affect sodium concentration in the saliva. This observation was corroborated by the salivary Na and K concentrations with the cows on the low NaCl diet having a salivary Na concentrations 20 mmol/L (P In Chapter 4, the assessment of the Na requirement of heat stressed lactating cows is hindered by accurate estimates of the Na losses through sweat. Direct studies, therefore, are needed on the time course of healthy animals to become Na depleted and the subsequent rate of repletion. The rate of Na depletion and subsequent rate of Na repletion with two levels of dietary Na to lactating dairy cows housed under tropical conditions was investigated using the salivary Na/K. The 12 lactating cows rapidly developed clinical signs of Na deficiency, including pica, polyuria and polydipsia, reduced body weight and reduced milk yield when fed a Na-low (0.33 g/kg DM) ration during 3 weeks. Deficiency symptoms were associated with a rapid decrease in salivary Na/K ratio to 6. A daily Na intake of heat-stressed lactating cows to a ration intake of 1.6 g Na/kg DM was insufficient to restore Na deficiency. One week was sufficient to deplete heat-stressed lactating cows of Na allowing for rapid dose-response studies utilizing the salivary Na/K ratio as a parameter for Na status of cows under tropical conditions. In Chapter 5, an experiment is reported where the effects of two levels of dietary CP in concentrates with similar proportions of rumen undegradable protein (RUP) on rumen metabolism, milk yield and composition in mid lactating cows was investigated. Concentrate feeds were formulated to provide low dietary CP [17.3%; LCP] or high dietary CP [19.0%; HCP]. The proportion of rumen degradable protein and RUP was 61% and 39% in both diets, respectively. Diets were isocaloric in terms of net energy for lactation. Milk yield, milk lactose yield, dry matter intake (DMI), and apparent digestibility of DM, CP and neutral detergent fibre (NDF) were greater in cows fed the HCP than in those fed the LCP. Concentration of blood urea nitrogen (N) was elevated in cows fed HCP diets. Rumen ammonia-N concentration and pH tended to increase in cows fed the HCP diet. Rumen microorganism counts and volatile fatty acids levels in the rumen did not differ between treatments. In conclusion, increasing CP content in mid-lactating cow was beneficial to increase DMI, apparent digestibility of DM, CP and NDF and therefore milk yield. In Chapter 6, the results of the experimental studies reported in this thesis are discussed in light of Thai dairy production conditions.

Sodium and protein nutrition of lactating cows under tropical conditions