Effect of supplementation on early lactation on secretion rate of milk constituents in B . taurus x B . indicus cattle

Eight cows grazing star grass (Cynodon nlemfl uensis) were divided into four supplement treatments: Control 3 kg/day soyabean meal/sorghum supplement (16% CP), HP: High protein (2 kg soyabean meal 1 kg sorghum 31.5% CP), HE: High energy (1 kg sorghum 2 kg Ca-soap) and HPE: High protein-High energy (2 kg Ca-soap, 1 kg soyabean meal). Treatments were balanced in two 4 × 4 latin squares groups as cows received or not supplement during the dry period (3 kg 18% CP). Milk secretion rate and effi ciency was not affected by treatment. However, supplementation during the dry period increased milk protein and lactose concentration. Protein and lactose secretion rate and udder effi ciency were also increased.


INTRODUCTION
Bos taurus x B. indicus cows are commonly used in tropical dairy systems.Production systems are based mainly on grazing tropical pastures and supplements.Low productivity levels achieved in tropical dairy systems has been usually attributed to nutritional limitations arising from the tropical feeds quality (Stobbs and Thompson, 1975).However, little research has been done on the biological constraints for increased milk production.The objective of the present work was to obtain biological indicators of milk production and udder effi ciency in tropical dairy systems and to evaluate if increased productivity can be achieved by increasing the nutrient supply.
Eight multiparous cows, calved within a 1.5 week interval, grazing star grass (Cynodon nlemfl uensis), were divided into four supplement treatments: Control 3 kg/day soyabean meal/sorghum supplement (16% CP), HP: High protein (2 kg soyabean meal 1 kg sorghum 31.5% CP), HE: High Energy (1 kg sorghum 2 kg Casoap) and HPE: High protein-High energy (2 kg Ca-soap, 1 kg soyabean meal).At the beginning of the experiment, ~45 days of lactation, cows had an average milk yield of 16±4 kg/d.Treatments were designed to cover in excess their nutrient requirements.Treatments were balanced in two 4 x 4 latin squares groups as cows received or not supplement during the dry period (3 kg 18% CP supplement).Experimental periods were 2-weeks long.During the second week milk yield was recorded with a fl ow meter (Waikato, inc) and samples were taken for protein, fat and lactose analysis.Oxytocin was used to achieved complete udder emptiness in both current and previous milking.Udder volume was measured as described by Magaña and Sandoval (2003).Data was analyzed according to the experimental design using the GLM procedure of Minitab 12 (Minitab, 1997).

RESULTS AND DISCUSSION
In spite of different theoretical levels of nutrient supply to the udder, milk composition, secretion rate and udder effi ciency were not affected (P>0.05) by any of the treatments (Table 1).Then, other factors apart from nutritional stress were infl uencing the productivity of these cows.In agreement, it has been shown that the potential productivity of cows in the tropics is mainly constraint by other stressing factors as the climate (Pongpiachan et al., 2000).
The effect of supplementation during the dry period on milk composition, secretion rate and udder effi ciency is summarized in Table 2. Milk protein and lactose content were increased by dry period supplementation but not fat.However, the apparent effi ciency of the udder was increased for all three constituents.
Looking at secretion and effi ciency an apparent dichotomy is observed.Secretion was higher for supplemented cows, but, effi ciency was lower.Possible explanations are that as result of supplementation during the dry period cows had a bigger udder volume due to fat depots, or, higher number of cells, but, lower number of actually differentiated secretory cells.Although, udder volume is only a relative measure of the amount of secretory tissue, the last hypothesis is supported by the biological model analysis of the lactation curve reported by Magaña-Sevilla and Sandoval-Castro (2002) using the model described by Pollot (2000) and it is also supported by the fi nding of Akers (2000) that non dairy breeds had lower number of secretory cell per alveoli and their cells are less differentiated.The crossbred cows in the present experiment could have a predominant non-dairy genetic make up, thus explaining the lack of response to the increased MAGAÑA SEVILLA H., SANDOVAL CASTRO C.A. amount of nutrients supplied to the udder by supplementation.Thus, once lactation has established nutrient supply do not seems to be constraint for milk production as high quality diets can be supplied to the cows as in the present experiment.On the other hand, as supplementation during the dry period had a signifi cant effect, it seems that might be nutritional constraints to udder development (prepuberal and/ or prepartum) which require further research in order to obtain proper mammary gland development and potentially increase productivity of crossbred cows.

CONCLUSIONS
Milk secretion rate and effi ciency of B. taurus x B. indicus cows was not affected by supplementation during early lactation, maybe because the treatment periods were too short for the effect of feeding to take action.However, supplementing during the dry period increased protein and lactose milk concentration.It also increased protein and lactose secretion rate as well as udder effi ciency during early lactation.

Table 1 .
Milk composition and secretion rate of milk and its main constituents in B. taurus x B.