Twenty-eight multiparous Holstein-Zebu crossbred cows in mid-lactation were randomly assigned to receive 4 levels of cottonseed meal (CSM) (2, 3, 4, 5 kg/head/day) in diets based on ad libitum rice straw and cassava chips (5 kg/head/day). Increasing the level of CSM from 2 to 5 kg/day linearly decreased straw intake from 1.25 to 0.88% liveweight. Cottonseed meal significantly increased milk yield up to the 4 kg level of supplementation (from 8.56 to 11.3 kg/d of 3.5% FCM).

It is concluded that protein supplementation with CSM up to 4 kg/head/day significantly enhances milk yield without affecting milk composition in crossbred dairy cows fed rice straw in the tropics.

The feeding of dairy cattle in the tropics is often difficult because of deficiencies in feed supply in both quality and quantity; there is also little farmer experience of milk production with improved (crossbred) cattle (Wanapat and Devendra 1992). The use of rice straw as a feed in the dry season, in spite of its low nutritive value (Wanapat 1994), has been a common system generally practiced by dairy farmers in the tropics when green forages are not available.

A large scale project in China demonstrated that intensive beef production could be achieved with rice straw diets provided these were adequately supplemented with a source of bypass protein such as cottonseed meal (Zhang Weixian et al 1994). The objective of the present experiment was to study the effect of increasing levels of cottonseed meal as a supplement for lactating crossbred cattle fed rice straw.

Twenty-eight multiparous Holstein Friesian crossbred cattle were randomly assigned during mid-lactation according to a randomized design to receive cottonseed meal supplementation at 2, 3, 4 and 5 kg/head/day in a diet in which rice straw was offered as the roughage on an ad libitum basis. In addition, cassava chip were given to all animals to provide non-structural carbohydrate at the rate of 5 kg/head/day. The animals were offered the feeds in two equal meals per day. Rice straw refusals were recorded on a daily basis. Milk yields were recorded daily. Samples of feeds, and of morning and afternoon milk were collected two times during the 60 day feeding period to be analyzed for their chemical composition using standard methods. All data were subjected to statistical analyses using SAS (1985).

The chemical composition of the feeds used in the experiment is shown in Table 1. The experimental results are shown in Table 2. Straw intake was significantly decreased from 1.25% to 0.88% of liveweight as the level of CSM was increased. Rumen pH was high and did not differ among treatments.

Table 1: Chemical compositions of rice straw, cassava chips and cottonseed meal
	DM	Ash	CP	NDF	ADF
	%	------ % dry matter---------
Rice straw	93.0	14.6	4.25	78.6	47.2
Cassava chips	91.2	3.7	2.96	16.0	9.2
CS meal	90.3	9.1	46.2	38.6	22.3

Table 2: Effect of cottonseed meal supplementation on rice straw intake, milk yield and milk composition.
	CSM, kg/day				SEM
	2	3	4	5
Straw intake,
kg/d	4.83	4.39	4.12	3.35	±0.39
% LWt	1.25a	1.14a	1.08a	0.88b	±0.08
Rumen pH	7.18	7.01	7.13	7.08	±0.04
Milk yield,
kg/d	7.50a	9.16b	10.1b	8.73bc	±0.56
3.5% F.M.	8.56a	10.3b	11.3b	9.96c	±0.69
Milk composition, %
Fat	4.38	4.49	4.24	4.38	±0.95
Protein	3.65	3.55	3.56	3.49	±0.13
Lactose	4.38	4.69	4.38	4.44	±0.06
SNF	8.72	8.94	8.63	8.63	±0.14

Milk yield was remarkably enhanced by CSM supplementation with a 33% increase in 3.5%F.M. as CSM was raised from 2 to 4 kg/head/day. With 5 kg/head/day of CSM the yield was depressed. Milk fat, protein and solids-not-fat contents were quite high and were similar among treatments.

The high values of rumen pH indicated that the rumen ecosystem had not been seriously affected by the feeding of 5 kg/day of cassava chips. Nevertheless, the depression in straw intake, and the reduction in milk yield at the 5 kg/day level of CSM, presumably reflected a depression in fibre digestion which was not compensated by the increased amount of supplement.

Milk production of crossbred Holstein-Zebu cows fed a low protein basal diet of rice straw and cassava chips was markedly improved when the cottonseed meal supplement was increased from 2 to 4 kg/day. This feeding system is an economical attractive alternative to farmers who traditionally use commercial concentrates. Both cassava chips and cottonseed meal are locally available.

The authors wish to express their sincere gratitude to the Thailand Research Fund (TRF) for financial support of this research. Thanks are also given to the staff at Khon Kaen University, the Dairy Farming Organization (DPO) of Thailand and the University of Newcastle upon Tyne, UK for their collaboration and technical support.

Received 10 September 1996